# Patent

## Title
Method of updating IC instruction and data cache

## Abstract

A method of updating a cache in an integrated circuit is provided. The integrated circuit incorporates the cache, memory and a memory interface connected to the cache and memory. Following a cache miss, the method fetches, using the memory interface, first data associated with the cache miss and second data from the memory, where the second data is stored in the memory adjacent the first data, and updates the cache with the fetched first and second data via the memory interface. The cache includes instruction and data cache, the method performing arbitration between instruction cache misses and data cache misses such that the fetching and updating are performed for data cache misses before instruction cache misses.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. Ser. No. 11/488,853 filed Jul. 19, 2006, which is a continuation of U.S. Ser. No. 10/727,178 filed on Dec. 2, 2003, now issued Pat. No. 7,181,572, all of which is herein incorporated by reference.


FIELD OF INVENTION
The present invention relates to a cache updating mechanism for use in a computer system. The invention has primarily been developed for use in a printer controller chip that controls a printhead comprising one or more printhead modules constructed using microelectromechanical systems (MEMS) techniques, and will be described with reference to this application. However, it will be appreciated that the invention can be applied to other types of printing technologies in which analogous problems are faced.
BACKGROUND OF INVENTION
Manufacturing a printhead that has relatively high resolution and print-speed raises a number of problems. Difficulties in manufacturing pagewidth printheads of any substantial size arise due to the relatively small dimensions of standard silicon wafers that are used in printhead (or printhead module) manufacture. For example, if it is desired to make an 8 inch wide pagewidth printhead, only one such printhead can be laid out on a standard 8-inch wafer, since such wafers are circular in plan. Manufacturing a pagewidth printhead from two or more smaller modules can reduce this limitation to some extent, but raises other problems related to providing a joint between adjacent printhead modules that is precise enough to avoid visible artefacts (which would typically take the form of noticeable lines) when the printhead is used. The problem is exacerbated in relatively high-resolution applications because of the tight tolerances dictated by the small spacing between nozzles. The quality of a joint region between adjacent printhead modules relies on factors including a precision with which the abutting ends of each module can be manufactured, the accuracy with which they can be aligned when assembled into a single printhead, and other more practical factors such as management of ink channels behind the nozzles. It will be appreciated that the difficulties include relative vertical displacement of the printhead modules with respect to each other.
Whilst some of these issues may be dealt with by careful design and manufacture, the level of precision required renders it relatively expensive to manufacture printheads within the required tolerances. It would be desirable to provide a solution to one or more of the problems associated with precision manufacture and assembly of multiple printhead modules to form a printhead, and especially a pagewidth printhead.
In some cases, it is desirable to produce a number of different printhead module types or lengths on a substrate to maximise usage of the substrate's surface area. However, different sizes and types of modules will have different numbers and layouts of print nozzles, potentially including different horizontal and vertical offsets. Where two or more modules are to be joined to form a single printhead, there is also the problem of dealing with different seam shapes between abutting ends of joined modules, which again may incorporate vertical or horizontal offsets between the modules. Printhead controllers are usually dedicated application specific integrated circuits (ASICs) designed for specific use with a single type of printhead module, that is used by itself rather than with other modules. It would be desirable to provide a way in which different lengths and types of printhead modules could be accounted for using a single printer controller.
Printer controllers face other difficulties when two or more printhead modules are involved, especially if it is desired to send dot data to each of the printheads directly (rather than via a single printhead connected to the controller). One concern is that data delivered to different length controllers at the same rate will cause the shorter of the modules to be ready for printing before any longer modules. Where there is little difference involved, the issue may not be of importance, but for large length differences, the result is that the bandwidth of a shared memory from which the dot data is supplied to the modules is effectively left idle once one of the modules is full and the remaining module or modules is still being filled. It would be desirable to provide a way of improving memory bandwidth usage in a system comprising a plurality of printhead modules of uneven length.
In any printing system that includes multiple nozzles on a printhead or printhead module, there is the possibility of one or more of the nozzles failing in the field, or being inoperative due to manufacturing defect. Given the relatively large size of a typical printhead module, it would be desirable to provide some form of compensation for one or more “dead” nozzles. Where the printhead also outputs fixative on a per-nozzle basis, it is also desirable that the fixative is provided in such a way that dead nozzles are compensated for.
A printer controller can take the form of an integrated circuit, comprising a processor and one or more peripheral hardware units for implementing specific data manipulation functions. A number of these units and the processor may need access to a common resource such as memory. One way of arbitrating between multiple access requests for a common resource is timeslot arbitration, in which access to the resource is guaranteed to a particular requester during a predetermined timeslot.
One difficulty with this arrangement lies in the fact that not all access requests make the same demands on the resource in terms of timing and latency. For example, a memory read requires that data be fetched from memory, which may take a number of cycles, whereas a memory write can commence immediately. Timeslot arbitration does not take into account these differences, which may result in accesses being performed in a less efficient manner than might otherwise be the case. It would be desirable to provide a timeslot arbitration scheme that improved this efficiency as compared with prior art timeslot arbitration schemes.
Also of concern when allocating resources in a timeslot arbitration scheme is the fact that the priority of an access request may not be the same for all units. For example, it would be desirable to provide a timeslot arbitration scheme in which one requestor (typically the memory) is granted special priority such that its requests are dealt with earlier than would be the case in the absence of such priority.
In systems that use a memory and cache, a cache miss (in which an attempt to load data or an instruction from a cache fails) results in a memory access followed by a cache update. It is often desirable when updating the cache in this way to update data other than that which was actually missed. A typical example would be a cache miss for a byte resulting in an entire word or line of the cache associated with that byte being updated. However, this can have the effect of tying up bandwidth between the memory (or a memory manager) and the processor where the bandwidth is such that several cycles are required to transfer the entire word or line to the cache. It would be desirable to provide a mechanism for updating a cache that improved cache update speed and/or efficiency.
Most integrated circuits an externally provided signal as (or to generate) a clock, often provided from a dedicated clock generation circuit. This is often due to the difficulties of providing an onboard clock that can operate at a speed that is predictable. Manufacturing tolerances of such on-board clock generation circuitry can result in clock rates that vary by a factor of two, and operating temperatures can increase this margin by an additional factor of two. In some cases, the particular rate at which the clock operates is not of particular concern. However, where the integrated circuit will be writing to an internal circuit that is sensitive to the time over which a signal is provided, it may be undesirable to have the signal be applied for too long or short a time. For example, flash memory is sensitive to being written too for too long a period. It would be desirable to provide a mechanism for adjusting a rate of an on-chip system clock to take into account the impact of manufacturing variations on clockspeed.
One form of attacking a secure chip is to induce (usually by increasing) a clock speed that takes the logic outside its rated operating frequency. One way of doing this is to reduce the temperature of the integrated circuit, which can cause the clock to race. Above a certain frequency, some logic will start malfunctioning. In some cases, the malfunction can be such that information on the chip that would otherwise be secure may become available to an external connection. It would be desirable to protect an integrated circuit from such attacks.
In an integrated circuit comprising non-volatile memory, a power failure can result in unintentional behaviour. For example, if an address or data becomes unreliable due to falling voltage supplied to the circuit but there is still sufficient power to cause a write, incorrect data can be written. Even worse, the data (incorrect or not) could be written to the wrong memory. The problem is exacerbated with multi-word writes. It would be desirable to provide a mechanism for reducing or preventing spurious writes when power to an integrated circuit is failing.
In an integrated circuit, it is often desirable to reduce unauthorised access to the contents of memory. This is particularly the case where the memory includes a key or some other form of security information that allows the integrated circuit to communicate with another entity (such as another integrated circuit, for example) in a secure manner. It would be particularly advantageous to prevent attacks involving direct probing of memory addresses by physically investigating the chip (as distinct from electronic or logical attacks via manipulation of signals and power supplied to the integrated circuit).
It is also desirable to provide an environment where the manufacturer of the integrated circuit (or some other authorised entity) can verify or authorize code to be run on an integrated circuit.
Another desideratum would be the ability of two or more entities, such as integrated circuits, to communicate with each other in a secure manner. It would also be desirable to provide a mechanism for secure communication between a first entity and a second entity, where the two entities, whilst capable of some form of secure communication, are not able to establish such communication between themselves.
In a system that uses resources (such as a printer, which uses inks) it may be desirable to monitor and update a record related to resource usage. Authenticating ink quality can be a major issue, since the attributes of inks used by a given printhead can be quite specific. Use of incorrect ink can result in anything from misfiring or poor performance to damage or destruction of the printhead. It would therefore be desirable to provide a system that enables authentication of the correct ink being used, as well as providing various support systems secure enabling refilling of ink cartridges.
In a system that prevents unauthorized programs from being loaded onto or run on an integrated circuit, it can be laborious to allow developers of software to access the circuits during software development. Enabling access to integrated circuits of a particular type requires authenticating software with a relatively high-level key. Distributing the key for use by developers is inherently unsafe, since a single leak of the key outside the organization could endanger security of all chips that use a related key to authorize programs. Having a small number of people with high-security clearance available to authenticate programs for testing can be inconvenient, particularly in the case where frequent incremental changes in programs during development require testing. It would be desirable to provide a mechanism for allowing access to one or more integrated circuits without risking the security of other integrated circuits in a series of such integrated circuits.
In symmetric key security, a message, denoted by M, is plaintext. The process of transforming M into ciphertext C, where the substance of M is hidden, is called encryption. The process of transforming C back into M is called decryption. Referring to the encryption function as E, and the decryption function as D, we have the following identities:

E[M]=C

D[C]=M

Therefore the following identity is true:

D[E[M]]=M

A symmetric encryption algorithm is one where:

    
    
        the encryption function E relies on key K1,
        the decryption function D relies on key K2,
        K2 can be derived from K2, and
        K1 can be derived from K2.
    
    


In most symmetric algorithms, K1 equals K2. However, even if K1 does not equal K2, given that one key can be derived from the other, a single key K can suffice for the mathematical definition. Thus:

EK[M]=C

DK[C]=M

The security of these algorithms rests very much in the key K. Knowledge of K allows anyone to encrypt or decrypt. Consequently K must remain a secret for the duration of the value of M. For example, M may be a wartime message “My current position is grid position 123-456”. Once the war is over the value of M is greatly reduced, and if K is made public, the knowledge of the combat unit's position may be of no relevance whatsoever. The security of the particular symmetric algorithm is a function of two things: the strength of the algorithm and the length of the key.
An asymmetric encryption algorithm is one where:

    
    
        the encryption function E relies on key K1,
        the decryption function D relies on key K2,
        K2 cannot be derived from K1 in a reasonable amount of time, and
        K1 cannot be derived from K2 in a reasonable amount of time.
    
    


Thus:

EK1[M]=C

DK2[C]=M

These algorithms are also called public-key because one key K1 can be made public. Thus anyone can encrypt a message (using K1) but only the person with the corresponding decryption key (K2) can decrypt and thus read the message.
In most cases, the following identity also holds:

EK2[M]=C

DK1[C]=M

This identity is very important because it implies that anyone with the public key K1 can see M and know that it came from the owner of K2. No-one else could have generated C because to do so would imply knowledge of K2. This gives rise to a different application, unrelated to encryption-digital signatures.
A number of public key cryptographic algorithms exist. Most are impractical to implement, and many generate a very large C for a given M or require enormous keys. Still others, while secure, are far too slow to be practical for several years. Because of this, many public key systems are hybrid—a public key mechanism is used to transmit a symmetric session key, and then the session key is used for the actual messages.
All of the algorithms have a problem in terms of key selection. A random number is simply not secure enough. The two large primes p and q must be chosen carefully—there are certain weak combinations that can be factored more easily (some of the weak keys can be tested for). But nonetheless, key selection is not a simple matter of randomly selecting 1024 bits for example. Consequently the key selection process must also be secure.
Symmetric and asymmetric schemes both suffer from a difficulty in allowing establishment of multiple relationships between one entity and a two or more others, without the need to provide multiple sets of keys. For example, if a main entity wants to establish secure communications with two or more additional entities, it will need to maintain a different key for each of the additional entities. For practical reasons, it is desirable to avoid generating and storing large numbers of keys. To reduce key numbers, two or more of the entities may use the same key to communicate with the main entity. However, this means that the main entity cannot be sure which of the entities it is communicating with. Similarly, messages from the main entity to one of the entities can be decrypted by any of the other entities with the same key. It would be desirable if a mechanism could be provided to allow secure communication between a main entity and one or more other entities that overcomes at least some of the shortcomings of prior art.
In a system where a first entity is capable of secure communication of some form, it may be desirable to establish a relationship with another entity without providing the other entity with any information related the first entity's security features. Typically, the security features might include a key or a cryptographic function. It would be desirable to provide a mechanism for enabling secure communications between a first and second entity when they do not share the requisite secret function, key or other relationship to enable them to establish trust.
A number of other aspects, features, preferences and embodiments are disclosed in the Detailed Description of the Preferred Embodiment below.
SUMMARY OR INVENTION
In accordance with the invention, there is provided a method of updating a cache in an integrated circuit comprising:

    
    
        the cache
        a processor connected to the cache via a cache bus;
        a memory interface connected to the cache via a first bus and to the processor via a second bus, the first bus being wider than the second bus or the cache bus; and
        memory connected to the memory interface via a memory bus;
        the method comprising the steps of:
        (a) following a cache miss, using the processor to issue a request for first data via a first address, the first data being that associated with the cache miss;
        (b) in response to the request, using the memory interface to fetch the first data from the memory, and sending the first data to the processor;
        (c) sending, from the memory interface and via the first bus, the first data and additional data, the additional data being that stored in the memory adjacent the first data;
        (d) updating the cache with the first data and the additional data via the first bus; and
        (e) updating flags in the cache associated with the first data and the additional data, such that the updated first data and additional data in the cache is valid.
    
    


Preferably, the processor is configured to attempt a cache update with the first data upon receiving it from the memory interface, the method further including the step of preventing the attempted cache update by the processor from being successful, thereby preventing interference with the cache update of steps (d) and/or (e).
More preferably, steps (c), (d), and (e) are performed substantially simultaneously.
In one embodiment, steps (d) and (e) are performed by the memory interface.
Preferably, steps (d) and (e) are performed in response to the processor attempting to update the cache following step (c). More preferably, the memory interface is configured to monitor the processor to determine when it attempts to update the cache following step (c).



BRIEF DESCRIPTION OF THE DRAWINGS
Preferred and other embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 is an example of a single printer controller (hereinafter “SoPEC”) A4 simplex printer system
FIG. 2 shows a SoPEC system top level partition
FIG. 3 shows a SoPEC CPU memory map (not to scale)
FIG. 4 is a block diagram of CPU
FIG. 5 shows CPU bus transactions
FIG. 6 shows a state machine for a CPU subsystem slave
FIG. 7 shows a SoPEC CPU memory map (not to scale)
FIG. 8 shows an external signal view of a memory management unit (hereinafter “MMU”) sub-block partition
FIG. 9 shows an internal signal view of an MMU sub-block partition
FIG. 10 shows a DRAM write buffer
FIG. 11 shows DIU waveforms for multiple transactions
FIG. 12 shows a SoPEC LEON CPU core
FIG. 13 shows a cache data RAM wrapper
FIG. 14 shows a realtime debug unit block diagram
FIG. 15 shows interrupt acknowledge cycles for single and pending interrupts



DETAILED DESCRIPTION OF EMBODIMENTS
Imperative phrases such as “must”, “requires”, “necessary” and “important” (and similar language) should be read as being indicative of being necessary only for the preferred embodiment actually being described. As such, unless the opposite is clear from the context, imperative wording should not be interpreted as such. Nothing in the detailed description is to be understood as limiting the scope of the invention, which is intended to be defined as widely as is defined in the accompanying claims.
The preferred of the present invention is implemented in a printer using microelectromechanical systems (MEMS) printheads. The printer can receive data from, for example, a personal computer such as an IBM compatible PC or Apple computer. In other embodiments, the printer can receive data directly from, for example, a digital still or video camera. The particular choice of communication link is not important, and can be based, for example, on USB, Firewire, Bluetooth or any other wireless or hardwired communications protocol.
A SoPEC (Small office home office Print Engine Controller) ASIC (Application Specific Integrated Circuit) is provided which suitable for use in, for example, SoHo printer products. The SoPEC ASIC is intended to be a low cost solution for bi-lithic printhead control, replacing the multichip solutions in larger more professional systems with a single chip. The increased cost competitiveness is achieved by integrating several systems such as a modified PEC1 printing pipeline, CPU control system, peripherals and memory sub-system onto one SoC ASIC, reducing component count and simplifying board design. The following terms are used throughout this specification:

    Bi-lithic printhead Refers to printhead constructed from 2 printhead ICs
    CPU Refers to CPU core, caching system and MMU.
    ISI-Bridge chip A device with a high speed interface (such as USB2.0, Ethernet or IEEE1394) and one or more ISI interfaces. The ISI-Bridge would be the ISIMaster for each of the ISI buses it interfaces to.
    ISIMaster The ISIMaster is the only device allowed to initiate communication on the Inter Sopec Interface (ISI) bus. The ISIMaster interfaces with the host.
    ISISlave Multi-SoPEC systems will contain one or more ISISlave SoPECs connected to the ISI bus. ISISlaves can only respond to communication initiated by the ISIMaster.
    LEON Refers to the LEON CPU core.
    LineSyncMaster The LineSyncMaster device generates the line synchronisation pulse that all SoPECs in the system must synchronise their line outputs to.
    Multi-SoPEC Refers to SoPEC based print system with multiple SoPEC devices
    Netpage Refers to page printed with tags (normally in infrared ink).
    PEC1 Refers to Print Engine Controller version 1, precursor to SoPEC used to control printheads constructed from multiple angled printhead segments.
    Printhead IC Single MEMS IC used to construct bi-lithic printhead
    PrintMaster The PrintMaster device is responsible for coordinating all aspects of the print operation. There may only be one PrintMaster in a system.
    QA Chip Quality Assurance Chip


Storage SoPEC An ISISlave SoPEC used as a DRAM store and which does not print.

    Tag Refers to pattern which encodes information about its position and orientation which allow it to be optically located and its data contents read.


A bi-lithic printhead produces 1600 dpi bi-level dots. On low-diffusion paper, each ejected drop forms a 22.5 m diameter dot. Dots are easily produced in isolation, allowing dispersed-dot dithering to be exploited to its fullest. Since the bi-lithic printhead is the width of the page and operates with a constant paper velocity, color planes are printed in perfect registration, allowing ideal dot-on-dot printing. Dot-on-dot printing minimizes ‘muddying’ of midtones caused by inter-color bleed.
A page layout may contain a mixture of images, graphics and text. Continuous-tone (contone) images and graphics are reproduced using a stochastic dispersed-dot dither. Unlike a clustered-dot (or amplitude-modulated) dither, a dispersed-dot (or frequency-modulated) dither reproduces high spatial frequencies (i.e. image detail) almost to the limits of the dot resolution, while simultaneously reproducing lower spatial frequencies to their full color depth, when spatially integrated by the eye. A stochastic dither matrix is carefully designed to be free of objectionable low-frequency patterns when tiled across the image. As such its size typically exceeds the minimum size required to support a particular number of intensity levels (e.g. 16×16×8 bits for 257 intensity levels).
Human contrast sensitivity peaks at a spatial frequency of about 3 cycles per degree of visual field and then falls off logarithmically, decreasing by a factor of 100 beyond about 40 cycles per degree and becoming immeasurable beyond 60 cycles per degree [25][25]. At a normal viewing distance of 12 inches (about 300 mm), this translates roughly to 200-300 cycles per inch (cpi) on the printed page, or 400-600 samples per inch according to Nyquist's theorem.
In practice, contone resolution above about 300 ppi is of limited utility outside special applications such as medical imaging. Offset printing of magazines, for example, uses contone resolutions in the range 150 to 300 ppi. Higher resolutions contribute slightly to color error through the dither.
Black text and graphics are reproduced directly using bi-level black dots, and are therefore not anti-aliased (i.e. low-pass filtered) before being printed. Text should therefore be supersampled beyond the perceptual limits discussed above, to produce smoother edges when spatially integrated by the eye. Text resolution up to about 1200 dpi continues to contribute to perceived text sharpness (assuming low-diffusion paper, of course).
A Netpage printer, for example, may use a contone resolution of 267 ppi (i.e. 1600 dpi), and a black text and graphics resolution of 800 dpi. A high end office or departmental printer may use a contone resolution of 320 ppi (1600 dpi/5) and a black text and graphics resolution of 1600 dpi. Both formats are capable of exceeding the quality of commercial (offset) printing and photographic reproduction.
The SoPEC device can be used in several printer configurations and architectures.
In the general sense every SoPEC based printer architecture will contain:

    
    
        One or more SoPEC devices.
        One or more bi-lithic printheads.
        Two or more Low Speed Serial interface (LSS) busses.
        Two or more QA chips.
        USB 1.1 connection to host or ISI connection to Bridge Chip.
        ISI bus connection between SoPECs (when multiple SoPECs are used).
    
    


The SoPEC device contains several system on a chip (SoC) components, as well as the print engine pipeline control application specific logic.
The Print Engine Pipeline (PEP) reads compressed page store data from the embedded memory, optionally decompresses the data and formats it for sending to the printhead. The print engine pipeline functionality includes expanding the page image, dithering the contone layer, compositing the black layer over the contone layer, rendering of Netpage tags, compensation for dead nozzles in the printhead, and sending the resultant image to the bi-lithic printhead.
SoPEC contains an embedded CPU for general purpose system configuration and management. The CPU performs page and band header processing, motor control and sensor monitoring (via the GPIO) and other system control functions. The CPU can perform buffer management or report buffer status to the host. The CPU can optionally run vendor application specific code for general print control such as paper ready monitoring and LED status update.
A 2.5Mbyte embedded memory buffer is integrated onto the SoPEC device, of which approximately 2Mbytes are available for compressed page store data. A compressed page is divided into one or more bands, with a number of bands stored in memory. As a band of the page is consumed by the PEP for printing a new band can be downloaded. The new band may be for the current page or the next page.
Using banding it is possible to begin printing a page before the complete compressed page is downloaded, but care must be taken to ensure that data is always available for printing or a buffer underrun may occur.
An Storage SoPEC acting as a memory buffer or an ISI-Bridge chip with attached DRAM could be used to provide guaranteed data delivery.
The embedded USB 1.1 device accepts compressed page data and control commands from the host PC, and facilitates the data transfer to either embedded memory or to another SoPEC device in multi-SoPEC systems.
The printhead is constructed by abutting 2 printhead ICs together. The printhead ICs can vary in size from 2 inches to 8 inches, so to produce an A4 printhead several combinations are possible. For example two printhead ICs of 7 inches and 3 inches could be used to create a A4 printhead (the notation is 7:3). Similarly 6 and 4 combination (6:4), or 5:5 combination. For an A3 printhead it can be constructed from 8:6 or an 7:7 printhead IC combination. For photographic printing smaller printheads can be constructed.
Each SoPEC device has 2 LSS system buses for communication with QA devices for system authentication and ink usage accounting. The number of QA devices per bus and their position in the system is unrestricted with the exception that PRINTER_QA and INK_QA devices should be on separate LSS busses.
Each SoPEC system can have several QA devices. Normally each printing SoPEC will have an associated PRINTER_QA. Ink cartridges will contain an INK_QA chip.
PRINTER_QA and INK_QA devices should be on separate LSS busses. All QA chips in the system are physically identical with flash memory contents defining PRINTER_QA from INK_QA chip.
The Inter-SoPEC Interface (ISI) provides a communication channel between SoPECs in a multi-SoPEC system. The ISIMaster can be SoPEC device or an ISI-Bridge chip depending on the printer configuration. Both compressed data and control commands are transferred via the interface.
A device, other than a SoPEC with a USB connection, which provides print data to a number of slave SoPECs. A bridge chip will typically have a high bandwidth connection, such as USB2.0, Ethernet or IEEE1394, to a host and may have an attached external DRAM for compressed page storage. A bridge chip would have one or more ISI interfaces. The use of multiple ISI buses would allow the construction of independent print systems within the one printer. The ISI-Bridge would be the ISIMaster for each of the ISI buses it interfaces to.
Several possible SoPEC based system architectures exist. It is possible to have extra SoPEC devices in the system used for DRAM storage. The QA chip configurations shown are indicative of the flexibility of LSS bus architecture, but not limited to those configurations.
The Small Office Home Office Print Engine Controller (SoPEC) is a page rendering engine ASIC that takes compressed page images as input, and produces decompressed page images at up to 6 channels of bi-level dot data as output. The bi-level dot data is generated for the Memjet bi-lithic printhead. The dot generation process takes account of printhead construction, dead nozzles, and allows for fixative generation.
A single SoPEC can control 2 bi-lithic printheads and up to 6 color channels at 10,000 lines/sec, equating to 30 pages per minute at 1600dpi. A single SoPEC can perform full-bleed printing of A3, A4 and Letter pages. The 6 channels of colored ink are the expected maximum in a consumer SOHO, or office Bi-lithic printing environment:

    
    
        CMY, for regular color printing.
        K, for black text, line graphics and gray-scale printing.
        IR (infrared), for Netpage-enabled applications.
        F (fixative), to enable printing at high speed. Because the bi-lithic printer is capable of printing so fast, a fixative may be required to enable the ink to dry before the page touches the page already printed. Otherwise the pages may bleed on each other. In low speed printing environments the fixative may not be required.
    
    


SoPEC is color space agnostic. Although it can accept contone data as CMYX or RGBX, where X is an optional 4th channel, it also can accept contone data in any print color space. Additionally, SoPEC provides a mechanism for arbitrary mapping of input channels to output channels, including combining dots for ink optimization, generation of channels based on any number of other channels etc. However, inputs are typically CMYK for contone input, K for the bi-level input, and the optional Netpage tag dots are typically rendered to an infra-red layer. A fixative channel is typically generated for fast printing applications.
SoPEC provides an interface for synchronization with other SoPECs. This allows simple multi-SoPEC solutions for simultaneous A3/A4/Letter duplex printing. However, SoPEC is also capable of printing only a portion of a page image. Combining synchronization functionality with partial page rendering allows multiple SoPECs to be readily combined for alternative printing requirements including simultaneous duplex printing and wide format printing.
From the highest point of view the SoPEC device consists of 3 distinct subsystems

    
    
        CPU Subsystem
        DRAM Subsystem
        PEP Subsystem
    
    


See FIG. 2 for a block level diagram of SoPEC.
The CPU subsystem controls and configures all aspects of the other subsystems. It provides general support for interfacing and synchronising the external printer with the internal print engine. It also controls the low speed communication to the QA chips. The CPU subsystem contains various peripherals to aid the CPU, such as GPIO (includes motor control), interrupt controller, LSS Master and general timers. The Serial Communications Block (SCB) on the CPU subsystem provides a full speed USB 1.1 interface to the host as well as an Inter SoPEC Interface (ISI) to other SoPEC devices.
The DRAM subsystem accepts requests from the CPU, Serial Communications Block (SCB) and blocks within the PEP subsystem. The DRAM subsystem (in particular the DIU) arbitrates the various requests and determines which request should win access to the DRAM. The DIU arbitrates based on configured parameters, to allow sufficient access to DRAM for all requesters. The DIU also hides the implementation specifics of the DRAM such as page size, number of banks, refresh rates etc.
The PEP subsystem accepts compressed pages from DRAM and renders them to bi-level dots for a given print line destined for a printhead interface that communicates directly with up to 2 segments of a bi-lithic printhead.
The first stage of the page expansion pipeline is the CDU, LBD and TE. The CDU expands the JPEG-compressed contone (typically CMYK) layer, the LBD expands the compressed bi-level layer (typically K), and the TE encodes Netpage tags for later rendering (typically in IR or K ink). The output from the first stage is a set of buffers: the CFU, SFU, and TFU. The CFU and SFU buffers are implemented in DRAM.
The second stage is the HCU, which dithers the contone layer, and composites position tags and the bi-level spot( ) layer over the resulting bi-level dithered layer. A number of options exist for the way in which compositing occurs. Up to 6 channels of bi-level data are produced from this stage. Note that not all 6 channels may be present on the printhead. For example, the printhead may be CMY only, with K pushed into the CMY channels and IR ignored. Alternatively, the position tags may be printed in K if IR ink is not available (or for testing purposes).
The third stage (DNC) compensates for dead nozzles in the printhead by color redundancy and error diffusing dead nozzle data into surrounding dots.
The resultant bi-level 6 channel dot-data (typically CMYK-IRF) is buffered and written out to a set of line buffers stored in DRAM via the DWU.
Finally, the dot-data is loaded back from DRAM, and passed to the printhead interface via a dot FIFO. The dot FIFO accepts data from the LLU at the system clock rate (pclk), while the PHI removes data from the FIFO and sends it to the printhead at a rate of 2⁄3 times the system clock rate. Looking at FIG. 2, the various units are described here in summary form:







TABLE 1







Units within SoPEC











Unit




Subsystem
Acronym
Unit Name
Description





DRAM
DIU
DRAM interface
Provides the interface for DRAM read and




unit
write access for the various SoPEC units,





CPU and the SCB block. The DIU





provides arbitration between competing





units controls DRAM access.



DRAM
Embedded DRAM
20 Mbits of embedded DRAM,


CPU
CPU
Central Processing
CPU for system configuration and control




Unit



MMU
Memory
Limits access to certain memory address




Management Unit
areas in CPU user mode



RDU
Real-time Debug
Facilitates the observation of the contents




Unit
of most of the CPU addressable registers in





SoPEC in addition to some pseudo-registers





in realtime.



TIM
General Timer
Contains watchdog and general system





timers



LSS
Low Speed Serial
Low level controller for interfacing with




Interfaces
the QA chips



GPIO
General Purpose IOs
General IO controller, with built-in Motor





control unit, LED pulse units and de-glitch





circuitry



ROM
Boot ROM
16 KBytes of System Boot ROM code



ICU
Interrupt Controller
General Purpose interrupt controller with




Unit
configurable priority, and masking.



CPR
Clock, Power and
Central Unit for controlling and generating




Reset block
the system clocks and resets and





powerdown mechanisms



PSS
Power Save Storage
Storage retained while system is powered





down



USB
Universal Serial Bus
USB device controller for interfacing with




Device
the host USB.



ISI
Inter-SoPEC
ISI controller for data and control




Interface
communication with other SoPEC's in a





multi-SoPEC system



SCB
Serial
Contains both the USB and ISI blocks.




Communication




Block


Print
PCU
PEP controller
Provides external CPU with the means to


Engine


read and write PEP Unit registers, and read


Pipeline


and write DRAM in single 32-bit chunks.


(PEP)



CDU
Contone decoder
Expands JPEG compressed contone layer




unit
and writes decompressed contone to





DRAM



CFU
Contone FIFO Unit
Provides line buffering between CDU and





HCU



LBD
Lossless Bi-level
Expands compressed bi-level layer.




Decoder



SFU
Spot FIFO Unit
Provides line buffering between LBD and





HCU



TE
Tag encoder
Encodes tag data into line of tag dots.



TFU
Tag FIFO Unit
Provides tag data storage between TE and





HCU



HCU
Halftoner
Dithers contone layer and composites the




compositor unit
bi-level spot 0 and position tag dots.



DNC
Dead Nozzle
Compensates for dead nozzles by color




Compensator
redundancy and error diffusing dead nozzle





data into surrounding dots.



DWU
Dotline Writer Unit
Writes out the 6 channels of dot data for a





given printline to the line store DRAM



LLU
Line Loader Unit
Reads the expanded page image from line





store, formatting the data appropriately for





the bi-lithic printhead.



PHI
PrintHead Interface
Is responsible for sending dot data to the bi-





lithic printheads and for providing line





synchronization between multiple SoPECs.





Also provides test interface to printhead





such as temperature monitoring and Dead





Nozzle Identification.









SoPEC must address

    
    
        20 Mbit DRAM.
        PCU addressed registers in PEP.
        CPU-subsystem addressed registers.
    
    


SoPEC has a unified address space with the CPU capable of addressing all CPU-subsystem and PCU-bus accessible registers (in PEP) and all locations in DRAM. The CPU generates byte-aligned addresses for the whole of SoPEC.
22 bits are sufficient to byte address the whole SoPEC address space.
The embedded DRAM is composed of 256-bit words. However the CPU-subsystem may need to write individual bytes of DRAM. Therefore it was decided to make the DIU byte addressable. 22 bits are required to byte address 20 Mbits of DRAM.
Most blocks read or write 256-bit words of DRAM. Therefore only the top 17 bits i.e. bits 21 to 5 are required to address 256-bit word aligned locations.
The exceptions are

    
    
        CDU which can write 64-bits so only the top 19 address bits i.e. bits 21-3 are required.
        The CPU-subsystem always generates a 22-bit byte-aligned DIU address but it will send flags to the DIU indicating whether it is an 8, 16 or 32-bit write.
    
    


All DIU accesses must be within the same 256-bit aligned DRAM word.
PEP Unit configuration registers which specify DRAM locations should specify 256-bit aligned DRAM addresses i.e. using address bits 21:5. Legacy blocks from PEC1 e.g. the LBD and TE may need to specify 64-bit aligned DRAM addresses if these reused blocks DRAM addressing is difficult to modify. These 64-bit aligned addresses require address bits 21:3. However, these 64-bit aligned addresses should be programmed to start at a 256-bit DRAM word boundary.
Unlike PEC1, there are no constraints in SoPEC on data organization in DRAM except that all data structures must start on a 256-bit DRAM boundary. If data stored is not a multiple of 256-bits then the last word should be padded.
The CPU subsystem bus supports 32-bit word aligned read and write accesses with variable access timings. The CPU subsystem bus does not currently support byte reads and writes but this can be added at a later date if required by imported IP.
The PCU only supports 32-bit register reads and writes for the PEP blocks. As the PEP blocks only occupy a subsection of the overall address map and the PCU is explicitly selected by the MMU when a PEP block is being accessed the PCU does not need to perform a decode of the higher-order address bits. The system wide memory map is shown in FIG. 3.
The address mapping for the peripherals attached to the CPU-bus includes the MMU performing the decode of cpu_adr[21:12] to generate the relevant cpu_block_select signal for each block. The addressed blocks decode however many of the lower order bits of cpu_adr[11:2] are required to address all the registers within the block.
The PEP blocks are addressed via the PCU. From FIG. 3, the PCU mapped registers are in the range 0x0002—0000 to 0x0002_BFFF. There are 12 sub-blocks within the PCU address space. Therefore, only four bits are necessary to address each of the sub-blocks within the PEP part of SoPEC. A further 12 bits may be used to address any configurable register within a PEP block. This gives scope for 1024 configurable registers per sub-block (the PCU mapped registers are all 32-bit addressed registers so the upper 10 bits are required to individually address them). This address will come either from the CPU or from a command stored in DRAM. The bus is assembled as follows:

    
    
        address[15:12]=sub-block address,
        address[n:2]=register address within sub-block, only the number of bits required to decode the registers within each sub-block are used,
        address[1:0]=byte address, unused as PCU mapped registers are all 32-bit addressed registers.
    
    


So for the case of the HCU, its addresses range from 0x7000 to 0x7FFF within the PEP subsystem or from 0x0002_7000 to 0x0002_7FFF in the overall system.
The CPU block consists of the CPU core, MMU, cache and associated logic. The principal tasks for the program running on the CPU to fulfill in the system are:
Communications:

    
    
        Control the flow of data from the USB interface to the DRAM and ISI
        Communication with the host via USB or ISI
        Running the USB device driver
    
    


PEP Subsystem Control:

    
    
        Page and band header processing (may possibly be performed on host PC)
        Configure printing options on a per band, per page, per job or per power cycle basis
        Initiate page printing operation in the PEP subsystem
        Retrieve dead nozzle information from the printhead interface (PHI) and forward to the host PC
        Select the appropriate firing pulse profile from a set of predefined profiles based on the printhead characteristics
        Retrieve printhead temperature via the PHI
    
    


Security:

    
    
        Authenticate downloaded program code
        Authenticate printer operating parameters
        Authenticate consumables via the PRINTER_QA and INK_QA chips
        Monitor ink usage
        Isolation of OEM code from direct access to the system resources
    
    


Other:

    
    
        Drive the printer motors using the GPIO pins
        Monitoring the status of the printer (paper jam, tray empty etc.)
        Driving front panel LEDs
        Perform post-boot initialisation of the SoPEC device
        Memory management (likely to be in conjunction with the host PC)
        Miscellaneous housekeeping tasks
    
    


To control the Print Engine Pipeline the CPU is required to provide a level of performance at least equivalent to a 16-bit Hitachi H8-3664 microcontroller running at 16 MHz. An as yet undetermined amount of additional CPU performance is needed to perform the other tasks, as well as to provide the potential for such activity as Netpage page assembly and processing, RIPing etc. The extra performance required is dominated by the signature verification task and the SCB (including the USB) management task. An operating system is not required at present. A number of CPU cores have been evaluated and the LEON P1754 is considered to be the most appropriate solution. A diagram of the CPU block is shown in FIG. 4.







TABLE 2







CPU Subsystem I/Os










Port name
Pins
I/O
Description










Clocks and Resets










prst_n
1
In
Global reset. Synchronous to pclk, active





low.


Pclk
1
In
Global clock







CPU to DIU DRAM interface










cpu_adr[21:2]
20
Out
Address bus for both DRAM and peripheral





access


cpu_dataout[31:0]
32
Out
Data out to both DRAM and peripheral





devices. This should be driven at the same





time as the cpu_adr and request signals.


dram_cpu_data[255:0]
256
In
Read data from the DRAM


cpu_diu_rreq
1
Out
Read request to the DIU DRAM


diu_cpu_rack
1
In
Acknowledge from DIU that read request has





been accepted.


diu_cpu_rvalid
1
In
Signal from DIU telling SoPEC Unit that





valid read data is on the dram_cpu_data bus


cpu_diu_wdatavalid
1
Out
Signal from the CPU to the DIU indicating





that the data currently on the cpu_diu_wdata





bus is valid and should be committed to the





DIU posted write buffer


diu_cpu_write_rdy
1
In
Signal from the DIU indicating that the posted





write buffer is empty


cpu_diu_wdadr[21:4]
18
Out
Write address bus to the DIU


cpu_diu_wdata[127:0]
128
Out
Write data bus to the DIU


cpu_diu_wmask[15:0]
16
Out
Write mask for the cpu_diu_wdata bus. Each





bit corresponds to a byte of the 128-bit





cpu_diu_wdata bus.







CPU to peripheral blocks










cpu_rwn
1
Out
Common read/not-write signal from the CPU


cpu_acode[1:0]
2
Out
CPU access code signals.





cpu_acode[0] - Program (0)/Data (1) access





cpu_acode[1] - User (0)/Supervisor (1)





access


cpu_cpr_sel
1
Out
CPR block select.


cpr_cpu_rdy
1
In
Ready signal to the CPU. When cpr_cpu_rdy





is high it indicates the last cycle of the access.





For a write cycle this means cpu_dataout has





been registered by the CPR block and for a





read cycle this means the data on





cpr_cpu_data is valid.


cpr_cpu_berr
1
In
CPR bus error signal to the CPU.


cpr_cpu_data[31:0]
32
In
Read data bus from the CPR block


cpu_gpio_sel
1
Out
GPIO block select.


gpio_cpu_rdy
1
In
GPIO ready signal to the CPU.


gpio_cpu_berr
1
In
GPIO bus error signal to the CPU.


gpio_cpu_data[31:0]
32
In
Read data bus from the GPIO block


cpu_icu_sel
1
Out
ICU block select.


icu_cpu_rdy
1
In
ICU ready signal to the CPU.


icu_cpu_berr
1
In
ICU bus error signal to the CPU.


icu_cpu_data[31:0]
32
In
Read data bus from the ICU block


cpu_lss_sel
1
Out
LSS block select.


lss_cpu_rdy
1
In
LSS ready signal to the CPU.


lss_cpu_berr
1
In
LSS bus error signal to the CPU.


lss_cpu_data[31:0]
32
In
Read data bus from the LSS block


cpu_pcu_sel
1
Out
PCU block select.


pcu_cpu_rdy
1
In
PCU ready signal to the CPU.


pcu_cpu_berr
1
In
PCU bus error signal to the CPU.


pcu_cpu_data[31:0]
32
In
Read data bus from the PCU block


cpu_scb_sel
1
Out
SCB block select.


scb_cpu_rdy
1
In
SCB ready signal to the CPU.


scb_cpu_berr
1
In
SCB bus error signal to the CPU.


scb_cpu_data[31:0]
32
In
Read data bus from the SCB block


cpu_tim_sel
1
Out
Timers block select.


tim_cpu_rdy
1
In
Timers block ready signal to the CPU.


tim_cpu_berr
1
In
Timers bus error signal to the CPU.


tim_cpu_data[31:0]
32
In
Read data bus from the Timers block


cpu_rom_sel
1
Out
ROM block select.


rom_cpu_rdy
1
In
ROM block ready signal to the CPU.


rom_cpu_berr
1
In
ROM bus error signal to the CPU.


rom_cpu_data[31:0]
32
In
Read data bus from the ROM block


cpu_pss_sel
1
Out
PSS block select.


pss_cpu_rdy
1
In
PSS block ready signal to the CPU.


pss_cpu_berr
1
In
PSS bus error signal to the CPU.


pss_cpu_data[31:0]
32
In
Read data bus from the PSS block


cpu_diu_sel
1
Out
DIU register block select.


diu_cpu_rdy
1
In
DIU register block ready signal to the CPU.


diu_cpu_berr
1
In
DIU bus error signal to the CPU.


diu_cpu_data[31:0]
32
In
Read data bus from the DIU block







Interrupt signals










icu_cpu_ilevel[3:0]
3
In
An interrupt is asserted by driving the





appropriate priority level on icu_cpu_ilevel.





These signals must remain asserted until the





CPU executes an interrupt acknowledge cycle.



3
Out
Indicates the level of the interrupt the CPU is





acknowledging when cpu_iack is high


cpu_iack
1
Out
Interrupt acknowledge signal. The exact





timing depends on the CPU core





implementation







Debug signals










diu_cpu_debug_valid
1
In
Signal indicating the data on the





diu_cpu_data bus is valid debug data.


tim_cpu_debug_valid
1
In
Signal indicating the data on the





tim_cpu_data bus is valid debug data.


scb_cpu_debug_valid
1
In
Signal indicating the data on the





scb_cpu_data bus is valid debug data.


pcu_cpu_debug_valid
1
In
Signal indicating the data on the





pcu_cpu_data bus is valid debug data.


lss_cpu_debug_valid
1
In
Signal indicating the data on the lss_cpu_data





bus is valid debug data.


icu_cpu_debug_valid
1
In
Signal indicating the data on the icu_cpu_data





bus is valid debug data.


gpio_cpu_debug_valid
1
In
Signal indicating the data on the





gpio_cpu_data bus is valid debug data.


cpr_cpu_debug_valid
1
In
Signal indicating the data on the





cpr_cpu_data bus is valid debug data.


debug_data_out
32
Out
Output debug data to be muxed on to the





GPIO & PHI pins


debug_data_valid
1
Out
Debug valid signal indicating the validity of





the data on debug_data_out. This signal is





used in all debug configurations


debug_cntrl
33
Out
Control signal for each PHI bound debug data





line indicating whether or not the debug data





should be selected by the pin mux









The SoPEC realtime requirements have yet to be fully determined but they may be split into three categories: hard, firm and soft
Hard requirements are tasks that must be completed before a certain deadline or failure to do so will result in an error perceptible to the user (printing stops or functions incorrectly). There are three hard realtime tasks:

    
    
        Motor control: The motors which feed the paper through the printer at a constant speed during printing are driven directly by the SoPEC device. Four periodic signals with different phase relationships need to be generated to ensure the paper travels smoothly through the printer. The generation of these signals is handled by the GPIO hardware but the CPU is responsible for enabling these signals (i.e. to start or stop the motors) and coordinating the movement of the paper with the printing operation of the printhead.
        Buffer management: Data enters the SoPEC via the SCB at an uneven rate and is consumed by the PEP subsystem at a different rate. The CPU is responsible for managing the DRAM buffers to ensure that neither overrun nor underrun occur. This buffer management is likely to be performed under the direction of the host.
        Band processing: In certain cases PEP registers may need to be updated between bands. As the timing requirements are most likely too stringent to be met by direct CPU writes to the PCU a more likely scenario is that a set of shadow registers will programmed in the compressed page units before the current band is finished, copied to band related registers by the finished band signals and the processing of the next band will continue immediately. An alternative solution is that the CPU will construct a DRAM based set of commands that can be executed by the PCU. The task for the CPU here is to parse the band headers stored in DRAM and generate a DRAM based set of commands for the next number of bands. The location of the DRAM based set of commands must then be written to the PCU before the current band has been processed by the PEP subsystem. It is also conceivable (but currently considered unlikely) that the host PC could create the DRAM based commands. In this case the CPU will only be required to point the PCU to the correct location in DRAM to execute commands from.
    
    


Firm requirements are tasks that should be completed by a certain time or failure to do so will result in a degradation of performance but not an error. The majority of the CPU tasks for SoPEC fall into this category including all interactions with the QA chips, program authentication, page feeding, configuring PEP registers for a page or job, determining the firing pulse profile, communication of printer status to the host over the USB and the monitoring of ink usage. The authentication of downloaded programs and messages will be the most compute intensive operation the CPU will be required to perform. Initial investigations indicate that the LEON processor, running at 160 MHz, will easily perform three authentications in under a second.
Soft requirements are tasks that need to be done but there are only light time constraints on when they need to be done. These tasks are performed by the CPU when there are no pending higher priority tasks. As the SoPEC CPU is expected to be lightly loaded these tasks will mostly be executed soon after they are scheduled.
There are different buses in the CPU block and different protocols are used for each bus. There are three buses in operation:
The LEON CPU core uses an AMBA2.0 AHB bus to communicate with memory and peripherals (usually via an APB bridge).
The CPU to DIU bus conforms to a DIU bus protocol. Note that the address bus used for DIU reads (i.e. cpu_adr(21:2)) is also that used for CPU subsystem with bus accesses while the write address bus (cpu_diu_wadr) and the read and write data buses (dram_cpu_data and cpu_diu_wdata) are private buses between the CPU and the DIU. The effective bus width differs between a read (256 bits) and a write (128 bits). As certain CPU instructions may require byte write access this will need to be supported by both the DRAM write buffer (in the AHB bridge) and the DIU.
With the CPU Subsystem bus, for access to the on-chip peripherals a simple bus protocol is used. The MMU must first determine which particular block is being addressed (and that the access is a valid one) so that the appropriate block select signal can be generated. During a write access CPU write data is driven out with the address and block select signals in the first cycle of an access. The addressed slave peripheral responds by asserting its ready signal indicating that it has registered the write data and the access can complete. The write data bus is common to all peripherals and is also used for CPU writes to the embedded DRAM. A read access is initiated by driving the address and select signals during the first cycle of an access. The addressed slave responds by placing the read data on its bus and asserting its ready signal to indicate to the CPU that the read data is valid. Each block has a separate point-to-point data bus for read accesses to avoid the need for a tri-stateable bus.
All peripheral accesses are 32-bit (Programming note: char or short C types should not be used to access peripheral registers). The use of the ready signal allows the accesses to be of variable length. In most cases accesses will complete in two cycles but three or four (or more) cycles accesses are likely for PEP blocks or IP blocks with a different native bus interface. All PEP blocks are accessed via the PCU which acts as a bridge. The PCU bus uses a similar protocol to the CPU subsystem bus but with the PCU as the bus master. The duration of accesses to the PEP blocks is influenced by whether or not the PCU is executing commands from DRAM. As these commands are essentially register writes the CPU access will need to wait until the PCU bus becomes available when a register access has been completed. This could lead to the CPU being stalled for up to 4 cycles if it attempts to access PEP blocks while the PCU is executing a command. The size and probability of this penalty is sufficiently small to have any significant impact on performance.
In order to support user mode (i.e. OEM code) access to certain peripherals the CPU subsystem bus propagates the CPU function code signals (cpu_acode[1:0]). These signals indicate the type of address space (i.e. User/Supervisor and Program/Data) being accessed by the CPU for each access. Each peripheral must determine whether or not the CPU is in the correct mode to be granted access to its registers and in some cases (e.g. Timers and GPIO blocks) different access permissions can apply to different registers within the block. If the CPU is not in the correct mode then the violation is flagged by asserting the block's bus error signal (block_cpu_berr) with the same timing as its ready signal (block_cpu_rdy) which remains deasserted. When this occurs invalid read accesses should return 0 and write accesses should have no effect.
FIG. 5 shows two examples of the peripheral bus protocol in action. A write to the LSS block from code running in supervisor mode is successfully completed. This is immediately followed by a read from a PEP block via the PCU from code running in user mode. As this type of access is not permitted the access is terminated with a bus error. The bus error exception processing then starts directly after this—no further accesses to the peripheral should be required as the exception handler should be located in the DRAM. Each peripheral acts as a slave on the CPU subsystem bus.
CPU subsystem bus slave operation is described by the state machine in FIG. 6. This state machine will be implemented in each CPU subsystem bus slave. The only new signals mentioned here are the valid_access and reg_available signals. The valid_access is determined by comparing the cpu_acode value with the block or register (in the case of a block that allow user access on a per register basis such as the GPIO block) access permissions and asserting valid_access if the permissions agree with the CPU mode. The reg_available signal is only required in the PCU or in blocks that are not capable of two-cycle access (e.g. blocks containing imported IP with different bus protocols). In these blocks the reg_available signal is an internal signal used to insert wait states (by delaying the assertion of block_cpu_rdy) until the CPU bus slave interface can gain access to the register.
When reading from a register that is less than 32 bits wide the CPU subsystems bus slave should return zeroes on the unused upper bits of the block_cpu_data bus.
To support debug mode the contents of the register selected for debug observation, debug reg, are always output on the block_cpu_data bus whenever a read access is not taking place.
The LEON processor is an open-source implementation of the IEEE-1754 standard (SPARC V8) instruction set. LEON is available from and actively supported by Gaisler Research (www.gaisler.com).
The following features of the LEON-2 processor will be utilised on SoPEC:

    
    
        IEEE-1754 (SPARC V8) compatible integer unit with 5-stage pipeline
        Separate instruction and data cache (Harvard architecture). 1 kbyte direct mapped caches will be used for both.
        Full implementation of AMBA-2.0 AHB on-chip bus
    
    


The standard release of LEON incorporates a number of peripherals and support blocks which will not be included on SoPEC. The LEON core as used on SoPEC will consist of: 1) the LEON integer unit, 2) the instruction and data caches (currently 1 kB each), 3) the cache control logic, 4) the AHB interface and 5) possibly the AHB controller (although this functionality may be implemented in the LEON AHB bridge).
The version of the LEON database that the SoPEC LEON components will be sourced from is LEON2-1.0.7 although later versions may be used if they offer worthwhile functionality or bug fixes that affect the SoPEC design.
The LEON core will be clocked using the system clock, pclk, and reset using the prst_n_section[1] signal. The ICU will assert all the hardware interrupts using a suitable protocol. The LEON hardware multipliers and floating-point unit are not required. SoPEC will use the recommended 8 register window configuration.
Only two of the registers described in the LEON manual are implemented on SoPEC—the LEON configuration register and the Cache Control Register (CCR). The addresses of these registers are shown in Table 3.
The LEON configuration register allows runtime software to determine the settings of LEONs various configuration options. This is a read-only register whose value for the SoPEC ASIC will be 0x1071_8C00. Further descriptions of many of the bitfields can be found in the LEON manual. The values used for SoPEC are highlighted in bold for clarity.







TABLE 3







LEON Configuration Register









Field Name
bit(s)
Description





WriteProtection
1:0
Write protection type.




00 - none




01 - standard


PCICore
3:2
PCI core type




00 - none




01 - InSilicon




10 - ESA




11 - Other


FPUType
5:4
FPU type.




00 - none




01 - Meiko


MemStatus
6
0 - No memory status and failing address register present




1 - Memory status and failing address register present


Watchdog
7
0 - Watchdog timer not present (Note this refers to the




LEON watchdog timer in the LEON timer block).




1 - Watchdog timer present


UMUL/SMUL
8
0 - UMUL/SMUL instructions are not implemented




1 - UMUL/SMUL instructions are implemented


UDIV/SDIV
9
0 - UMUL/SMUL instructions are not implemented




1 - UMUL/SMUL instructions are implemented


DLSZ
11:10
Data cache line size in 32-bit words:




00 - 1 word




01 - 2 words




10 - 4 words




11 - 8 words


DCSZ
14:12
Data cache size in kBbytes = 2DCSZ. SoPEC DCSZ = 0.


ILSZ
16:15
Instruction cache line size in 32-bit words:




00 - 1 word




01 - 2 words




10 - 4 words




11 - 8 words


ICSZ
19:17
Instruction cache size in kBbytes = 2ICSZ. SoPEC ICSZ = 0.


RegWin
24:20
The implemented number of SPARC register windows - 1.




SoPEC value = 7.


UMAC/SMAC
25 
0 - UMAC/SMAC instructions are not implemented




1 - UMAC/SMAC instructions are implemented


Watchpoints
28:26
The implemented number of hardware watchpoints. SoPEC




value = 4.


SDRAM
29 
0 - SDRAM controller not present




1 - SDRAM controller present


DSU
30 
0 - Debug Support Unit not present




1 - Debug Support Unit present


Reserved
31 
Reserved. SoPEC value = 0.









Memory Management Units (MMUs) are typically used to protect certain regions of memory from invalid accesses, to perform address translation for a virtual memory system and to maintain memory page status (swapped-in, swapped-out or unmapped)
The SoPEC MMU is a much simpler affair whose function is to ensure that all regions of the SoPEC memory map are adequately protected. The MMU does not support virtual memory and physical addresses are used at all times. The SoPEC MMU supports a full 32-bit address space. The SoPEC memory map is depicted in FIG. 7.
The MMU selects the relevant bus protocol and generates the appropriate control signals depending on the area of memory being accessed. The MMU is responsible for performing the address decode and generation of the appropriate block select signal as well as the selection of the correct block read bus during a read access. The MMU will need to support all of the bus transactions the CPU can produce including interrupt acknowledge cycles, aborted transactions etc.
When an MMU error occurs (such as an attempt to access a supervisor mode only region when in user mode) a bus error is generated. While the LEON can recognise different types of bus error (e.g. data store error, instruction access error) it handles them in the same manner as it handles all traps i.e it will transfer control to a trap handler. No extra state information is be stored because of the nature of the trap. The location of the trap handler is contained in the TBR (Trap Base Register). This is the same mechanism as is used to handle interrupts.
The MMU performs the decode of the high order bits to generate the relevant cpu_block_select signal. Apart from the PCU, which decodes the address space for the PEP blocks, each block only needs to decode as many bits of cpu adr[11:2] as required to address all the registers within the block.
The embedded DRAM is broken into 8 regions, with each region defined by a lower and upper bound address and with its own access permissions.
The association of an area in the DRAM address space with a MMU region is completely under software control. Table 4 below gives one possible region mapping. Regions should be defined according to their access requirements and position in memory. Regions that share the same access requirements and that are contiguous in memory may be combined into a single region. The example below is purely for indicative purposes—real mappings are likely to differ significantly from this. Note that the RegionBottom and RegionTop fields in this example include the DRAM base address offset (0x4000—0000) which is not required when programming the RegionNTop and RegionNBottom registers.
As shown in FIG. 7 the DRAM occupies only 2.5 MBytes of the total 4 GB SoPEC address space. The non-DRAM regions of SoPEC are handled by the MMU as follows: ROM (0x00000000 to 0x0000_FFFF): The ROM block will control the access types allowed. The cpu_acode[1:0] signals will indicate the CPU mode and access type and the ROM block will assert rom_cpu_berr if an attempted access is forbidden. The ROM block access permissions are hard wired to allow all read accesses except to the FuseChipID registers which may only be read in supervisor mode.
MMU Internal Registers (0x0001—0000 to 0x0001—0FFF): The MMU is responsible for controlling the accesses to its own internal registers and will only allow data reads and writes (no instruction fetches) from supervisor data space. All other accesses will result in the mmu_cpu_berr signal being asserted in accordance with the CPU native bus protocol. CPU Subsystem Peripheral Registers (0x0001—1000 to 0x0001_FFFF): Each peripheral block will control the access types allowed. Every peripheral will allow supervisor data accesses (both read and write) and some blocks (e.g. Timers and GPIO) will also allow user data space accesses as outlined in the relevant chapters of this specification. Neither supervisor nor user instruction fetch accesses are allowed to any block as it is not possible to execute code from peripheral registers
PCU Mapped Registers (0x0002—0000 to 0x0002_BFFF): All of the PEP blocks registers which are accessed by the CPU via the PCU will inherit the access permissions of the PCU. These access permissions are hard wired to allow supervisor data accesses only and the protocol used is the same as for the CPU peripherals.
Unused address space (0x0002_C000 to 0x3FFF_FFFF and 0x4028—0000 to 0xFFFF_FFFF): All accesses to the unused portion of the address space will result in the mmu_cpu_berr signal being asserted in accordance with the CPU native bus protocol. These accesses will not propagate outside of the MMU i.e. no external access will be initiated.
When a reset occurs the LEON processor starts executing code from address 0x0000—0000. A common software bug is zero-referencing or null pointer de-referencing (where the program attempts to access the contents of address 0x0000—0000). To assist software debug the MMU will assert a bus error every time the locations 0x0000—0000 to 0x0000—000F (i.e. the first 4 words of the reset trap) are accessed after the reset trap handler has legitimately been retrieved immediately after reset.
The MMU configuration registers include the RDU configuration registers and two LEON registers. Note that all the MMU configuration registers may only be accessed when the CPU is running in supervisor mode.
The 20 Mbit of embedded DRAM on SoPEC is arranged as 81920 words of 256 bits each. All region boundaries need to align with a 256-bit word. Thus only 17 bits are required for the RegionNTop and RegionNBottom registers. Note that the bottom 5 bits of the RegionNTop and RegionNBottom registers cannot be written to and read as ‘0’ i.e. the RegionNTop and RegionNBottom registers represent byte-aligned DRAM addresses
Both the RegionNTop and RegionNBottom registers are inclusive i.e. the addresses in the registers are included in the region. Thus the size of a region is (RegionNTop−RegionNBottom)+1 DRAM words.
If DRAM regions overlap (there is no reason for this to be the case but there is nothing to prohibit it either) then only accesses allowed by all overlapping regions are permitted. That is if a DRAM address appears in both Region1 and Region3 (for example) the cpu_acode of an access is checked against the access permissions of both regions. If both regions permit the access then it will proceed but if either or both regions do not permit the access then it will not be allowed.
The MMU does not support negatively sized regions i.e. the value of the RegionNTop register should always be greater than or equal to the value of the RegionNBottom register. If RegionNTop is lower in the address map than RegionNTop then the region is considered to be zero-sized and is ignored.
When both the RegionNTop and RegionNBottom registers for a region contain the same value the region is then simply one 256-bit word in length and this corresponds to the smallest possible active region.
Each memory region has a control register associated with it. The RegionNControl register is used to set the access conditions for the memory region bounded by the RegionNTop and RegionNBottom registers. Table 4 describes the function of each bit field in the RegionNControl registers. All bits in a RegionNControl register are both readable and writable by design. However, like all registers in the MMU, the RegionNControl registers can only be accessed by code running in supervisor mode.







TABLE 4







Region Control Register









Field Name
bit(s)
Description





SupervisorAccess
2:0
Denotes the type of access allowed when the




CPU is running in Supervisor mode. For




each access type a 1 indicates the access




is permitted and a 0 indicates the access is




not permitted.




bit0 - Data read access permission




bit1 - Data write access permission




bit2 - Instruction fetch access permission


UserAccess
5:3
Denotes the type of access allowed when the




CPU is running in User mode. For each




access type a 1 indicates the access is




permitted and a 0 indicates the access is not




permitted.




bit3 - Data read access permission




bit4 - Data write access permission




bit5 - Instruction fetch access permission









The SPARC V8 architecture allows for a number of types of memory access error to be trapped. However on the LEON processor only data_store_error and data_access_exception trap types will result from an external (to LEON) bus error.
According to the SPARC architecture manual the processor will automatically move to the next register window (i.e. it decrements the current window pointer) and copies the program counters (PC and nPC) to two local registers in the new window. The supervisor bit in the PSR is also set and the PSR can be saved to another local register by the trap handler (this does not happen automatically in hardware). The ExceptionSource register aids the trap handler by identifying the source of an exception. Each bit in the ExceptionSource register is set when the relevant trap condition and should be cleared by the trap handler by writing a ‘1’ to that bit position.
As can be seen from FIGS. 8 and 9 the MMU consists of three principal sub-blocks. For clarity the connections between these sub-blocks and other SoPEC blocks and between each of the sub-blocks are shown in two separate diagrams.
The LEON AHB bridge consists of an AHB bridge to DIU and an AHB to CPU subsystem bus bridge. The AHB bridge will convert between the AHB and the DIU and CPU subsystem bus protocols but the address decoding and enabling of an access happens elsewhere in the MMU. The AHB bridge will always be a slave on the AHB. Note that the AMBA signals from the LEON core are contained within the ahbso and ahbsi records. Glue logic may be required to assist with enabling memory accesses, endianness coherency, interrupts and other miscellaneous signalling.
The LEON AHB bridge must ensure that all CPU bus transactions are functionally correct and that the timing requirements are met. The AHB bridge also implements a 128-bit DRAM write buffer to improve the efficiency of DRAM writes, particularly for multiple successive writes to DRAM. The AHB bridge is also responsible for ensuring endianness coherency i.e. guaranteeing that the correct data appears in the correct position on the data buses (hrdata, cpu_dataout and cpu_mmu_wdata) for every type of access. This is a requirement because the LEON uses big-endian addressing while the rest of SoPEC is little-endian.
The LEON AHB bridge will assert request signals to the DIU if the MMU control block deems the access to be a legal access. The validity (i.e. is the CPU running in the correct mode for the address space being accessed) of an access is determined by the contents of the relevant RegionNControl register. As the SPARC standard requires that all accesses are aligned to their word size (i.e. byte, half-word, word or double-word) and so it is not possible for an access to traverse a 256-bit boundary (as required by the DIU). Invalid DRAM accesses are not propagated to the DIU and will result in an error response (ahbso.hresp=‘01’) on the AHB. The DIU will return a 256-bit dataword on dram_cpu_data[255:0] for every read access.
While the LEON AHB bridge performs the protocol translation between AHB and the CPU subsystem bus the select signals for each block are generated by address decoding in the CPU subsystem bus interface. The CPU subsystem bus interface also selects the correct read data bus, ready and error signals for the block being addressed and passes these to the LEON AHB bridge which puts them on the AHB bus.
It is expected that some signals (especially those external to the CPU block) will need to be registered here to meet the timing requirements. Careful thought will be required to ensure that overall CPU access times are not excessively degraded by the use of too many register stages.
The DRAM write buffer improves the efficiency of DRAM writes by aggregating a number of CPU write accesses into a single DIU write access. This is achieved by checking to see if a CPU write is to an address already in the write buffer and if so the write is immediately acknowledged (i.e. the ahbsi.hready signal is asserted without any wait states) and the DRAM write buffer updated accordingly. When the CPU write is to a DRAM address other than that in the write buffer then the current contents of the write buffer are sent to the DIU (where they are placed in the posted write buffer) and the DRAM write buffer is updated with the address and data of the CPU write. The DRAM write buffer consists of a 128-bit data buffer, an 18-bit write address tag and a 16-bit write mask. Each bit of the write mask indicates the validity of the corresponding byte of the write buffer as shown in FIG. 10.
The operation of the DRAM write buffer is summarised by the following set of rules:
1) The DRAM write buffer only contains DRAM write data i.e. peripheral writes go directly to the addressed peripheral.
2)CPU writes to locations within the DRAM write buffer or to an empty write buffer (i.e. the write mask bits are all 0) complete with zero wait states regardless of the size of the write (byte/half-word/word/ double-word).
3)The contents of the DRAM write buffer are flushed to DRAM whenever a CPU write to a location outside the write buffer occurs, whenever a CPU read from a location within the write buffer occurs or whenever a write to a peripheral register occurs.
4)A flush resulting from a peripheral write will not cause any extra wait states to be inserted in the peripheral write access.
5)Flushes resulting from a DRAM accesses will cause wait states to be inserted until the DIU posted write buffer is empty. If the DIU posted write buffer is empty at the time the flush is required then no wait states will be inserted for a flush resulting from a CPU write or one wait state will be inserted for a flush resulting from a CPU read (this is to ensure that the DIU sees the write request ahead of the read request). Note that in this case further wait states will also be inserted as a result of the delay in servicing the read request by the DIU. FIG. 11 depicts the operation of the AHB bridge over a sample sequence of DRAM transactions consisting of a read into the DCache, a double-word store to an address other than that currently in the DRAM write buffer followed by an ICache line refill. To avoid clutter a number of AHB control signals that are inputs to the MMU have been grouped together as ahbsi.CONTROL and only the ahbso.HREADY is shown of the output AHB control signals.
The first transaction is a single word load (‘LD’). The MMU (specifically the MMU control block) uses the first cycle of every access (i.e. the address phase of an AHB transaction) to determine whether or not the access is a legal access. The read request to the DIU is then asserted in the following cycle (assuming the access is a valid one) and is acknowledged by the DIU a cycle later. Note that the time from cpu_diu_rreq being asserted and diu_cpu_rack being asserted is variable as it depends on the DIU configuration and access patterns of DIU requesters. The AHB bridge will insert wait states until it sees the diu_cpu_rvalid signal is high, indicating the data (‘LD1’) on the dram_cpu_data bus is valid. The AHB bridge terminates the read access in the same cycle by asserting the ahbso.HREADY signal (together with an ‘OKAY’ HRESP code). The AHB bridge also selects the appropriate 32 bits (‘RD1’) from the 256-bit DRAM line data (‘LD1’) returned by the DIU corresponding to the word address given by A1.
The second transaction is an AHB two-beat incrementing burst issued by the LEON acache block in response to the execution of a double-word store instruction. As LEON is a big endian processor the address issued (‘A2’) during the address phase of the first beat of this transaction is the address of the most significant word of the double-word while the address for the second beat (‘A3’) is that of the least significant word i.e. A3=A2+4. The presence of the DRAM write buffer allows these writes to complete without the insertion of any wait states. This is true even when, as shown here, the DRAM write buffer needs to be flushed into the DIU posted write buffer, provided the DIU posted write buffer is empty. If the DIU posted write buffer is not empty (as would be signified by diu_cpu_write rdy being low) then wait states would be inserted until it became empty. The cpu_diu_wdata buffer builds up the data to be written to the DIU over a number of transactions (‘BD1’ and ‘BD2’ here) while the cpu_diu_wmask records every byte that has been written to since the last flush—in this case the lowest word and then the second lowest word are written to as a result of the double-word store operation.
The final transaction shown here is a DRAM read caused by an ICache miss. Note that the pipelined nature of the AHB bus allows the address phase of this transaction to overlap with the final data phase of the previous transaction. All ICache misses appear as single word loads (‘LD’) on the AHB bus. In this case we can see that the DIU is slower to respond to this read request than to the first read request because it is processing the write access caused by the DRAM write buffer flush. The ICache refill will complete just after the window shown in FIG. 11.
The CPU Subsystem Interface block handles all valid accesses to the peripheral blocks that comprise the CPU Subsystem.
The CPU Subsystem Bus Interface block performs simple address decoding to select a peripheral and multiplexing of the returned signals from the various peripheral blocks. Note that access to the MMU configuration registers are handled by the MMU Control Block rather than the CPU Subsystem Bus Interface block.
The MMU Control Block determines whether every CPU access is a valid access. No more than one cycle is to be consumed in determining the validity of an access and all accesses must terminate with the assertion of either mmu_cpu_rdy or mmu_cpu_berr. To safeguard against stalling the CPU a simple bus timeout mechanism will be supported.
The MMU Control Block is responsible for the MMU's core functionality, namely determining whether or not an access to any part of the address map is valid. An access is considered valid if it is to a mapped area of the address space and if the CPU is running in the appropriate mode for that address space. Furthermore the MMU control block must correctly handle the special cases that are: an interrupt acknowledge cycle, a reset exception vector fetch, an access that crosses a 256-bit DRAM word boundary and a bus timeout condition.
The version of LEON implemented on SoPEC features 1 kB of ICache and 1 kB of DCache. Both caches are direct mapped and feature 8 word lines so their data RAMs are arranged as 32×256-bit and their tag RAMs as 32×30-bit (itag) or 32×32-bit (dtag). Like most of the rest of the LEON code used on SoPEC the cache controllers are taken from the leon2-1.0.7 release. The LEON cache controllers and cache RAMs have been modified to ensure that an entire 256-bit line is refilled at a time to make maximum use out of the memory bandwidth offered by the embedded DRAM organization (DRAM lines are also 256-bit). The data cache controller has also been modified to ensure that user mode code cannot access the DCache contents unless it is authorised to do so. A block diagram of the LEON CPU core as implemented on SoPEC is shown in FIG. 12.
In this diagram dotted lines are used to indicate hierarchy and red items represent signals or wrappers added as part of the SoPEC modifications. LEON makes heavy use of VHDL records and the records used in the CPU core are described in Table 5. Unless otherwise stated the records are defined in the iface.vhd file (part of the LEON release) and this should be consulted for a complete breakdown of the record elements.







TABLE 5







Relevant LEON records








Record Name
Description





rfi
Register File Input record. Contains address, datain and control signals



for the register file.


rfo
Register File Output record. Contains the data out of the dual read port



register file.


ici
Instruction Cache In record. Contains program counters from different



stages of the pipeline and various control signals


ico
Instruction Cache Out record. Contains the fetched instruction data and



various control signals. This record is also sent to the DCache (i.e.



icol) so that diagnostic accesses (e.g. lda/sta) can be serviced.


dci
Data Cache In record. Contains address and data buses from different



stages of the pipeline (execute & memory) and various control signals


dco
Data Cache Out record. Contains the data retrieved from either memory



or the caches and various control signals. This record is also sent to the



ICache (i.e. dcol) so that diagnostic accesses (e.g. lda/sta) can be



serviced.


iui
Integer Unit In record. This record contains the interrupt request level



and a record for use with LEONs Debug Support Unit (DSU)


iuo
Integer Unit Out record. This record contains the acknowledged



interrupt request level with control signals and a record for use with



LEONs Debug Support Unit (DSU)


mcii
Memory to Cache Icache In record. Contains the address of an Icache



miss and various control signals


mcio
Memory to Cache Icache Out record. Contains the returned data from



memory and various control signals


mcdi
Memory to Cache Dcache In record. Contains the address and data of a



Dcache miss or write and various control signals


mcdo
Memory to Cache Dcache Out record. Contains the returned data from



memory and various control signals


ahbi
AHB In record. This is the input record for an AHB master and contains



the data bus and AHB control signals. The destination for the



signals in this record is the AHB controller. This record is defined in



the amba.vhd file


ahbo
AHB Out record. This is the output record for an AHB master and



contains the address and data buses and AHB control signals. The AHB



controller drives the signals in this record. This record is defined in the



amba.vhd file


ahbsi
AHB Slave In record. This is the input record for an AHB slave and



contains the address and data buses and AHB control signals. It is used



by the DCache to facilitate cache snooping (this feature is not enabled in



SoPEC). This record is defined in the amba.vhd file


crami
Cache RAM In record. This record is composed of records of records



which contain the address, data and tag entries with associated control



signals for both the ICache RAM and DCache RAM


cramo
Cache RAM Out record. This record is composed of records of records



which contain the data and tag entries with associated control signals for



both the ICache RAM and DCache RAM


iline_rdy
Control signal from the ICache controller to the instruction cache



memory. This signal is active (high) when a full 256-bit line (on



dram_cpu_data) is to be written to cache memory.


dline_rdy
Control signal from the DCache controller to the data cache memory.



This signal is active (high) when a full 256-bit line (on dram_cpu_data)



is to be written to cache memory.


dram_cpu_data
256-bit data bus from the embedded DRAM









The LEON cache module consists of three components: the ICache controller (icache.vhd), the DCache controller (dcache.vhd) and the AHB bridge (acache.vhd) which translates all cache misses into memory requests on the AHB bus.
In order to enable full line refill operation a few changes had to be made to the cache controllers. The ICache controller was modified to ensure that whenever a location in the cache was updated (i.e. the cache was enabled and was being refilled from DRAM) all locations on that cache line had their valid bits set to reflect the fact that the full line was updated. The iline_rdy signal is asserted by the ICache controller when this happens and this informs the cache wrappers to update all locations in the idata RAM for that line. A similar change was made to the DCache controller except that the entire line was only updated following a read miss and that existing write through operation was preserved. The DCache controller uses the dline_rdy signal to instruct the cache wrapper to update all locations in the ddata RAM for a line. An additional modification was also made to ensure that a double-word load instruction from a non-cached location would only result in one read access to the DIU i.e. the second read would be serviced by the data cache. Note that if the DCache is turned off then a double-word load instruction will cause two DIU read accesses to occur even though they will both be to the same 256-bit DRAM line.
The DCache controller was further modified to ensure that user mode code cannot access cached data to which it does not have permission (as determined by the relevant RegionNControl register settings at the time the cache line was loaded). This required an extra 2 bits of tag information to record the user read and write permissions for each cache line. These user access permissions can be updated in the same manner as the other tag fields (i.e. address and valid bits) namely by line refill, STA instruction or cache flush. The user access permission bits are checked every time user code attempts to access the data cache and if the permissions of the access do not agree with the permissions returned from the tag RAM then a cache miss occurs. As the MMU evaluates the access permissions for every cache miss it will generate the appropriate exception for the forced cache miss caused by the errant user code. In the case of a prohibited read access the trap will be immediate while a prohibited write access will result in a deferred trap. The deferred trap results from the fact that the prohibited write is committed to a write buffer in the DCache controller and program execution continues until the prohibited write is detected by the MMU which may be several cycles later. Because the errant write was treated as a write miss by the DCache controller (as it did not match the stored user access permissions) the cache contents were not updated and so remain coherent with the DRAM contents (which do not get updated because the MMU intercepted the prohibited write). Supervisor mode code is not subject to such checks and so has free access to the contents of the data cache.
In addition to AHB bridging, the ACache component also performs arbitration between ICache and DCache misses when simultaneous misses occur (the DCache always wins) and implements the Cache Control Register (CCR). The leon2-1.0.7 release is inconsistent in how it handles cacheability: For instruction fetches the cacheability (i.e. is the access to an area of memory that is cacheable) is determined by the ICache controller while the ACache determines whether or not a data access is cacheable. To further complicate matters the DCache controller does determine if an access resulting from a cache snoop by another AHB master is cacheable (Note that the SoPEC ASIC does not implement cache snooping as it has no need to do so). This inconsistency has been cleaned up in more recent LEON releases but is preserved here to minimise the number of changes to the LEON RTL. The cache controllers were modified to ensure that only DRAM accesses (as defined by the SoPEC memory map) are cached.
The only functionality removed as a result of the modifications was support for burst fills of the ICache. When enabled burst fills would refill an ICache line from the location where a miss occurred up to the end of the line. As the entire line is now refilled at once (when executing from DRAM) this functionality is no longer required. Furthermore more substantial modifications to the ICache controller would be needed if we wished to preserve this function without adversely affecting full line refills. The CCR was therefore modified to ensure that the instruction burst fetch bit (bit16) was tied low and could not be written to. The CCR controls the operation of both the I and D caches. Note that the bitfields used on the SoPEC implementation of this register are based on the LEON v1.0.7 implementation and some bits have their values tied off.







TABLE 6







LEON Cache Control Register









Field




Name
bit(s)
Description





ICS
1:0
Instruction cache state:




00 - disabled




01 - frozen




10 - disabled




11 - enabled


Reserved
13:6 
Reserved. Reads as 0.


DCS
3:2
Data cache state:




00 - disabled




01 - frozen




10 - disabled




11 - enabled


IF
 4
ICache freeze on interrupt




0 - Do not freeze the ICache contents on taking an interrupt




1 - Freeze the ICache contents on taking an interrupt


DF
 5
DCache freeze on interrupt




0 - Do not freeze the DCache contents on taking an interrupt




1 - Freeze the DCache contents on taking an interrupt


Reserved
13:6 
Reserved. Reads as 0.


DP
14
Data cache flush pending.




0 - No DCache flush in progress




1 - DCache flush in progress




This bit is ReadOnly.


IP
15
Instruction cache flush pending.




0 - No ICache flush in progress




1 - ICache flush in progress




This bit is ReadOnly.


IB
16
Instruction burst fetch enable. This bit is tied low on SoPEC




because it would interfere with the operation of the cache wrappers.




Burst refill functionality is automatically provided in




SoPEC by the cache wrappers.


Reserved
20:17
Reserved. Reads as 0.


FI
21
Flush instruction cache. Writing a 1 this bit will flush the




ICache. Reads as 0.


FD
22
Flush data cache. Writing a 1 this bit will flush the DCache.




Reads as 0.


DS
23
Data cache snoop enable. This bit is tied low in SoPEC as there




is no requirement to snoop the data cache.


Reserved
31:24
Reserved. Reads as 0.









The cache RAMs used in the leon2-1.0.7 release needed to be modified to support full line refills and the correct IBM macros also needed to be instantiated. Although they are described as RAMs throughout this document (for consistency), register arrays are actually used to implement the cache RAMs. This is because IBM SRAMs were not available in suitable configurations (offered configurations were too big) to implement either the tag or data cache RAMs. Both instruction and data tag RAMs are implemented using dual port (1 Read & 1 Write) register arrays and the clocked write-through versions of the register arrays were used as they most closely approximate the single port SRAM LEON expects to see.
The itag and dtag RAMs differ only in their width—the itag is a 32×30 array while the dtag is a 32×32 array with the extra 2 bits being used to record the user access permissions for each line. When read using a LDA instruction both tags return 32-bit words. The tag fields are described in Tables 7 and 8 below. Using the IBM naming conventions the register arrays used for the tag RAMs are called RA032X30D2P2W1R1M3 for the itag and RA032X32D2P2W1R1M3 for the dtag. The ibm_syncram wrapper used for the tag RAMs is a simple affair that just maps the wrapper ports on to the appropriate ports of the IBM register array and ensures the output data has the correct timing by registering it. The tag RAMs do not require any special modifications to handle full line refills.







TABLE 7







LEON Instruction Cache Tag









Field Name
bit(s)
Description





Valid
7:0
Each valid bit indicates whether or not the




corresponding word of the cache line contains valid




data


Reserved
9:8
Reserved - these bits do not exist in the itag RAM.




Reads as 0.


Address
31:10
The tag address of the cache line
















TABLE 8







LEON Data Cache Tag









Field Name
bit(s)
Description





Valid
7:0
Each valid bit indicates whether or not the




corresponding word of the cache line contains valid




data


URP
8
User read permission.




0 - User mode reads will force a refill of this line




1 - User mode code can read from this cache line.


UWP
9
User write permission.




0 - User mode writes will not be written to the cache




1 - User mode code can write to this cache line.


Address
31:10
The tag address of the cache line









The cache data RAM contains the actual cached data and nothing else. Both the instruction and data cache data RAMs are implemented using 8 32×32-bit register arrays and some additional logic to support full line refills. Using the IBM naming conventions the register arrays used for the tag RAMs are called RA032X32D2P2W1R1M3. The ibm_cdram_wrap wrapper used for the tag RAMs is shown in FIG. 13.
To the cache controllers the cache data RAM wrapper looks like a 256×32 single port SRAM (which is what they expect to see) with an input to indicate when a full line refill is taking place (the line_rdy signal). Internally the 8-bit address bus is split into a 5-bit lineaddress, which selects one of the 32 256-bit cache lines, and a 3-bit wordaddress which selects one of the 8 32-bit words on the cache line. Thus each of the 8 32×32 register arrays contains one 32-bit word of each cache line. When a full line is being refilled (indicated by both the line_rdy and write signals being high) every register array is written to with the appropriate 32 bits from the linedatain bus which contains the 256-bit line returned by the DIU after a cache miss. When just one word of the cache line is to be written (indicated by the write signal being high while the line_rdy is low) then the wordaddress is used to enable the write signal to the selected register array only—all other write enable signals are kept low. The data cache controller handles byte and half-word write by means of a read-modify-write operation so writes to the cache data RAM are always 32-bit.
The wordaddress is also used to select the correct 32-bit word from the cache line to return to the LEON integer unit.
The RDU facilitates the observation of the contents of most of the CPU addressable registers in the SoPEC device in addition to some pseudo-registers in realtime. The contents of pseudo-registers, i.e. registers that are collections of otherwise unobservable signals and that do not affect the functionality of a circuit, are defined in each block as required. Many blocks do not have pseudo-registers and some blocks (e.g. ROM, PSS) do not make debug information available to the RDU as it would be of little value in realtime debug.
Each block that supports realtime debug observation features a DebugSelect register that controls a local mux to determine which register is output on the block's data bus (i.e. block_cpu_data). One small drawback with reusing the blocks data bus is that the debug data cannot be present on the same bus during a CPU read from the block. An accompanying active high block cpu debug_valid signal is used to indicate when the data bus contains valid debug data and when the bus is being used by the CPU. There is no arbitration for the bus as the CPU will always have access when required. A block diagram of the RDU is shown in FIG. 25.







TABLE 9







RDU I/Os










Port name
Pins
I/O
Description













diu_cpu_data
32
In
Read data bus from the DIU block


cpr_cpu_data
32
In
Read data bus from the CPR block


gpio_cpu_data
32
In
Read data bus from the GPIO block


icu_cpu_data
32
In
Read data bus from the ICU block


lss_cpu_data
32
In
Read data bus from the LSS block


pcu_cpu_debug_data
32
In
Read data bus from the PCU block


scb_cpu_data
32
In
Read data bus from the SCB block


tim_cpu_data
32
In
Read data bus from the TIM block


diu_cpu_debug_valid
1
In
Signal indicating the data on the diu_cpu_data bus





is valid debug data.


tim_cpu_debug_valid
1
In
Signal indicating the data on the tim_cpu_data bus





is valid debug data.


scb_cpu_debug_valid
1
In
Signal indicating the data on the scb_cpu_data bus





is valid debug data.


pcu_cpu_debug_valid
1
In
Signal indicating the data on the pcu_cpu_data





bus is valid debug data.


lss_cpu_debug_valid
1
In
Signal indicating the data on the lss_cpu_data bus





is valid debug data.


icu_cpu_debug_valid
1
In
Signal indicating the data on the icu_cpu_data bus





is valid debug data.


gpio_cpu_debug_valid
1
In
Signal indicating the data on the gpio_cpu_data





bus is valid debug data.


cpr_cpu_debug_valid
1
In
Signal indicating the data on the cpr_cpu_data bus





is valid debug data.


debug_data_out
32
Out
Output debug data to be muxed on to the





PHI/GPIO/other pins


debug_data_valid
1
Out
Debug valid signal indicating the validity of the





data on debug_data_out. This signal is used in all





debug configurations


debug_cntr1
33
Out
Control signal for each debug data line indicating





whether or not the debug data should be selected





by the pin mux









As there are no spare pins that can be used to output the debug data to an external capture device some of the existing I/Os will have a debug multiplexer placed in front of them to allow them be used as debug pins. Furthermore not every pin that has a debug mux will always be available to carry the debug data as they may be engaged in their primary purpose e.g. as a GPIO pin. The RDU therefore outputs a debug_cntrl signal with each debug data bit to indicate whether the mux associated with each debug pin should select the debug data or the normal data for the pin. The DebugPinSel1 and DebugPinSel2 registers are used to determine which of the 33 potential debug pins are enabled for debug at any particular time.
As it may not always be possible to output a full 32-bit debug word every cycle the RDU supports the outputting of an n-bit sub-word every cycle to the enabled debug pins. Each debug test would then need to be re-run a number of times with a different portion of the debug word being output on the n-bit sub-word each time. The data from each run should then be correlated to create a full 32-bit (or whatever size is needed) debug word for every cycle. The debug_data_valid and pclk_out signals will accompany every sub-word to allow the data to be sampled correctly. The pclk out signal is sourced close to its output pad rather than in the RDU to minimise the skew between the rising edge of the debug data signals (which should be registered close to their output pads) and the rising edge of pclk_out.
As multiple debug runs will be needed to obtain a complete set of debug data the n-bit sub-word will need to contain a different bit pattern for each run. For maximum flexibility each debug pin has an associated DebugDataSrc register that allows any of the 32 bits of the debug data word to be output on that particular debug data pin. The debug data pin must be enabled for debug operation by having its corresponding bit in the DebugPinSel registers set for the selected debug data bit to appear on the pin.
The size of the sub-word is determined by the number of enabled debug pins which is controlled by the DebugPinSel registers. Note that the debug_data_valid signal is always output. Furthermore debug_cntrl[0] (which is configured by DebugPinSel1) controls the mux for both the debug_data_valid and pclk_out signals as both of these must be enabled for any debug operation.
The mapping of debug_data_out[n] signals onto individual pins will take place outside the RDU.
The interrupt controller unit generates an interrupt request by driving interrupt request lines with the appropriate interrupt level. LEON supports 15 levels of interrupt with level 15 as the highest level. The CPU will begin processing an interrupt exception when execution of the current instruction has completed and it will only do so if the interrupt level is higher than the current processor priority. If a second interrupt request arrives with the same level as an executing interrupt service routine then the exception will not be processed until the executing routine has completed.
When an interrupt trap occurs the LEON hardware will place the program counters (PC and nPC) into two local registers. The interrupt handler routine is expected, as a minimum, to place the PSR register in another local register to ensure that the LEON can correctly return to its pre-interrupt state. The 4-bit interrupt level (irl) is also written to the trap type (tt) field of the TBR (Trap Base Register) by hardware. The TBR then contains the vector of the trap handler routine the processor will then jump. The TBA (Trap Base Address) field of the TBR must have a valid value before any interrupt processing can occur so it should be configured at an early stage.
Interrupt pre-emption is supported while ET (Enable Traps) bit of the PSR is set. This bit is cleared during the initial trap processing. In initial simulations the ET bit was observed to be cleared for up to 30 cycles. This causes significant additional interrupt latency in the worst case where a higher priority interrupt arrives just as a lower priority one is taken. The interrupt acknowledge cycles shown in FIG. 15 are derived from simulations of the LEON processor. The SoPEC toplevel interrupt signals used in this diagram map directly to the LEON interrupt signals in the iui and iuo records. An interrupt is asserted by driving its (encoded) level on the icu_cpu_ilevel[3:0] signals (which map to iui_irl[3:0]). The LEON core responds to this, with variable timing, by reflecting the level of the taken interrupt on the cpu_icu_ilevel[3:0] signals (mapped to iuo.irl[3:0]) and asserting the acknowledge signal cpu_iack (iuo.intack).The interrupt controller then removes the interrupt level one cycle after it has seen the level been acknowledged by the core. If there is another pending interrupt (of lower priority) then this should be driven on icu_cpu_ilevel[3:0] and the CPU will take that interrupt (the level 9 interrupt in the example below) once it has finished processing the higher priority interrupt. The cpu_icu_ilevel[3:0] signals always reflect the level of the last taken interrupt, even when the CPU has finished processing all interrupts.

## Claims

1. A method of updating a cache in an integrated circuit, the integrated circuit incorporating the cache, memory and a memory interface connected to the cache and memory, the method comprising the steps of, following a cache miss:
(a) fetching, using the memory interface, first data associated with the cache miss and second data from the memory, the second data being stored in the memory adjacent the first data; and
(b) updating the cache with the fetched first and second data via the memory interface; and
wherein the cache includes instruction and data cache, the method including performing arbitration between instruction cache misses and data cache misses such that steps (a) and (b) are performed for data cache misses before instruction cache misses.

2. A method according to claim 1, the integrated further comprising a processor connected to the memory interface, the processor being configured to attempt a cache update with the first data upon receiving the first data from the memory interface, the method further comprising the step of preventing the attempted cache update by the processor from being successful, thereby preventing interference with the cache update of step (b).

3. A method according to claim 2, wherein step (b) is performed in response to the processor attempting to update the cache following step (a).

4. A method according to claim 3, wherein the memory interface is configured to monitor the processor to determine when it attempts to update the cache following step (a).