Abstract:
Packet format configurability is extended for packets transported on physical links of an Intellectual Property (IP) core interconnect by using at least two independent parameters: one parameter governing data-width and one parameter governing latency penalty. The at least two independent parameters allow creation of transport protocol packets without additional latency insertion, which is useful for low-latency applications. The at least two independent parameters also allow creation of narrow packets with multi-cycle additional latency, which is useful for latency tolerant, area sensitive applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of a pending foreign priority application filed in France on Oct. 13, 2009, entitled “Zero-Latency Network on Chip (NOC),” and assigned French Patent Application No. 09 57137, the contents of which are incorporated herein in its entirety. 
     TECHNICAL FIELD 
     This subject matter is related generally to semiconductor Intellectual Property (IP) interconnect technology. 
     BACKGROUND 
     Many semiconductor designs, both in Integrated Circuit (IC) and in Field Programmable Gate Array (FPGA) applications are constructed in a modular fashion by combining a set of IP cores, such as Central Processing Units (CPUs), Digital Signal Processors (DSPs), video and networking processing blocks, memory controllers and others with an interconnect system. The interconnect system implements the system-level communications of the particular design. The IP cores are typically designed using a standard IP interface protocol, either public or proprietary. These IP interface protocols are referred to as transaction protocols. An example transaction protocol is Open Core Protocol (OCP) from OCP-IP, and Advanced Extensible Interface (AXI™) and Advanced High-performance Bus (AHB™) from Arm Inc. As semiconductor designs have evolved from relatively small, simple designs with a few IP cores into large, complex designs which may contain hundreds of IP cores, the IP core interconnect technology has also evolved. 
     The first generation of IP core interconnect technology consisted of a hierarchical set of busses and crossbars. The interconnect itself consists mostly of a set of wires, connecting the IP cores together, and one or more arbiters which arbitrate access to the communication system. A hierarchical approach is used to separate high-speed, high performance communications from lower-speed, lower performance subsystems. This solution is an appropriate solution for simple designs. A common topology used for these interconnects is either a bus or a crossbar. The trade-off between these topologies is straightforward. The bus topology has fewer physical wires which saves area and hence cost, but it is limited in bandwidth. The wire-intensive crossbar approach provides a higher aggregate communication bandwidth. 
     The above approach has a severe limitation in that the re-use of the IP cores is limited. The interfaces of all the IP cores connecting to the same interconnect are required to be the same. This can result in the re-design of the interface of an IP core or the design of bridge logic when a particular IP core needs to be used in another system. 
     This first generation of interconnect also implements a limited amount of system-level functions. This first generation of IP core interconnect technology can be described as a coupled solution. Since the IP interfaces are logically and physically not independent from each other, they are coupled such that modifying one interface requires modifying all the interfaces. 
     The second generation of IP interconnect is a partially decoupled implementation of the above described bus and crossbar topologies. In these solutions, the internal communication protocol of the communications system, or transport protocol, is decoupled from the IP interface protocol, or transaction protocol. These solutions are more flexible with regards to IP reuse as in these solutions the semiconductor system integrator can connect IP cores with different interfaces to the same communication system through some means of configurability. 
     The third generation of IP core interconnect technology is the Network-on-a-chip (NoC) which implements not only decoupling between transaction and transport layers, but also a clean decoupling between transport and physical layers. The key innovation enabling this solution is the packetization of the transaction layer information. The command and data information that is to be transported is encapsulated in a packet and the transport of the packet over the physical medium is independent of the physical layer. 
     In existing NoC solutions, bursts of information at the transaction layer are converted into packets, which are transported over physical links. A NoC packet is constructed of two parts: a header and a payload. The payload usually includes, but is not limited to, data information, with optionally other data-related information such as byte-enable and/or protection or security information. The header contains two types of information: first, transaction protocol level information that is transferred end-to-end without being changed by the interconnect and secondly, transport protocol level information needed and used by the interconnect to transport the payload correctly from one IP core to another through the interconnect. The term “correctly” does not refer only to routing, but also implies meeting other system level requirements, such as latency, quality of service, bandwidth requirements, etc. 
     In many transaction layer protocols, the command and data information of the transaction in a burst are presented in the same clock cycle. In the conversion process from transaction layer to transport layer, the header is created and used as the first one of several words in the packet. This packetization may insert one or more cycles of latency since the header is transported over the physical links during one or more clock cycles before the data is transported. 
     While an IP core can be re-used from design to design, the implementation of a NoC is likely to change as the NoC implements the system-level communications which are design specific. The number of IP cores, latency, bandwidth, power, and clock-speed are some of the variables that impact the requirements on the NoC. Hence, mechanisms have been developed to automate the design of an instance of a NoC to rapidly construct a NoC instance. During instantiation of a NoC, the width of the link, and therefore the width of the packet, is configurable by the user and this configurability allows the user to make an optimal trade-off between the number of wires in the interconnect system, latency and bandwidth. 
     In existing NoC solutions, the configurability that is available in the construction of a packet is limited to the selection of a value of one parameter. Using this single-parameter, a packet width can be selected. The values of the width of the packet and the latency penalty due to the header of the packet are intrinsically linked. Accordingly, a change in value of the packet width parameter can result in additional latency insertion. 
     SUMMARY 
     The disclosed implementations include packet formats for use in NoC interconnect solutions. In some implementations, packet format configurability is extended for packets transported on physical links of an IP core interconnect by using at least two independent parameters: one parameter governing data-width and one parameter governing latency penalty. The at least two independent parameters allow creation of transport protocol packets without additional latency insertion, which is useful for low-latency applications. The at least two independent parameters also allow creation of narrow packets with multi-cycle additional latency, which is useful for latency tolerant, area sensitive applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example NoC. 
         FIG. 2  is a block diagram of an example physical link connecting a transmitter and a receiver in the NoC of  FIG. 1 . 
         FIG. 3  is an example sequence of packet transport over the example link shown in  FIG. 2 . 
         FIG. 4  is an example packet for use with the NoC of  FIG. 1 . 
         FIG. 5  illustrates example packet formats for use with the NoC of  FIG. 1 . 
         FIG. 6  illustrates an example mechanism of padding unused bits in the packet formats of  FIG. 5 . 
         FIG. 7  is an example process for transmitting information over the link of  FIG. 2  using the packet formats shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Example NoC 
       FIG. 1  is a block diagram of an example NoC  100 . In some implementations, NoC  100  can be constructed out of a set of IP elements  102  which communicate with each other through a packet-based transport-protocol. Examples of IP elements  102  include but are not limited to: switches  102   a , clock converters  102   b , bandwidth regulators  102   c , sync First In First Out (FIFO)  102   d , width converters  102   e , Endian converters  102   f , rate adaptors  102   g , power isolators  102   h  and other IP elements. 
     In some implementations, at the edges of NoC  100 , Network Interface Units (NIUs)  104  implement a conversion between transaction protocol and transport protocol (ingress) and vice versa (egress). Some examples of NIUs for transaction protocols include but are not limited to: OCP NIU  104   a , AXI™ NIU  104   b , AHB™ NIU  104   c , memory scheduler  104   d  and a proprietary NIU  104   e . The NIUs  104  couple to various IP cores  110 . Some examples of IP cores are DSP  110   a , CPU  110   b , Direct Memory Access  110   c , OCP subsystem  110   d , DRAM Controller  110   e , SRAM  110   f  and other types of IP cores. 
     In NoC  100 , the transport protocol is packet-based. The commands of the transaction layer can include load and store instructions of one or more words of data that are converted into packets for transmission over physical links. Physical links form connections between the IP elements. An implementation of a transport port protocol used by NoC  100  is described in reference to  FIG. 2 . 
     Example Physical Link 
       FIG. 2  is a block diagram of an example physical link  200  connecting a transmitter  202  (TX) and a receiver  204  (RX) in NoC  100  of  FIG. 1 . A transport protocol socket can be used to transfer a packet from transmitter  202  to receiver  204  over physical link  200 . The socket can contain flow control signals (Vld, Rdy), framing signals (Head, Tail) and information signals (Data). The socket can be a synchronous interface working on rising edges of a clock signal (Clk). One active low reset signal (RStN) can also be included in the physical link  200 . The logical meaning of the different signals in this particular implementation is described next.
         Vld: Indicates that transmitter  202  presents valid information (Head, Tail and Data) in a current clock cycle. When Vld is negated, transmitter  202  drives an X value on Head, Tail and Data and receiver  204  discards these signals. Once transmitter  202  asserts Vld, the signals Head, Tail, Data and Vld remain constant until Rdy is asserted by receiver  204 . In this particular implementation, the width of Vld can be 1. Other widths can also be used.   Rdy: Indicates that receiver  204  is ready to accept Data in a current clock cycle. Rdy can depend (in combination) on Vld, Head, Tail and Data, or can only depend on the internal state of receiver  204 . In this particular implementation, the width of Rdy can be 1. Other widths can also be used.   Head: Indicates a first clock cycle of a packet. In this particular implementation, the width of Head is 1. Other widths can also be used.   Tail: Indicates a last clock cycle of a packet. In this particular implementation, the width of Tail is 1. Other widths can also be used.   Data: Effective information transferred from transmitter  202  to receiver  204 . Data contains a header and a payload. A data word transfer can occur when the condition Vld AND Rdy is true. The width of Data can be configurable.       

     Example Packet Transport Sequence 
       FIG. 3  is an example sequence of packet transport over the link of  FIG. 2 . In some implementations, a packet starts when Vld and Head are asserted, and completes when Vld and Tail are asserted. A single cycle packet can have both Head and Tail asserted. Inside a packet, Head is negated when Vld is asserted, and outside a packet, Head is asserted simultaneously with Vld. Packet content is carried on the Data signals. In this particular implementation, two packet formats exist: packets with payload (e.g., write requests, read responses), and packets without payload (e.g., all other packet types). 
     Example Packet 
       FIG. 4  is an example packet for use with NoC  100  of  FIG. 1 . More particularly,  FIG. 4  illustrates an example packet format  400  including a header  402  and a payload  404 . The example packet format  400  can be defined by four bytes (with byte-enables) of payload width and one cycle header penalty. In some implementations of the packet format  400 , some fields may be optional. The total width of header  402  can be referred to as wHeader. In some implementations, header  402  includes a header field containing a RouteID, an Address field (Addr) and several Control fields. The Control fields in the header  402  can carry additional end-to-end or transport protocol information. The particular use and meaning of the Control fields in header  402  is not relevant to the discussion of the disclosed implementations. The meaning of the other fields in header  402  is explained next.
         Addr: This header field indicates the start address of a transaction, expressed in bytes, in the target address space.   RouteId: This header field uniquely identifies a “initiator-mapping, target-mapping” pair. The pair can be unique information used by routing tables to steer a packet inside NoC  100 .
 
The fields in the payload of the packet can be Byte-Enable (BE) field and Data field (Byte). The meaning of these fields is explained next.
   BE: Indicates one Byte Enable bit per payload byte.   Byte: This field contains the payload part of the packet. The width of this field is configurable, and in some implementations, contains at least 8 bits of data. The width of a Byte can be extended to contain additional information such as protection or security information. The width of the Byte field is defined by wByte.
 
The way packets are transmitted on the Data signals can be defined by the two following independent parameters:
   nBytePerWord: This parameter indicates a number of payload bytes transferred per clock cycle. Example legal values are 0, 1, 2, 4, 8, 16, 32, 64 and 128 bytes.   hdrPenalty: This parameter indicates how a header is transmitted on the Data signals.
 
In some implementations, the parameter hdrPenalty can have the following values:
   1. NONE: A header is sent simultaneously with a first payload if any. The effective width of the Data signals (wData) is equal to (wByte+1)*nBytePerWord+wHeader.   2. ONE: A header occupies exactly one clock cycle. The effective width of the Data signals is equal to max((wByte+1)*nBytePerWord, wHeader).   3. TWO: A header occupies exactly two clock cycles, even when the header is smaller than (wByte+1)*nBytePerWord. The effective width of the Data signals is equal to max((wByte+1)*nBytePerWord, ceil(wHeader/2)).   4. AUTO: A header is automatically split to be transmitted on the (wByte+1)*nBytePerWord bits of the Data signals. The number of cycles for the header is equal to ceil(wHeader/((wByte+1)*nBytePerWord)).       

     Example Packet Formats 
       FIG. 5  illustrates example packet formats for use with NoC  100  of  FIG. 1 . More particularly,  FIG. 5  shows examples of different packet format combinations based on values of independent parameters hdrPenalty and nBytePerWord. In some implementations, a header can be split each time wHeader is greater than wData. Most significant bits can be sent first (big-endian), and least significant bits of a last Data word can be padded with zeroes when necessary. A payload can be padded with zeroes when (wByte+1)*nBytePerWord+wC&lt;wData. Padding can occur on the least significant bits 
     Example Padding Mechanism 
       FIG. 6  illustrates an example mechanism of padding unused bits. The shorthand “wH” is used as an abbreviation of wHeader and “wP” of (wByte+1)*nBytePerWord. 
     The flexibility of the packet formats of  FIG. 5  provides significant advantages to an implementation of a NoC or IP core interconnect structure. The additional flexibility allows NoC designers to implement a simple solution while still meeting latency and bandwidth requirements of the interconnect. The resulting area savings can translate directly into cost reduction of an IC or FPGA. 
     The option hdrPenalty=NONE can be used when low latency and/or high bandwidth are desired. This solution can be expensive in terms of wire usage, but in a modern System on a Chip (SoC) design, the number of links that require these stringent requirements is limited. 
     The option hdrPenalty=ONE is an option that can be used by a main interconnect in an SoC. This solution can be wire efficient while still providing high bandwidth and acceptable latency numbers. 
     The option hdrPenalty=TWO is an option that can be used by control and peripheral interconnect structures. This solution can provide high wire efficiency combined with somewhat reduced performance. 
     The option hdrPenalty=AUTO is an option that can be used by service interconnect structures where wire efficiency is an important design parameter. 
     In one implementation, the selection of a packet format can be made on a link-per-link basis. For example, the two independent parameters (hdrPenalty, nBytePerWord) defining the packet format can be selected differently for every link, allowing further optimization of the NoC  100 . Since physical links in a NoC may have different performance requirements, a packet format can be chosen optimally for each link reducing area and power consumption. 
     Since a NoC for a complex SoC, such as a cell-phone application processor or video processing chip can contain many links, there are many configuration decisions that need to be managed. In one implementation, the selection of the parameters can be made through a Graphical User Interface (GUI), allowing for a quick and efficient configuration of the packet-formats and associated links. 
     Example Process for Transmitting Information Over Link 
       FIG. 7  is an example process  700  for transmitting information over the physical link of  FIG. 2  using the packet format shown in  FIG. 5 . In some implementations, the process  700  includes configuring a packet for transport over a physical link, the configuring including specifying values for at least two independent parameters ( 702 ), and transmitting at least a portion of the header and the payload over the physical link during one or more clock cycles based on the values of the parameters ( 702 ). In some implementations, a first parameter indicates a width of the physical link or a width of the payload, and a second parameter specifies a minimal latency to transport the header over the physical link that is in addition to a minimal latency to transport the payload over the physical link. 
     While this specification contains many specifics, these should not be construed as limitations on the scope of what is claimed or of what can be claimed, but rather as descriptions of features specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. 
     Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single product or packaged into multiple products. 
     Thus, particular implementations have been described. Other implementations are within the scope of the following claims.