Abstract:
A method for reducing the number of interface lines and non-volatile memory devices in a computer system includes providing a non-volatile memory having a SDRAM style interface. A system having both non-volatile memory and SDRAM has reduced interface lines by providing only one memory interface. A system where the SDRAM interface logic is initialized by code stored in the non-volatile memory having a SDRAM style interface, eliminating any requirement for other non-volatile memory, independent of the SDRAM interface, from which to initialize the system.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/273,616, filed Mar. 5, 2001. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a computer system and a method of implementing a computer system having a non-volatile memory and a volatile memory. In particular, it relates to a computer system in which the non-volatile memory and volatile memory have a sequential access mode of operation. 
     BACKGROUND OF THE INVENTION 
     Computer systems typically include both non-volatile memory, for storing information that has to be retained at power-off, as well as volatile memory. 
     Dynamic random access memory (DRAM) is a commonly used volatile system memory. It uses an interface with address lines that are typically multiplexed in time. Thus, an address having twice the number of bits as there are address lines is supplied in two successive stages. The most and least significant portions of a complete address are presented sequentially. This arrangement reduces the total number of address lines required. In order to identify which address portion is being sent, each address portion is distinguished by separate control signals. Thus, the DRAM requires control information to access the appropriate memory location. 
     Synchronous dynamic random access memory (SDRAM) extends this approach by providing a mode in which only the initial address in a sequence of accesses is sent to the memory. Subsequent data locations are read by using additional control signals and a clock signal to indicate when each subsequent data value should be delivered. The SDRAM has internal logic used to advance the data address. In addition to the timing signals, certain control registers of the internal logic of the SDRAM must be loaded with timing control parameters before the sequential access mode may be used. 
     In addition to the control information needed by the memory itself, the interface logic to the SDRAM also requires initialization. Information is loaded into control registers of the logic subsystem which provides control signals to the SDRAM, to permit access to the SDRAM. Traditionally, this initialization information for the SDRAM memory interface logic and internal logic of the SDRAM memory itself, is provided by instructions read from a separate non-volatile memory in the system. 
     The disadvantage of these traditional systems is that both the SDRAM and the non-volatile memory in the system have separate interfaces, thus, significantly increasing the number of interface lines and control logic in a system. The present invention seeks to address this problem by creating a non-volatile memory using the same interface as SDRAM thereby providing a system in which both the non-volatile and volatile memories in the system have a common interface. However, using a common: interface, which needs software-controlled initialization, presents a paradox. The instructions for initializing the memory interface logic must be read through the very interface for which the logic has yet to be initialized. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to reduce the number of interface lines and non-volatile memory devices in a computer system. The present invention provides for a computer system wherein the non-volatile memory shares a common interface with SDRAM. In particular, the present invention provides for a non-volatile memory that has an SDRAM style memory interface, being used to provide initialization (boot) code to the computer system; eliminating any need for an additional random access non-volatile memory to provide boot code. 
     SDRAM interface logic has to be initialized before the SDRAM can be accessed. The initializing information is stored in the non-volatile memory, e.g., Flash memory, which creates a dilemma since the non-volatile memory uses an SDRAM interface for which the logic has yet to be initialized. The present invention addresses this problem by making use of the sequential access logic already incorporated in the SDRAM style memory interface, and a limited set of control signals that are independent of the SDRAM interface. 
     According to the invention there is provided a non-volatile memory having an SDRAM style interface. 
     Further according to the invention, there is provided a method of initializing a computer system, comprising reading boot code stored in a non-volatile memory, e.g., Flash memory, wherein the memory has an interface compatible with SDRAM, and wherein the first memory location in the non-volatile memory is read using one or more control signals that function independently of the SDRAM style interface. Subsequent memory locations are read by means of one or more control signals that function independently of the SDRAM style interface. 
     Further, according to the invention there is provided a method of reducing control and address lines in a computer system having non-volatile memory and SDRAM, comprising providing the non-volatile memory with an interface compatible with an SDRAM interface. 
     Still further, according to the invention, there is provided a method of configuring a SDRAM interface in a computer system wherein the computer system has a non-volatile memory with an SDRAM interface, comprising storing interface initialization code in the first location of the first accessed memory row of the non-volatile memory, providing a control signal independent of the interface for performing a read operation from the first location in the first accessed memory row in the non-volatile memory and providing a second control signal independent of the interface for incrementing the internal address of the non-volatile memory. The first portion of boot code, used for SDRAM interface initialization is thus delivered on the data bus to the computer system for sequential execution. The code performs no branch operations until the system is ready for random access code execution. Once the SDRAM interface logic has been initialized, a jump instruction may be executed to a location in the non-volatile memory where the remaining system boot code is located. The initialization code may alternatively include instructions for copying a portion of a boot code in the non-volatile memory into a random access memory, and a final instruction for branching to the portion of the boot code in the random access memory. 
     Still further, according to the invention, there is provided a computer system comprising non-volatile memory and volatile memory, wherein the non-volatile memory and volatile memory have a common interface, which, preferably, is a SDRAM style interface. Preferably, the code for initializing the interface logic is stored in the first location in the first accessed memory row of the non-volatile memory. The system typically includes a first control line that is independent of the interface, for reading from the first memory location in the first accessed memory row, and a second control line that is independent of the interface for incrementing the internal address register of the non-volatile memory. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a computer system of the invention, and 
         FIG. 2  is a more detailed block diagram of the system of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical computer system  10  is shown in  FIG. 1 . The system  10  is implemented on a motherboard  12 , and includes a processor or central processing unit (CPU)  14  also known as a micro-controller or micro-processor, a volatile random access memory  16 , a volatile sequential access memory in the form of SDRAM  18 , and non-volatile memory  20 . A power supply  22  is provided on the motherboard  12  in a conventional manner, and an input/output (I/O) unit  24  provides an interconnection of the motherboard  12  to typical I/O devices such as a display monitor  26 , keyboard  28 , and one or more disk drives  30 . These components are interconnected by signal buses, power lines, connectors, etc., as collectively indicated at  32 . The non-volatile memory  20  has the same interface as the SDRAM. 
     The boot code for initializing the SDRAM interface logic, the internal logic of the SDRAM, and the rest of the system is stored in the non-volatile memory  20 , which may be Flash memory. 
     Referring to  FIG. 2 , during power-up (reset time), e.g., using the system reset generator  60  asserts the system reset signal  61 , the internal state machine for sequential boot code access  70  for the system non-volatile memory  20  pre-reads the first row of the memory array and has it ready for read by the end of system reset time. A control signal such as chip select signal  64  (CS#), which functions independently of SDRAM interface initialization requirements, is used to initiate a read operation at the first location in the first accessed memory row. A second control signal, in this embodiment, read enable (RE#) signal  66  is used to indicate when the read data is to be delivered on the system data bus. The deassertion of the RE# signal  66  by the ROM/Flash controller  62  is used to increment the internal address to select the next location in the first memory row. 
     The system processor  14  starts the system initialization process by issuing reads to the non-volatile memory address range. A boot program address range decoder in the system control logic ASIC  68  responds by triggering the Flash controller  62  to output the CS# signal  64 . Thus, the first address in non-volatile memory  20  is accessed by means of an SDRAM interface independent control signal (CS# signal  64 ), whereafter the sequential access logic  70  incorporated into the logic subsystem of the non-volatile memory  20  with its SDRAM interface, receives the RE# control signal  66 , causing it to deliver sequential words to the data bus. These first words of boot code are constructed so as to do no branching and only perform the basic configuration of the SDRAM interface so as to initialize the SDRAM interface for normal operation. Thereafter, the remaining system initialization boot code may be execute in place (XIP), or instead, copied to random access memory  16  ( FIG. 1 ), whereafter a transfer branch is made from the non-volatile memory  20  to the boot code copied into the random access memory  16  for standard system initialization. 
     Ideally, the non-volatile memory  20  is able to support reading an unlimited number of sequential rows during the initial boot operation to allow unlimited coding for the boot process. However, it may be acceptable to only be able to read out a single row of words. 
     With this scheme there is no dependence on lower significant addresses from the processor. The memory  20  will read the same sequence of initial words independent of whether the processor uses a lower or high memory starting address. The processor simply fetches the code, and the non-volatile memory  20 , under control of the sequential access logic  70 , delivers purely sequential code, at least until the system is operational for normal code execution. Other than that the address range decoder in the system control logic ASIC  68  decodes the initial address range to trigger the Flash controller  62  to output the CS# signal  64 , the processor-supplied address is ignored by the memory since no non-sequential access is attempted by the code being executed. The processor simply needs an initial sequence of code to execute and assumes that the code delivered by the memory in response to the CS# signal  64  and RE# signal  66  is coming from the requested initialization code address location specified by the processor. The absolute location of the code in the memory array is unimportant as long as the initialization code is delivered by the non-volatile memory  20  following a system reset. 
     It will be appreciated that a non-volatile memory must be able to recognize a system reset event in order to know when to access the initial code sequence. The recognition of system reset could be through a separate (RESET#) signal pin or through the assertion of an unusual combination of standard SDRAM interface pins, such as the simultaneous assertion of CS#, RAS#, CAS#, and WE#. 
     Furthermore, the non-volatile memory  20  must be able to recognize that it is being selected to deliver code. The chip selection could be through the standard SDRAM interface pin CS# or through a separate pin dedicated to indicate the selection for sequential boot code access, (BOOTCS#). 
     The non-volatile memory  20  must also be able to recognize when to deliver each successive data word. The output enable and address advancing function (RE# signal in this embodiment) could be a separate pin or be served by the standard SDRAM interface data strobe (DQS) signal pin during the sequential initialization process. The pin could revert to its normal function following the first assertion of RAS after a reset. 
     In another embodiment, a single signal pin such as a Boot# pin could serve all these functions. The Boot# signal could be asserted for several clock cycles or some otherwise relatively long period to indicate a system reset. Once the memory completes its internal reset, the same Boot# signal could be asserted to indicate that data should be driven onto the bus. Deasserting the Boot# signal would advance the internal address to select the next sequential data word. In any event, in order to implement the system initialization control signals, a preferred embodiment would not require any additional control signals beyond those currently defined by the standard SDRAM interface and as few as one additional control line. 
     By adopting the use of interface independent control lines and the existing sequential access logic used in SDRAM interfaces, to access the SDRAM interface initialization code, the problem is avoided of how to initialize the SDRAM interface logic via an SDRAM interface that has not yet been initialized. The process can also be asynchronous since the timing of all the initial instruction read accesses is directly controlled by the signal used to perform the read enable function. 
     It will be appreciated that the invention is not limited to the specific embodiment described and that various configurations could be implemented making use of a non-volatile memory having an interface that is common with a volatile memory.