Abstract:
Apparatus and method emulating a parallel interface to effect parallel data transfer from serial flash memory are provided. A field-programmable gate array (FPGA) may be coupled to a processor via a data bus. A serial flash memory may be coupled to the FPGA via a serial interface. The FPGA may be programmed to emulate a parallel interface by converting a serial data stream of boot code or operating software received from the serial flash memory to a parallel data stream to effect parallel data transfer over the data bus to the processor. The FPGA may be responsive to respective logic signals set by the processor to start access to the serial flash memory by pointing to at least one predefined location corresponding to at least one starting address of data to be transferred to the processor without using a plurality of address lines to access the serial flash memory.

Description:
This application claims benefit of the Apr. 19, 2012 filing date of U.S. Provisional Application No. 61/635,415, titled “Enhanced Booting of Processor/FPGAs in a Confined Space”, which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     Disclosed embodiments relate to computing devices, and, more particularly, to computing devices that may require an external memory to boot or configure. 
     BACKGROUND 
     Across a wide spectrum of computing applications involving signal processing, the growth in signal processing complexity can exceed the processing capabilities of stand-alone processor, such as without limitation, digital signal processors (DSPs), PowerPC™ processors and microprocessors. In some of these applications, one may use additional devices to meet the signal processing needs of a given application. 
     Field-programmable gate array (FPGA) co-processing is well-suited for such applications. When a FPGA chip is connected to a separate processor chip, an interface is needed. The interface selection between the processor and the FPGA may be driven by the application characteristics as well as the available interfaces on the processor. For example, interfaces available may include an external memory interface (EMIF) bus. 
     Known CCA-based computing devices generally involve a relatively large number of circuit board traces (e.g., copper traces including separate traces to deliver each bit in connection with data and addresses for the data). Due to physical separation (e.g., electrical isolation) that may be needed between the traces, this may result in a need of a relatively large footprint of the CCA, more complex trace analysis, as well as complexity of the CCA layout to avoid racing conditions. 
     Flash memory devices may be used in a variety of applications to store configuration, program, and/or memory data. Parallel flash memory transmits and receives a plurality of bits at a time, such as 16 or 32 bits. Parallel flash memory may be contrasted with serial flash memory which transfers data one bit at a time (per channel). Serial flash memory may permit a reduction in board space relative to parallel flash memory. However, for conventional computing devices having a processor and a separate FPGA, serial flash memory would result in a substantial loss of speed during a data transfer. In view of the foregoing considerations, it would be desirable to provide improved methodology and apparatus for effecting fast and reliable data transfer of boot code or operating software. 
     BRIEF SUMMARY 
     Generally, one non-limiting embodiment may provide apparatus including a field-programmable gate array (FPGA) coupled to a processor by way of a data bus. A serial flash memory may be coupled to the FPGA by way of a serial interface. The FPGA may be programmed to emulate a parallel interface by converting a serial data stream comprising boot code or operational software received from the serial flash memory to a parallel data stream to effect parallel data transfer over the data bus to the processor. 
     Another non-limiting embodiment may provide apparatus including a field-programmable gate array (FPGA) coupled to a processor by way of a data bus. A serial flash memory may be coupled to the FPGA by way of a serial interface. The FPGA may be programmed to emulate a parallel interface by converting a serial data stream comprising boot code or operational software received from the serial flash memory to a parallel data stream to effect parallel data transfer over the data bus to the processor. The FPGA may be responsive to respective logic signals set by the processor to start access to the serial flash memory by pointing to at least one predefined location corresponding to at least one starting address of data to be transferred to the processor without using a plurality of address lines to access the serial flash memory. 
     Still a further non-limiting embodiment may provide a method which allows coupling a field-programmable gate array (FPGA) to a processor by way of a data bus. Coupling a serial flash memory to the FPGA by way of a serial interface, and programming the FPGA to emulate a parallel interface by converting a serial data stream comprising boot code or operational software received from the serial flash memory to a parallel data stream to effect parallel data transfer over the data bus to the processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are explained in the following description in view of the drawings that show: 
         FIG. 1  is a block representation of a disclosed embodiment of an apparatus having a processor, a separate FPGA, and a serial flash memory. 
         FIG. 2  is a flow chart depicting a process of a disclosed embodiment. 
         FIG. 3  is a block, diagram illustrating certain aspects of an embodiment in connection with selectable address jumps, which allow data transfers without involvement of parallel data addresses. 
         FIG. 4  is a block diagram illustrating further aspects of an embodiment in connection with management of a data buffer coupled to receive data being accessed by the processor from the serial flash memory. 
     
    
    
     DETAILED DESCRIPTION 
     A CCA-based computing device embodiment may include a processor and a separate FPGA and may further include a single serial flash memory which may be used to boot both the FPGA and the processor, instead of a parallel flash memory and a separate serial flash memory. Improved FPGA firmware has been designed to reduce the boot time of the processor by booting the processor through the FPGA, where the firmware may be arranged to emulate (e.g., mimic) a parallel flash chip interface to speed data transfer, such as boot code or operating software, from the FPGA to the processor. 
       FIG. 1  is a block representation of an embodiment of a computing device  20 . Computing device  20  may include a processor  22  (such as a without limitation, a DSP) and a separate FPGA  24  coupled to one another by a data bus  26  (such as an EMIF bus), and a separate memory that may include a single serial flash memory  28  and may be implemented on a circuit card assembly (CCA), according to a non-limiting embodiment of the present invention. The lack of parallel flash memory in computing device  20  is noted, with the processor being coupled to flash memory  28  by way of FPGA  24 , where FPGA  24  may be configured to boot processor  22 . The FPGA may include a co-processor. 
     Serial flash memory  28  may comprise a NOR-based flash memory. Advantageous features of disclosed embodiments may be the ability to speed-up the boot process and simplify the routing design. By way of comparison NAND flash (as opposed to NOR flash) may involve relatively longer read access times. Moreover, an interface  29  between FPGA  24  and flash memory  28  may be a serial synchronous interface, such as a Serial Peripheral Interface (SPI). A benefit of a synchronous interface (as contrasted with an asynchronous interface) is faster data reads, less traces to route, and smaller footprint. FPGA  24  may be configured to reformat the serial synchronous data into a stream of parallel data, such as 16-bit or higher bit-width. 
     As will be appreciated by those skilled in the art, typical serial flash interfaces send one bit of data every clock cycle. However, in one non-limiting embodiment, interface  29  may be a quad SRI interface to maximize throughput by sending four bits of data every clock cycle. Therefore, in this embodiment the firmware algorithm may convert a synchronous quad. SPI to an asynchronous 16-bit (or higher) parallel. 
     Using a quad SPI to boot FPGA  24  and processor  22  saves costs on parts, traces and over 50% board space on the CCA as compared to conventional parallel booting. The firmware was found to address the slower boot times due to use of a serial interface, by having FPGA  24  convert a quad serial stream from the SPI flash to a 16 bit parallel stream, tricking processor  22  to function as if it was coupled to a standard parallel flash. This was found to make the boot time approximately at least three times faster than the standard SPI interface. 
     The description below elaborates details in connection with a process of a non-limiting embodiment, where the FPGA may be tasked to boot the processor from a serial flash while an internal boot loader of the processor expects a parallel stream of data, such as 16-bit wide, 32-bit wide, etc. Thus, the FPGA may be configured to emulate a parallel stream of data to the processor. Table 1 below lists respective logical states of two general purpose input/output GPIO FPGA pins (e.g., GPOIs ( 1 ,  2 )) which may be set by the processor to determine respective modes of the FPGA for implementing the disclosed process. Table 2 below lists respective logical states of further FPGA GPIO pins (e.g., GPOIs ( 3 ,  4 ) which may be set by the FPGA during the process. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 FPGA Logic Signals Set by Processor 
               
             
          
           
               
                 GPIO 2 
                 GPIO 1 
                 FPGA Mode 
                 Description 
               
               
                   
               
               
                 0 
                 0 
                 Normal Memory 
                 When both GPIOs are low, 
               
               
                   
                   
                 Access 
                 processor has access to 
               
               
                   
                   
                   
                 standard memory map 
               
               
                 0 
                 1 
                 Flash Buffer Access 
                 Any change to GPIOs resulting 
               
               
                   
                   
                 Only 
                 in this state starts access 
               
               
                   
                   
                   
                 to SPI flash at a predefined 
               
               
                   
                   
                   
                 starting address location 
               
               
                   
                   
                   
                 for operating software (OFP) 
               
               
                 1 
                 0 
                 Flash Buffer Access 
                 Any change to GPIOs resulting 
               
               
                   
                   
                 Only 
                 in this state starts access 
               
               
                   
                   
                   
                 to SPI flash at a predefined 
               
               
                   
                   
                   
                 starting address location 
               
               
                   
                   
                   
                 for backup software (MLV) 
               
               
                 1 
                 1 
                 Flash Buffer Access 
                 Any change to the GPIOs 
               
               
                   
                   
                 Only 
                 resulting in this state (includes 
               
               
                   
                   
                   
                 release from reset with pull-ups 
               
               
                   
                   
                   
                 by CCA of GPIOs) starts access 
               
               
                   
                   
                   
                 to the SPI at a at a predefined 
               
               
                   
                   
                   
                 starting address location 
               
               
                   
                   
                   
                 for Boot code. 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 FPGA Logic Signals for FPGA to SET 
               
             
          
           
               
                 GPIO 
                 Description 
               
               
                   
               
               
                 GPIO 3 
                 When set High, inactive - Data has not been loaded into the 
               
               
                   
                 buffer from the flash for boot, 
               
               
                   
                 When set Low, active - FPGA has buffered a predefined number 
               
               
                   
                 words (e.g., 256) and ready for the processor to read. 
               
               
                 GPIO 4 
                 When set High, inactive - The FPGA is in boot mode 
               
               
                   
                 When set Low, active - FPGA has released the EMIF bus out of 
               
               
                   
                 boot mode and normal register accesses are permitted. 
               
               
                   
               
             
          
         
       
     
       FIG. 2  is a flow chart depicting a process of a non-limiting embodiment. Presuming FPGA is active, and GPIOs ( 2 ,  1 ) have been set to (1, 1) so that, for example, GPIOs are tri-stated by the processor and pulled high by the CCA, then subsequent to a start step  50 , as shown in block  52 , FPGA holds processor in a reset condition. As shown in block  54 , FPGA sets SPI flash to a quad mode. As shown in block  56 , FPGA sets a predefined start address for boot data to start a preload of boot data from SPI to FPGA. As shown in block  58 , FPGA releases processor out of the reset condition and sets GPIO  3  to an active status (See Table 2 above) 
     In one non-limiting embodiment, the FPGA may store the boot data in a data buffer, (e.g., circular buffer 4-bits wide or more), for the processor boot loader to sequentially access parallel streams of boot data (e.g., such as 16-bits or 32-bits at a time). To handle a special case of the processor in connection with the first word being accessed by the processor, as illustrated in block  60 , FPGA may provide the first four bytes of data as 8-bit reads, and then as shown in block  62 , FPGA continues to sequentially read parallel streams of data, such as 16-bits, 32-bits wide, etc. 
     As shown in block  64 , the FPGA may be configured to control an appropriate loading of the buffer to ensure optimized data flow as the processor requests data. In one non-limiting embodiment, accesses to the buffer may be automatically incremented by way of strobes (e.g., rd_en strobes). The FPGA may release the processor from reset when the buffer registers a predefined target number of words (e.g., 256 words of data, 4 pages of data) to reduce the possibility of sending repeat data to the processor during boot. The FPGA may provide data flow control by way of a WAIT line on the EMIF bus to ensure fresh data (not repeat data) is supplied to the processor. If the data in the buffer is below a lower range threshold (e.g., approximately 2 pages of data), then one may command a number of wait cycles (e.g., 52 wait cycles) to the EMIF bus with respect to the processor. This equates to 64 wait states with respect to a processor operating at a non-limiting example clocking rate of 125 MHz. Once the FPGA detects a sufficient refilling of data stored in the buffer (e.g., 3 pages or more), then the FPGA can stop the wait states. 
     If the buffer reaches an upper range threshold (e.g., 5 pages) of stored data, then one may halt reading the SPI flash until the amount of data in the buffer falls below 4 pages as seen in Table 3 below regarding Management of the data buffer. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Data Buffer Management 
               
             
          
           
               
                 Number of words in 
                   
               
               
                 Buffer 
                 Action 
               
               
                   
               
               
                 Target Number Reached 
                 Remove Reset 
               
               
                 (e.g., 256 Words, 4 pages) 
               
               
                 Below Lower End 
                 Add wait states to EMIF Bus 
               
               
                 Threshold (e.g., 127 
               
               
                 Words, 2 pages) 
               
               
                 Refill Target Reached 
                 Remove wait states 
               
               
                 (e.g., 193 Words, 3 pages) 
               
               
                 Reaches Upper End 
                 Stop reading from SDI Flash, continue 
               
               
                 Threshold (e.g., 320 
                 reading when data in buffer decreases 
               
               
                 Words, 5 pages) 
                 to target number (e.g., four pages) 
               
               
                   
               
             
          
         
       
     
     As shown in decision block  66 , further iterations of data loading may be performed at block  64  till processor sets GPIOs ( 2 , 1 )=0,0. In this case, as shown in block  68 , FPGA sets quad SPI flash in a serial mode and sets GPIO  3  to an inactive status. As shown in decision block  70 , a further iteration will be performed at block  68  till SPI flash has changed mode and FGPA has emptied its buffer. In this case, as shown in block  72 , FGPA sets GPIO  4  to an active status and continues to block  74 , indicating boot mode is done and FGPA resumes normal operational access to memory. 
     Decision  76  evaluates Whether processor has set GPIOs ( 2 , 1 ) to one of three choices: (0,1); (1,0) or (1,1). If processor has set GPIOs ( 2 , 1 ) to (1,1), then the process returns to block  52  in connection with a further access of boot data. If processor .has set GPIOs ( 2 , 1 ) to (1,0), then the process continues at block  78 , where FPGA sets an initial address for accessing operating software, sets GPIO  4  to an inactive status and preloads a number of words (e.g., 256) of data from SPI flash to FGPA. If processor has set GPIOs ( 2 , 1 ) to (0,1), then the process continues at block  80 , where FPGA sets an address for backup software, sets GPIO  4  to an inactive and preloads a number of words (e.g.,256) of data from SPI flash to FGPA and continues to block  82  where FGPA sets GPIO  3  to an active status prior to continuing to block  74 . 
       FIG. 3  is a block diagram illustrating certain aspects of an embodiment in connection with programmable address jumps, which allow data transfers without involvement of parallel data addresses. In one non-limiting embodiment, FPGA  24  may be programmed to be responsive to respective logic signals set by the processor (see Table 1) to start accessing data in serial flash memory  28  by pointing (e.g., jumping) to at least one predefined location corresponding to at least one starting address of the data without using a plurality of address lines to access the data. For example, block  32  may represent boot data and location  34  may correspond to a starting address for boot data  32 . As listed in Table 1, when GPOIs ( 2 ,  1 ) are set by processor to 1,1, then the processor will start accessing data at location  34  and will continue to sequentially access such data without having to use a plurality of address lines. In a second non-limiting example, block  36  may represent operational software and location  38  may correspond to a starting address for the operational software. In this case, when GPOIs ( 2 ,  1 ) are set by processor to 0,1, then the processor will start accessing operational software data at location  38  and will continue to sequentially access such operational software without having to use address lines. In a third non-limiting example, block  40  may represent backup software and location  40  may correspond to a starting address for the backup software. In this case, when GPOIs ( 2 ,  1 ) are set by processor to 1,0, then processor will start accessing data at location  40  and will continue to sequentially access such operational software without having to use address lines. In one non-limiting example embodiment, the predefined location may be based on a type of data to be transferred. This embodiment may provide at least the following advantages. For example, not having to use address lines (e.g., parallel address lines) further reduces the number of traces in the CCA and thus further saves costs on parts, traces and board space on the CCA, as compared to conventional parallel addressing. Additionally, not having to wait for processing of parallel address data substantially reduces the time used to implement data transfers. 
       FIG. 4  is a block diagram illustrating further aspects of an embodiment regarding a data buffer  84  (e.g., a circular data buffer) coupled to receive data being accessed by the processor from the serial flash memory. It is noted that the amount and/or speed at which data is requested from the serial flash is driven by the processor. In one non-limiting example, FPGA  24  may be configured to provide flow control to data being requested by the processor. The storage level of buffer  84  may be monitored, as illustrated in block  86 . If decision block  88  determines that the storage level of buffer  84  (e.g., a number of data words) is below a Lower End Threshold (e.g., &lt;L.E.Th), then FPGA  24  may issue a plurality of wait commands  92  to the data bus until the number of words in the data buffer refills to a predefined number of words. See Table 3. If decision block  88  determines that the storage level of buffer  84  is above an Upper End Threshold (e.g., &gt;U.E.Th) then FPGA  24  may issue a plurality of halt commands  92  to halt reading of data from flash memory until the number of words in the data buffer has been reduced to a predefined number of words. If decision block  88  determines that the storage level of buffer  84  is within a target level, then data transfer to processor will continue, as requested by the processor. This allows keeping data ready for the processor regardless of the speed at which data may be requested by the processor from the serial flash. 
     While various embodiments have been shown and described herein, it will be apparent that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the embodiments herein. Accordingly, it is intended that such embodiments be limited only by the spirit and scope of the appended claims.