Synchronous Interface to XScale CPU

Hi everyone,

I am trying to design an interface between an XScale PXA255 CPU and an Altera FPGA - we have a working SRAM-like asynchronous interface, but would like to make it synchronous for improved performance and lower latency. I have been considering making the FPGA look like an SDRAM device and using the CPU's internal SDRAM controller to interface, but due to some issues with bust mode and so on, that approach could result in even higher latencies and lower throughput that the SRAM-like interface. Has anyone solved this problem before or know of a suitable way to do it? Kind regards, Hugo Vincent, Bluewater Systems

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Reply to
hugo
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I've interfaced the PXA255 and FPGAs, but not synchronously. You never mentioned your bandwidth and latency requirements. Can you use the DMA channels on the xscale? Maybe you could describe your application in more detail.

-Jeff

Reply to
Jeff Cunningham

I did this on a Virtex2 with four Chip-selects from the CPU:

- the first one was used for reading/writing registers we built inside the FPGA, fixed/low latency, just adress-decoding

- the next was used for a fast read-only SRAM access with bursts-of-8 - whenever the CPU wanted to read the other modules in the FPGA had to wait. Fixed latency, something around 8 cycles but with burst access. A little complicated priority handling inside the ram-controller but it worked :-)

- on a second SRAM we could not afford any priority for the CPU - only if one of the other 2 modules had no writes the transfer (single rd/wr) from the CPU was possible. The CPU was blocked with the ready-pin. In our case this was possible as this was only for debugging purposes. The priority issue for ram-access was much easier that way.

- The last chip-select was used for a second low-speed bus that came out of the fpga. There we attached some more peripheral there as the PXA-FPGA bus had to run at 100 MHz ...

As Jeff stated - it's possible but the solution depends on what you want to do ..

bye, Michael

Reply to
Michael Schöberl

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