Reading back SRAM content via JTAG?

Discussion in 'VHDL' started by moe, Nov 14, 2003.

  1. moe

    moe Guest

    I hope I'm posting it in the right groups. I've been designing for a
    while, but with minimal JTAG knowledge.

    Q: Can I use JTAG interface to verify what I wrote into the SRAM,
    instead of the traditional
    read-back method?

    My setup and the reason for wanting to do it this way is :
    An FPGA interfaces to a sync SRAM (QDR with separate write/read port).
    The FPGA can write to the SRAM
    using the processor interface, but does not have the read-back
    capability, because the read port from the SRAM goes elsewhere (no
    read-back capability there, either). Why do it this way? So that the
    each of the write/read data bus is point-to-point, without stubs on the
    board (150+Mhz). I could therotically bring the read bus back into the
    FPGA, then send it out again, but I'm fairly pin-limited. There are
    several SRAM interfaces, so whatever I do for each SRAM gets multipled
    by N. So, what I'd like to do is load the SRAM in a conventional
    manner, then test it via the JTAG interface. The amount of time it
    takes to verify doesn't matter.

    The SRAM I'm thinking of using is Cypress, will be running it at
    150+Mhz, and the device supports these JTAG instructions : EXTEST,
    IDCODE, SAMPLE Z, SAMPLE/PRELOAD, BYPASS (Preload isn't supported,
    actually). Looking at this, it looks like I need to use the FPGA to
    supply the read controls at a slower rate, to match JTAG speed, then use
    SAMPLE instruction. Or, am I way off base here? Would it get too messy
    and should I just provite the read-back path at the cost of higher

    Thank you in advance for any input/suggestion.

    To reply via email, change 'hard' to 'easy'.
    moe, Nov 14, 2003
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  2. moe

    Jim Wu Guest

    The SRAM I'm thinking of using is Cypress, will be running it at
    Are the SRAM read control signals connected to the FPGA? If not, I think you
    will need to use INTEST instruction, but looks like the SRAM does not
    support it.

    Jim Wu
    (remove capital letters)
    Jim Wu, Nov 14, 2003
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  3. moe

    moh Guest

    Thanks for your response.

    The read control is/can be attached to the SRAM from the FPGA, but
    do I need to run the read state machine to match the JTAG interface

    Is this going to turn into a project of its own? (Don't I need to
    somehow synchronize hw and sw - use hw to initiate read at a slower
    speed, then check with SAMPLE instruction via sw, back to hw to
    increment addr, etc.?)

    Thanks again.

    (replace 'hard' with 'easy' in email)
    moh, Nov 14, 2003
  4. moe

    Jim Wu Guest

    Is this going to turn into a project of its own? (Don't I need to
    You can also do it purely using JTAG with a combination of EXTEST and SAMPLE
    commands. Use EXTEST to set up read controls and address via FPGA pins and
    then use SAMPLE to read the SRAM data back.

    Jim Wu
    (remove capital letters)
    Jim Wu, Nov 16, 2003
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