Non-integer FIFO aspect ratio

[darin_i]

Hi,

I'm working on an application where I have a 96-bit word coming in from a bank of ADC, and I want to stream this through a 128-bit PCIe 2.0/3.0 xdma (integrated IP) interface.

If I just shove the 96-bits into a FIFO buffer and then into the xdma streaming port, I essentially "waste" 32 bits of bandwidth on the PCIe bus and downstream buffer space.

Would a reasonable approach be to use TWO back-to back asymmetric FIFOs, one 96:384 bits, and the other 384:128 (with commensurate clock division between) to pack the data?

Is there a better way?

Thanks,

    -darin

[avrumw]

Why not just pack the data in RTL? To do this you only 96 bits of storage and a simple state machine.

As each 96 bit value comes in you concatenate it with whatever remainder you have from the last clock cycle. If the total number of bits is >= 128 you push it into your FIFO. So, for example

Bits held          +  bits added  - bits sent out = new bits held

    0                           96                   0                          96      (no word pushed)

   96                          96               128                          64      (word pushed)

   64                          96               128                          32      (word pushed)

   32                          96               128                            0      (word pushed)

    0                            96                  0                         96       (no word pushed)

  ...

Avrum

[dpaul24]

@darin_i,

I support the method described by avrum.