Nexys4 -- an Obituary

I bought a Nexys4 board in January 2015. It was my first board with a Series-7 FPGA. The initial development was done with ISE 14.7 because early Vivado versions did not properly synthesize the VHDL coding style used in the w11 project. Vivado quickly matured, and with this the Nexys4 board became the main development platform for the w11 project in the last four and a half years.

Out of the blue, on July 20th 2019, the Nexys4 board stopped working. No functional changes on firmware or backend code had been done, only lots of SPDX comment changes. Board configuration worked, but any further rlink communication failed after a few commands with

  -E- 18:15:48.122022 : DecodeResponse: NAK seen, code Ccrc

Further analysis showed that the 2nd command of a chain was aborted with a CRC NAK.

    ti_w11 -tuD,1M,break,cts -ll3 -tl3
    -I- 20:47:26.946166 : port write nchar=  18 dt_rd=0.000042  dt_wr=0.000973
           0: ca 78 22 20 40 03 00 87 33 2a 20 40 01 ff 11 27 ca 71 
    -I- 20:47:26.947124 : port  read nchar=  11 dt_rd=0.001000  dt_wr=0.000958
           0: ca 78 22 00 84 60 ca 6a b8 ca 71 
              ^^^^^ ^^^^^^^^^^^ ^^^^^^^^
              SOP   wreg-resp   NAK-ccrc
    -E- 20:47:26.947145 : DecodeResponse: NAK seen, code Ccrc 

A NAK comma symbol (represented as ca 6a) is send by the rlink core when a received command can't be processed. The following byte, the NAK code, encodes one of eight abort conditions. The code b8 means ccrc abort, or in other words, that the CRC check for the command request failed.

A lot of systematic testing was done

At this point is was natural to ask whether the serial data path, starting at the FTDI FT2232H, was somehow broken. The FT2232H as such seems to be OK because FPGA configuration, which uses one of the two channels, always worked. So I ran the sys_tst_serloop2_n4 design

    cd $RETROBASE/rtl/sys_gen/tst_serloop/nexys4
    make sys_tst_serloop2_n4.vconfig

    cd $RETROBASE/rtl/sys_gen/tst_serloop

    tst_serloop USB2 460800  1 -break -trace  -loop 1 10
      2019-07-29:20:34:03.062471: rx   10 char:   d4 d5 d6 d7 d8 d9 da db dc dd
      2019-07-29:20:34:03.062485: tx   10 char:   de df e0 e1 e2 e3 e4 e5 e6 e7
      2019-07-29:20:34:03.063471: rx   10 char:   de df e0 e1 e2 e3 e4 e5 e6 e7
      2019-07-29:20:34:03.063485: tx   10 char:   e8 e9 ea eb ec ed ee ef f0 f1
      2019-07-29:20:34:03.064472: rx   10 char:   e8 e9 ea eb ec ed ee ef f0 f1
      2019-07-29:20:34:03.064487: tx   10 char:   f2 f3 f4 f5 f6 f7 f8 f9 fa fb
      2019-07-29:20:34:03.065472: rx   10 char:   f2 f3 f4 f5 f6 f7 f8 f9 fa fb
      2019-07-29:20:34:03.065484:  1.00s  998 l     9980 c: 997.5 lps   9975 cps
    # --> seems to work
    # do systematical tests
    tst_serloop USB2   460800 1 -break  -loop 10 1024
    tst_serloop USB2  1000000 1 -break  -loop 10 1024
    tst_serloop USB2  4000000 1 -break  -loop 10 1024
    tst_serloop USB2 12000000 1 -break  -loop 10 4
    tst_serloop USB2 12000000 1 -break  -loop 10 1024
    tst_serloop USB2 12000000 1 -break  -loop 10 4096
    # --> all work fine !!

After this puzzling finding another round of testing started

Given all the evidence listed above all plausible error scenarios I could come up with are somehow excluded:

The sad bottom line is that the Nexys4 board is apparently broken. It is quite unclear what exactly is damaged. The only thing I can do is to put the Nexys4 board aside and buy a Nexys4 A7 as replacement.