This document describes the PPS catcher state machine that has been implemented
in the SIMtrace3 sniffer FPGA, using a sort of verbal pseudocode.

INITIAL:

This state will be entered upon reset (held in this state while RST is low).
Must receive 8'h3B to proceed to state T0, otherwise transition to DONE.

ATR_T0:

Whatever byte we receive in this state, we latch the upper nibble in the Y
register and the lower nibble in the K register - then proceed to TAn.

ATR_TAn:

Advance to TBn if Y register indicates no TAn OR we got a byte.

ATR_TBn:

Advance to TCn if Y register indicates no TBn OR we got a byte.

ATR_TCn:

Advance to TDn if Y register indicates no TCn OR we got a byte.

ATR_TDn:

If Y register indicates no TDn, advance to state ATR_HIST.

 ELSE wait for byte; upon byte Rx, do the following:

- latch the upper nibble in the Y register;
- if lower nibble != 0, set TCK present flag;
- always go back to TAn.

ATR_HIST:

If K == 0, advance to state ATR_TCK.

 ELSE wait for byte; upon byte Rx, decrement K and remain in this state.

ATR_TCK:

Advance to READY_FOR_PPS if Have_TCK flag is false OR we got a byte.

READY_FOR_PPS:

Wait for Rx byte.  If the byte equals 8'hFF, proceed to REQ_PPS0,
otherwise DONE.

REQ_PPS0:

Wait for Rx byte; latch 3 bits of the received byte and proceed to REQ_PPS1.

REQ_PPS1:

Advance to REQ_PPS2 if register indicates no PPS1 OR we got a byte.

REQ_PPS2:

Advance to REQ_PPS3 if register indicates no PPS2 OR we got a byte.

REQ_PPS3:

Advance to REQ_PCK if register indicates no PPS3 OR we got a byte.

REQ_PCK:

Wait for Rx byte; ignore the received value and go to READY_FOR_RESP.

READY_FOR_RESP:

Wait for Rx byte.  If the byte equals 8'hFF, proceed to RESP_PPS0,
otherwise DONE.

RESP_PPS0:

Wait for Rx byte.  If PPS1 indicator bit is cleared, go to DONE.  Otherwise,
latch the 3 bits like in REQ_PPS0 and proceed to RESP_PPS1.

RESP_PPS1:

Wait for Rx byte.  The upper 6 bits must match 6'b100101 - if mismatch, go to
DONE.  If the upper bits match, latch the two lsbs - they will be the speed
enhancement multiplier bits - and proceed to RESP_PPS2.

RESP_PPS2:

Advance to RESP_PPS3 if register indicates no PPS2 OR we got a byte.

RESP_PPS3:

Advance to RESP_PCK if register indicates no PPS3 OR we got a byte.

RESP_PCK:

Wait for Rx byte.  Upon getting this final byte, trigger the speed enhancement
switch and go to DONE.

DONE:

Final state, nothing more happens.