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doc/PPS-catcher-FSM: initial description
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Tue, 22 Aug 2023 08:55:33 +0000 |
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children | c2fc75655937 |
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This document describes the PPS catcher state machine (to be implemented in the sniffer FPGA) in 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.