Full ARP round trip on hardware: 5 requests out, 5 replies in

The PC's replies are unicast to our own MAC, which it could only learn from our requests, so the loop is closed independently of any packet capture. Records the four W5100 fixes it took and the caveat that TI does not mean the chip transmitted. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
2026-09-30 19:25:58 +00:00
parent 5f60c011cb
commit b0e8c32619
1 changed files with 28 additions and 0 deletions
@@ -277,6 +277,34 @@ Pi over J4 with the board on 12 V — device ID `04 02 02 00`, NCRA reset
 self-clearing, PAR0–5, MAR0–7, ANALOG, NWAYC, IMR `0x86` readback, NCRA SR,
 and NWAYS `0x17`. See `hardware/re-bba-rb/exi_fullpath_rpi.py`.
 ✅ **FULL ROUND TRIP ON HARDWARE 2026-09-30.** The board sent 5 ARP requests
 over EXI and received all 5 replies back: `op=2`, unicast to our own MAC
 `00:09:bf:00:00:01`, sender 10.0.0.50 / target 10.0.0.99, read back out of the
 SPRAM ring by EXI DMA with RWP advancing exactly one page per exchange. The PC
 could only reply, and only address it to our MAC, if our request reached it —
 so this closes the loop independently of any packet capture.
 Getting there took four W5100 fixes, none of which simulation could catch
 because the bus model reproduces neither the chip's reset timing nor its
 interrupt latching. Each masked the next:
 1. `tx_bytes` overflow — the drain waited for a length that only arrived at the
   ST trigger, so frames truncated at exactly 16 bytes (see TWD fix below).
 2. Post-reset settle was 1 ms; the W5100S needs 60.3 ms. Marginal, so it
   *sometimes* worked — which is what made everything look flaky.
 3. `w5100_rst_n` was declared and NEVER DRIVEN. The only reset was the MR
   software reset, which a wedged chip cannot answer because it arrives over
   the stuck bus — so a wedge survived NCRA reset, reflashing and even FPGA
   reconfiguration, and needed a 12 V power cycle.
 4. `RX_RSR_CHK` returned to IDLE on RSR==0 without clearing `Sn_IR`. SEND_OK
   (bit 4) fires after every transmit and only RECV (bit 2) was ever cleared,
   so INT_N latched low; since IDLE tests `~w5100_int_n` BEFORE `tx_valid`, the
   FSM span and TX starved. Symptom: first frame transmits, all later ones
   never set TI while RX keeps working.
 ⚠️ Note TI only means `TXFrameDrain` handed the frame to the W5100 — it does
 NOT mean the chip put it on the wire. Do not treat TI as proof of transmission;
 the round trip above is the real test.
 ✅ **BOTH DIRECTIONS PROVEN ON HARDWARE 2026-09-30 — the BBA data path works.**
 *GC → wire* (after the TWD fix below): a full **60-byte ARP request**, captured