fix(w5100): actually drive RSTn on init so a wedged chip can recover

w5100_rst_n was declared and never driven anywhere -- the pin sat at its init
value of 1 forever.  The only reset the chip ever got was the MR software
reset, which arrives over the very bus a wedged chip has stopped answering, so
a wedged W5100 could not be recovered at all: not by NCRA reset, not by
reflashing, not even by reconfiguring the FPGA (RSTn just floats to its R36
pull-up while unconfigured).  Only a 12 V power cycle worked.

Init now pulses RSTn low (1 ms, datasheet minimum is ~2 us) and waits the full
post-reset settle before touching the bus.  RSTn low time is clamped to
reset_cycles so the testbenches' small values stay reachable by wait_ctr.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

exi_bba/w5100_parallel_master.py

@@ -50,6 +50,10 @@ _SUBR0     = 0x0005   # Subnet mask, 4 bytes
 _SHAR0     = 0x0009   # Source MAC, 6 bytes
 _SIPR0     = 0x000F   # Source IP, 4 bytes
 _NETLCKR   = 0x0071   # W5100S Network Lock Register (WO) — GWR/SUBR/SHAR/SIPR are
+
+# Default RSTn low time before release (sync cycles).  24_000 @ 24 MHz = 1 ms;
+# the W5100S datasheet minimum is ~2 us, so this is generous on purpose.
+_RST_LOW_DEFAULT = 24_000
 _NETLCK_UNLOCK = 0x3A #   writable ONLY while NETLCKR==0x3A (default 0x00 = locked)
 _IR        = 0x0015   # Interrupt register
 _IMR       = 0x0016   # Interrupt mask
@@ -150,6 +154,10 @@ class W5100ParallelMaster(Elaboratable):
         self._strobe = strobe_cycles
         # MR-reset settle wait; testbench overrides with a small value.
         self._reset_cycles = reset_cycles
+        # RSTn low time.  Clamped to reset_cycles so the testbenches (which
+        # pass a tiny reset_cycles to stay fast) keep wait_ctr wide enough to
+        # actually reach it -- an unreachable target would hang the FSM.
+        self._rst_low = min(_RST_LOW_DEFAULT, reset_cycles)
 
         # ── Optional socket-1 UDP bring-up test ───────────────────────────────
         # When enabled, init also configures the W5100's IP stack (GAR/SUBR/
@@ -556,7 +564,8 @@ class W5100ParallelMaster(Elaboratable):
                 with m.If(self.init_req):
                     for i in range(6):
                         m.d.sync += mac_shadow[i].eq(self.par[i*8:(i+1)*8])
-                    m.next = "MR_RST"
+                    m.d.sync += [self.w5100_rst_n.eq(0), wait_ctr.eq(0)]
+                    m.next = "HW_RST"
                 with m.Elif(~self.w5100_int_n):
                     m.next = "RX_CHECK"
                 with m.Elif(self.tx_valid & self.tx_sof):
@@ -582,6 +591,32 @@ class W5100ParallelMaster(Elaboratable):
                                  self.dbg_rd_done.eq(1)]
                     m.next = "IDLE"
 
+            # Pulse the W5100's RSTn pin, then let it boot, BEFORE touching the
+            # bus.  This used to be missing entirely: w5100_rst_n was declared
+            # and never driven, so the only reset the chip ever got was the MR
+            # software reset -- which a wedged chip does not answer, because it
+            # arrives over the very bus that is stuck.  The result was a W5100
+            # that could only be recovered by a 12 V power cycle; even
+            # reconfiguring the FPGA did not help, since RSTn just floats to its
+            # pull-up (R36) while the FPGA is unconfigured.  Driving it here
+            # makes an NCRA reset able to recover the chip on its own.
+            # RSTn low: datasheet minimum is ~2 us; _RST_LOW is far longer and
+            # costs nothing on a path that runs once per init.
+            with m.State("HW_RST"):
+                with m.If(wait_ctr == self._rst_low):
+                    m.d.sync += [self.w5100_rst_n.eq(1), wait_ctr.eq(0)]
+                    m.next = "HW_RST_WAIT"
+                with m.Else():
+                    m.d.sync += wait_ctr.eq(wait_ctr + 1)
+
+            # RSTn released: the W5100S needs 60.3 ms before it answers at all.
+            with m.State("HW_RST_WAIT"):
+                with m.If(wait_ctr == self._reset_cycles):
+                    m.d.sync += wait_ctr.eq(0)
+                    m.next = "MR_RST"
+                with m.Else():
+                    m.d.sync += wait_ctr.eq(wait_ctr + 1)
+
             # MR = 0x80 software reset (direct A=00), then settle.
             #
             # reset_cycles MUST cover the W5100S's post-reset dead time (the