From a99fe56aae96b58ff313c43bab99b0c87288aed8 Mon Sep 17 00:00:00 2001
From: Allen Hill <allenofthehills@gmail.com>
Date: Tue, 21 Jul 2026 13:22:19 -0700
Subject: [PATCH] Fix intermittent, early boot short start bit errors

---
 src/avclan/avclan.h                        |  3 +-
 src/avclan/bus.cc                          |  7 +++--
 src/avclan/target/avr-attiny3216/phy_avr.c | 36 ++++++++++++++++++----
 3 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/src/avclan/avclan.h b/src/avclan/avclan.h
index 303b16a..a64fe15 100644
--- a/src/avclan/avclan.h
+++ b/src/avclan/avclan.h
@@ -143,7 +143,8 @@ struct Error {
     BAD_CONTROLLER_PARITY,
     BAD_CONTROL_PARITY,
     BAD_PARITY, // generic bad parity has max severity
-    STARTBIT_TOO_SHORT,
+    STARTBIT_MISSED,
+    STARTBIT_MALFORMED,
     STARTBIT_TOO_LONG,
     BAD_STARTBIT,
     POOL_EMPTY, // non-bus error
diff --git a/src/avclan/bus.cc b/src/avclan/bus.cc
index 47b4a1d..aaa11c5 100644
--- a/src/avclan/bus.cc
+++ b/src/avclan/bus.cc
@@ -289,7 +289,10 @@ auto Bus::read(uint16_t address, Frame::Print print)
     switch (err.errno) {
       case POOL_EMPTY: puts("failed Frame alloc"); break;
       case BAD_STARTBIT: fputs("bad start bit (other)", stdout); break;
-      case STARTBIT_TOO_SHORT: fputs("bad start bit (short)", stdout); break;
+      case STARTBIT_MISSED: fputs("missed start bit", stdout); break;
+      case STARTBIT_MALFORMED:
+        fputs("malformed start bit (external cause)", stdout);
+        break;
       case STARTBIT_TOO_LONG: fputs("bad start bit (long)", stdout); break;
       case BAD_CONTROLLER_PARITY:
         fputs("reading controller addr.", stdout);
@@ -312,7 +315,7 @@ auto Bus::read(uint16_t address, Frame::Print print)
   }
 
   // Only print if some data has been correctly received
-  if (print.print && (err.errno < STARTBIT_TOO_SHORT)) {
+  if (print.print && (err.errno < STARTBIT_MISSED)) {
     if (err.errno > BAD_DATA_PARITY)
       in->length = 0;
     in->print(print);
diff --git a/src/avclan/target/avr-attiny3216/phy_avr.c b/src/avclan/target/avr-attiny3216/phy_avr.c
index 9e79abc..cef4e8d 100644
--- a/src/avclan/target/avr-attiny3216/phy_avr.c
+++ b/src/avclan/target/avr-attiny3216/phy_avr.c
@@ -344,6 +344,14 @@ void phy_init() {
 // error reporting; no printing happens here.
 Read phy_read_startbit() {
   uint16_t startbitlen = TCB1.CNT = 0;
+
+  // Reset the ~atomic `pulsewidth` variable to detect the post-pulse update
+  // from the TCB0_INT_vect ISR
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    if (!BUS_IS_IDLE) // Only reset if bus is actively driven (i.e. current
+      pulsewidth = 0; // value is stale/already been used)
+  }
+
   while (!BUS_IS_IDLE) {
     startbitlen = TCB1.CNT;
     if (startbitlen > (uint16_t)AVCLAN_STARTBIT_LOGIC_0 * 1.2) {
@@ -367,18 +375,34 @@ Read phy_read_startbit() {
       return result;
     }
   }
+
+  // `pulsewidth` updates once the TCB0_INT_vect ISR runs for this pulse.
+  TCB1.CNT = 0;
+  do {
+    if (TCB1.CNT > (uint16_t)AVCLAN_BIT0_LOGIC_1) // Wait a max of ~6μs for ISR
+      return BAD_STARTBIT; // ISR/other implementation bug; abort
+
+    // Read ~atomically, to prevent torn reads
+    ATOMIC_BLOCK(ATOMIC_RESTORESTATE) { startbitlen = pulsewidth; }
+  } while (startbitlen == 0);
+
   if (startbitlen < (uint16_t)(AVCLAN_STARTBIT_LOGIC_0 * 0.8)) {
-    // We missed the beginning of this message; wait for it to finish (bus
-    // continuously idle for >1 bit length) before returning, so we don't have
-    // multiple false-starts while the in-progress message keeps sending more
-    // bits.
+    // Not a start bit; wait for the message to finish (bus continuously idle
+    // for >1 bit length) before returning, so we only report one error (instead
+    // of e.g. repeated "bad (short) start bit" errors)
     TCB1.CNT = 0;
     while (TCB1.CNT < (uint16_t)(AVCLAN_BIT_LENGTH_MAX * 1.2)) {
       if (!BUS_IS_IDLE)
-        TCB1.CNT = 0;
+        TCB1.CNT = 0; // Reset counter after each bit pulse
     }
-    return STARTBIT_TOO_SHORT;
+    // A pulse no wider than a normal bit means we merely tuned in mid-frame and
+    // this was a data bit; a wider-but-still-sub-start pulse means some other
+    // device emitted a wonky pulse.
+    return (startbitlen < (uint16_t)AVCLAN_BIT_LENGTH_MAX) ? STARTBIT_MISSED
+                                                           : STARTBIT_MALFORMED;
   }
+  if (startbitlen > (uint16_t)(AVCLAN_STARTBIT_LOGIC_0 * 1.2))
+    return STARTBIT_TOO_LONG;
   return (Read)0; // that was a start bit
 }