From 0332b728e60de52b035555cdc4d4a99e736d1db8 Mon Sep 17 00:00:00 2001
From: Mateusz Kowalczyk <mateusz.kowalczyk@universalunit.net>
Date: Wed, 10 Jun 2026 01:37:18 +0000
Subject: [PATCH] Move I2C work out of interrupt context to fix display freeze

The tube library calls delay(100) after every display write and the
BME280 library calls delay(100) when re-initializing after a failed
transfer. Inside the TC3 timer ISR the SysTick interrupt is blocked,
so micros() stops advancing past ~1ms and delay() can spin forever
when its start sample lands exactly at the bottom of the micros()
sawtooth. All clocks derive from the same 48MHz source, so the ISR
phase is locked and one specific code path (display update on a
falling temperature reading) hits the fatal window reproducibly.

ISRs now only raise flags; sensor reads, display updates and timer
rescheduling run from loop(), where delay() works normally.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 button.ino   |  4 ++--
 metronom.ino | 32 +++++++++++++++++++++++---------
 2 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/button.ino b/button.ino
index b422bda..88e8bb8 100644
--- a/button.ino
+++ b/button.ino
@@ -18,11 +18,11 @@ void buttonISRstate() {
     else if (countdown == 0) {
       // TODO: restart metronome timer to align countdown
       countdown = COUNTDOWN;
-      updateTube();
+      displayNeedsUpdate = true;
     } else {
       countdown = -1;
       digitalWrite(BUTTON_LED_PIN, LOW);
-      updateTube();
+      displayNeedsUpdate = true;
       // TODO: disable metronome timer
     }
   }
diff --git a/metronom.ino b/metronom.ino
index f6e8db9..67b9f41 100644
--- a/metronom.ino
+++ b/metronom.ino
@@ -37,18 +37,19 @@ volatile float sensorValue[SENSORS] = {};
 volatile uint displaySensor = TEMPERATURE;
 volatile bool displayNeedsUpdate = false;
 volatile bool tubeRotated = false;
+volatile bool sensorsDue = false;
 int tubeTimerID;
 
+/* I2C (BME280 reads, tube writes) must never run in interrupt context:
+   both libraries call delay(), which inside an ISR can spin forever
+   because SysTick (millis/micros) is blocked. ISRs only raise flags;
+   loop() does the actual bus work. */
 void TasksHandler(void) {
   TasksISRs.run();
+}
 
-  if (displayNeedsUpdate || tubeRotated)
-    updateTube();
-
-  if (tubeRotated) {
-    TasksISRs.changeInterval(tubeTimerID, SENSOR_TIMESHARE[displaySensor]);
-    tubeRotated = false;
-  }
+void scheduleSensors() {
+  sensorsDue = true;
 }
 
 void setup() {
@@ -62,7 +63,7 @@ void setup() {
 
   TaskTimer.attachInterruptInterval_MS(TASK_HANDLER_INTERVAL, TasksHandler);
   TasksISRs.setInterval(METRONOME_INTERVAL, runMetronome);
-  TasksISRs.setInterval(SENSOR_INTERVAL, readSensors);
+  TasksISRs.setInterval(SENSOR_INTERVAL, scheduleSensors);
   tubeTimerID = TasksISRs.setInterval(SENSOR_TIMESHARE[displaySensor], rotateTube);
 }
 
@@ -79,4 +80,17 @@ void runMetronome() {
   }
 }
 
-void loop() {}
+void loop() {
+  if (sensorsDue) {
+    sensorsDue = false;
+    readSensors();
+  }
+
+  if (displayNeedsUpdate || tubeRotated)
+    updateTube();
+
+  if (tubeRotated) {
+    TasksISRs.changeInterval(tubeTimerID, SENSOR_TIMESHARE[displaySensor]);
+    tubeRotated = false;
+  }
+}