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;
+  }
+}