Timer: add demo (untested)

firmware/src/main.cpp

@@ -7,6 +7,7 @@
 #include "keyboard.h"
 #include "encoder.h"
 #include "debug.h"
+#include "timer.h"
 
 void setup() {
   setupDebug();
@@ -27,6 +28,8 @@ void loop() {
   // debugScanI2C();
   // demoScreen();
 
+  demoTimer();
+
   auto key = handleKeyboard();
   if (key != KEY_NONE) {
     tick();

firmware/src/timer.cpp

@@ -0,0 +1,46 @@
+#include <Arduino.h>
+
+#define DEMO_TRIGGER_MS 1500
+
+void setupTimer() {
+  // Set up Timer 0 to trigger interrupt every 1ms
+  // Timer 0 is already used and configured by Arduino for millis()
+  // however it uses TOIE0 (overflow interrupt), so we'll use OCIE0A:
+  //   When this bit is written to one, and the I-flag in the Status Register is set
+  //   (interrupts globally enabled), the Timer/Counter0 Output Compare A Match interrupt is enabled
+  //   The corresponding Interrupt Vector is executed when the OCF0A Flag, located in TIFR0, is set.
+  OCR0A = 0xAF; // any number is OK, it will trigger at some point during the counter
+  TIMSK0 |= _BV(OCIE0A);
+}
+
+// incremented every DEMO_TRIGGER_MS by interrupt,
+// read and decremented by main loop
+volatile uint8_t timerEvents = 0;
+
+void demoTimer() {
+  bool shouldTrigger = false;
+
+  noInterrupts();
+  if (timerEvents > 0) {
+    timerEvents--;
+    shouldTrigger = true;
+  }
+  interrupts();
+
+  if (shouldTrigger) {
+    Serial.println("Timer tick!");
+  }
+}
+
+volatile uint16_t timerMillis = 0;
+
+// triggers every 1ms
+ISR(TIMER0_COMPA_vect) {
+  timerMillis++;
+  if (timerMillis >= DEMO_TRIGGER_MS) {
+    timerMillis -= DEMO_TRIGGER_MS;
+    timerEvents++;
+  }
+
+  // TODO: Add motor encoder reading, calculate required carriage offset, control stepper
+}

firmware/src/timer.h

@@ -0,0 +1,2 @@
+void setupTimer();
+void demoTimer();