fix(timer): TOV flag does not update correctly #75

fix #75

2 files changed, 56 insertions, 20 deletions

diff --git a/src/peripherals/timer.spec.ts b/src/peripherals/timer.spec.ts
index 11c0264..583e759 100644
--- a/src/peripherals/timer.spec.ts
+++ b/src/peripherals/timer.spec.ts
@@ -239,7 +239,7 @@ describe('timer', () => {
     expect(cpu.cycles).toEqual(2);
   });
 
-  it('should clear the timer in CTC mode if it equals to OCRA', () => {
+  it('should reset the counter in CTC mode if it equals to OCRA', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
     new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x10);
@@ -248,12 +248,44 @@ describe('timer', () => {
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
     cpu.cycles = 1;
     cpu.tick();
-    cpu.cycles = 2;
+    cpu.cycles = 3;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(2);
+    expect(cpu.cycles).toEqual(3);
+  });
+
+  it('should not set the TOV bit when TOP < MAX in CTC mode (issue #75)', () => {
+    const cpu = new CPU(new Uint16Array(0x1000));
+    new AVRTimer(cpu, timer0Config);
+    cpu.writeData(TCNT0, 0x1e);
+    cpu.writeData(OCR0A, 0x1f);
+    cpu.writeData(TCCR0A, WGM01); // WGM: CTC
+    cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
+    cpu.cycles = 1;
+    cpu.tick();
+    cpu.cycles = 2;
+    cpu.tick();
+    const tcnt = cpu.readData(TCNT0);
+    expect(tcnt).toEqual(0x1f);
+    expect(cpu.data[TIFR0]).toEqual(OCF0A); // TOV0 clear
+  });
+
+  it('should set the TOV bit when TOP == MAX in CTC mode (issue #75)', () => {
+    const cpu = new CPU(new Uint16Array(0x1000));
+    new AVRTimer(cpu, timer0Config);
+    cpu.writeData(TCNT0, 0xfe);
+    cpu.writeData(OCR0A, 0xff);
+    cpu.writeData(TCCR0A, WGM01); // WGM: CTC
+    cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
+    cpu.cycles = 1;
+    cpu.tick();
+    cpu.cycles = 2;
+    cpu.tick();
+    const tcnt = cpu.readData(TCNT0);
+    expect(tcnt).toEqual(0xff);
+    expect(cpu.data[TIFR0]).toEqual(OCF0A | TOV0);
   });
 
   it('should set OCF0B flag when timer equals OCRB', () => {
diff --git a/src/peripherals/timer.ts b/src/peripherals/timer.ts
index 0f4d837..f10a49b 100644
--- a/src/peripherals/timer.ts
+++ b/src/peripherals/timer.ts
@@ -238,12 +238,14 @@ function compToOverride(comp: CompBitsValue) {
 }
 
 export class AVRTimer {
+  private readonly MAX = this.config.bits === 16 ? 0xffff : 0xff;
   private lastCycle = 0;
   private ocrA: u16 = 0;
   private nextOcrA: u16 = 0;
   private ocrB: u16 = 0;
   private nextOcrB: u16 = 0;
   private ocrUpdateMode = OCRUpdateMode.Immediate;
+  private tovUpdateMode = TOVUpdateMode.Max;
   private icr: u16 = 0; // only for 16-bit timers
   private timerMode: TimerMode;
   private topValue: TimerTopValue;
@@ -403,10 +405,11 @@ export class AVRTimer {
 
   private updateWGMConfig() {
     const wgmModes = this.config.bits === 16 ? wgmModes16Bit : wgmModes8Bit;
-    const [timerMode, topValue, ocrUpdateMode] = wgmModes[this.WGM];
+    const [timerMode, topValue, ocrUpdateMode, tovUpdateMode] = wgmModes[this.WGM];
     this.timerMode = timerMode;
     this.topValue = topValue;
     this.ocrUpdateMode = ocrUpdateMode;
+    this.tovUpdateMode = tovUpdateMode;
   }
 
   count = (reschedule = true) => {
@@ -417,28 +420,34 @@ export class AVRTimer {
       const counterDelta = Math.floor(delta / divider);
       this.lastCycle += counterDelta * divider;
       const val = this.tcnt;
-      const { timerMode } = this;
+      const { timerMode, TOP } = this;
       const phasePwm =
         timerMode === TimerMode.PWMPhaseCorrect || timerMode === TimerMode.PWMPhaseFrequencyCorrect;
-      const limit = this.TOP + 1;
       const newVal = phasePwm
         ? this.phasePwmCount(val, counterDelta)
-        : (val + counterDelta) % limit;
-      const overflow = val + counterDelta >= limit;
+        : (val + counterDelta) % (TOP + 1);
+      const overflow = val + counterDelta > TOP;
       // A CPU write overrides (has priority over) all counter clear or count operations.
       if (!this.tcntUpdated) {
         this.tcnt = newVal;
         this.timerUpdated();
       }
 
-      if (!phasePwm && this.ocrUpdateMode == OCRUpdateMode.Bottom && overflow) {
-        // OCRUpdateMode.Top only occurs in Phase Correct modes, handled by phasePwmCount()
-        this.ocrA = this.nextOcrA;
-        this.ocrB = this.nextOcrB;
-      }
+      if (!phasePwm) {
+        if (this.ocrUpdateMode == OCRUpdateMode.Bottom && overflow) {
+          // OCRUpdateMode.Top only occurs in Phase Correct modes, handled by phasePwmCount()
+          this.ocrA = this.nextOcrA;
+          this.ocrB = this.nextOcrB;
+        }
 
-      if ((timerMode === TimerMode.Normal || timerMode === TimerMode.FastPWM) && val > newVal) {
-        cpu.setInterruptFlag(this.OVF);
+        // OCRUpdateMode.Bottom only occurs in Phase Correct modes, handled by phasePwmCount().
+        // Thus we only handle TOVUpdateMode.Top or TOVUpdateMode.Max here.
+        if (
+          (newVal === TOP || overflow) &&
+          (this.tovUpdateMode == TOVUpdateMode.Top || TOP === this.MAX)
+        ) {
+          cpu.setInterruptFlag(this.OVF);
+        }
       }
     }
     if (this.tcntUpdated) {
@@ -492,11 +501,6 @@ export class AVRTimer {
 
     if (this.ocrA && value === this.ocrA) {
       this.cpu.setInterruptFlag(this.OCFA);
-      if (this.timerMode === TimerMode.CTC) {
-        // Clear Timer on Compare Match (CTC) Mode
-        this.tcnt = 0;
-        this.cpu.setInterruptFlag(this.OVF);
-      }
       if (this.compA) {
         this.updateCompPin(this.compA, 'A');
       }