feat: initial timer implementation

8-bit timers basic functionality + tests:
1. basic counting + prescaler
2. timer overflow
3. timer overflow interrupt

1 files changed, 84 insertions, 0 deletions

diff --git a/src/timer.spec.ts b/src/timer.spec.ts
new file mode 100644
index 0000000..b7d96a9
--- /dev/null
+++ b/src/timer.spec.ts
@@ -0,0 +1,84 @@
+import { CPU } from './cpu';
+import { AVRTimer, timer0Config } from './timer';
+
+describe('timer', () => {
+  let cpu: CPU;
+
+  beforeEach(() => {
+    cpu = new CPU(new Uint16Array(0x1000));
+  });
+
+  it('should update timer every tick when prescaler is 1', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x45] = 0x1; // TCCR0B.CS <- 1
+    cpu.cycles = 1;
+    timer.tick();
+    expect(cpu.data[0x46]).toEqual(1); // TCNT should be 1
+  });
+
+  it('should update timer every 64 ticks when prescaler is 3', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x45] = 0x3; // TCCR0B.CS <- 3
+    cpu.cycles = 64;
+    timer.tick();
+    expect(cpu.data[0x46]).toEqual(1); // TCNT should be 1
+  });
+
+  it('should not update timer if it has been disabled', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x45] = 0; // TCCR0B.CS <- 0
+    cpu.cycles = 100000;
+    timer.tick();
+    expect(cpu.data[0x46]).toEqual(0); // TCNT should stay 0
+  });
+
+  it('should set TOV if timer overflows', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x46] = 0xff; // TCNT0 <- 0xff
+    cpu.data[0x45] = 0x1; // TCCR0B.CS <- 1
+    cpu.cycles = 1;
+    timer.tick();
+    expect(cpu.data[0x46]).toEqual(0); // TCNT should be 0
+    expect(cpu.data[0x35]).toEqual(1); // TOV bit in TIFR
+  });
+
+  it('should generate an overflow interrupt if timer overflows and interrupts enabled', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x46] = 0xff; // TCNT0 <- 0xff
+    cpu.data[0x45] = 0x1; // TCCR0B.CS <- 1
+    cpu.data[0x6e] = 0x1; // TIMSK0: TOIE0
+    cpu.data[95] = 0x80; // SREG: I-------
+    cpu.cycles = 1;
+    timer.tick();
+    expect(cpu.data[0x46]).toEqual(0); // TCNT should be 0
+    expect(cpu.data[0x35]).toEqual(0); // TOV bit in TIFR should be clear
+    expect(cpu.pc).toEqual(0x20);
+    expect(cpu.cycles).toEqual(3);
+  });
+
+  it('should not generate an overflow interrupt when global interrupts disabled', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x46] = 0xff; // TCNT0 <- 0xff
+    cpu.data[0x45] = 0x1; // TCCR0B.CS <- 1
+    cpu.data[0x6e] = 0x1; // TIMSK0: TOIE0
+    cpu.data[95] = 0x0; // SREG: --------
+    cpu.cycles = 1;
+    timer.tick();
+    expect(cpu.data[0x35]).toEqual(1); // TOV bit in TIFR should be set
+    expect(cpu.pc).toEqual(0);
+    expect(cpu.cycles).toEqual(1);
+  });
+
+  it('should not generate an overflow interrupt when TOIE0 is clear', () => {
+    const timer = new AVRTimer(cpu, timer0Config);
+    cpu.data[0x46] = 0xff; // TCNT0 <- 0xff
+    cpu.data[0x45] = 0x1; // TCCR0B.CS <- 1
+    cpu.data[0x6e] = 0; // TIMSK0: clear
+    cpu.data[95] = 0x80; // SREG: I-------
+    cpu.cycles = 1;
+    timer.tick();
+    expect(cpu.data[0x35]).toEqual(1); // TOV bit in TIFR should be set
+    expect(cpu.pc).toEqual(0);
+    expect(cpu.cycles).toEqual(1);
+  });
+});