perf!: centeral timekeeping

This should improve performance, especially when running simulations with
multiple peripherals. For instance, the demo project now runs at ~322%,
up from ~185% in AVR8js 0.13.1.

BREAKING CHANGE: `tick()` methods were removed from individual peripherals.
You now need to call `cpu.tick()` instead.

12 files changed, 222 insertions, 221 deletions

diff --git a/src/cpu/cpu.ts b/src/cpu/cpu.ts
index 4b70efb..fb98786 100644
--- a/src/cpu/cpu.ts
+++ b/src/cpu/cpu.ts
@@ -55,6 +55,13 @@ export interface AVRInterruptConfig {
   constant?: boolean;
 }
 
+export type AVRClockEventCallback = () => void;
+
+interface AVRClockEventEntry {
+  cycles: number;
+  callback: AVRClockEventCallback;
+}
+
 export class CPU implements ICPU {
   readonly data: Uint8Array = new Uint8Array(this.sramBytes + registerSpace);
   readonly data16 = new Uint16Array(this.data.buffer);
@@ -63,6 +70,7 @@ export class CPU implements ICPU {
   readonly readHooks: CPUMemoryReadHooks = [];
   readonly writeHooks: CPUMemoryHooks = [];
   private readonly pendingInterrupts: AVRInterruptConfig[] = [];
+  private readonly clockEvents: AVRClockEventEntry[] = [];
   readonly pc22Bits = this.progBytes.length > 0x20000;
 
   // This lets the Timer Compare output override GPIO pins:
@@ -71,6 +79,7 @@ export class CPU implements ICPU {
   pc: u32 = 0;
   cycles: u32 = 0;
   nextInterrupt: i16 = -1;
+  private nextClockEvent: u32 = 0;
 
   constructor(public progMem: Uint16Array, private sramBytes = 8192) {
     this.reset();
@@ -164,10 +173,44 @@ export class CPU implements ICPU {
     }
   }
 
+  private updateClockEvents() {
+    this.clockEvents.sort((a, b) => a.cycles - b.cycles);
+    this.nextClockEvent = this.clockEvents[0]?.cycles ?? 0;
+  }
+
+  addClockEvent(callback: AVRClockEventCallback, cycles: number) {
+    const entry = { cycles: this.cycles + Math.max(1, cycles), callback };
+    this.clockEvents.push(entry);
+    this.updateClockEvents();
+    return callback;
+  }
+
+  updateClockEvent(callback: AVRClockEventCallback, cycles: number) {
+    const entry = this.clockEvents.find((item) => (item.callback = callback));
+    if (entry) {
+      entry.cycles = this.cycles + Math.max(1, cycles);
+      this.updateClockEvents();
+      return true;
+    }
+    return false;
+  }
+
+  clearClockEvent(callback: AVRClockEventCallback) {
+    const index = this.clockEvents.findIndex((item) => (item.callback = callback));
+    if (index >= 0) {
+      this.clockEvents.splice(index, 1);
+      this.updateClockEvents();
+    }
+  }
+
   tick() {
+    if (this.nextClockEvent && this.nextClockEvent <= this.cycles) {
+      const clockEvent = this.clockEvents.shift();
+      clockEvent?.callback();
+      this.nextClockEvent = this.clockEvents[0]?.cycles ?? 0;
+    }
     if (this.interruptsEnabled && this.nextInterrupt >= 0) {
       const interrupt = this.pendingInterrupts[this.nextInterrupt];
-      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
       avrInterrupt(this, interrupt.address);
       if (!interrupt.constant) {
         this.clearInterrupt(interrupt);
diff --git a/src/peripherals/eeprom.spec.ts b/src/peripherals/eeprom.spec.ts
index d293bd9..8f08e47 100644
--- a/src/peripherals/eeprom.spec.ts
+++ b/src/peripherals/eeprom.spec.ts
@@ -23,11 +23,10 @@ describe('EEPROM', () => {
   describe('Reading the EEPROM', () => {
     it('should return 0xff when reading from an empty location', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const eeprom = new AVREEPROM(cpu, new EEPROMMemoryBackend(1024));
+      new AVREEPROM(cpu, new EEPROMMemoryBackend(1024));
       cpu.writeData(EEARL, 0);
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EERE);
-      eeprom.tick();
       cpu.tick();
       expect(cpu.cycles).toEqual(4);
       expect(cpu.data[EEDR]).toEqual(0xff);
@@ -36,12 +35,11 @@ describe('EEPROM', () => {
     it('should return the value stored at the given EEPROM address', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
       eepromBackend.memory[0x250] = 0x42;
       cpu.writeData(EEARL, 0x50);
       cpu.writeData(EEARH, 0x2);
       cpu.writeData(EECR, EERE);
-      eeprom.tick();
       cpu.tick();
       expect(cpu.data[EEDR]).toEqual(0x42);
     });
@@ -51,13 +49,12 @@ describe('EEPROM', () => {
     it('should write a byte to the given EEPROM address', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
       cpu.writeData(EEDR, 0x55);
       cpu.writeData(EEARL, 15);
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EEMPE);
       cpu.writeData(EECR, EEPE);
-      eeprom.tick();
       cpu.tick();
       expect(cpu.cycles).toEqual(2);
       expect(eepromBackend.memory[15]).toEqual(0x55);
@@ -84,10 +81,10 @@ describe('EEPROM', () => {
 
       const cpu = new CPU(program);
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
       eepromBackend.memory[9] = 0x0f; // high four bits are cleared
 
-      const runner = new TestProgramRunner(cpu, eeprom);
+      const runner = new TestProgramRunner(cpu);
       runner.runInstructions(instructionCount);
 
       // EEPROM was 0x0f, and our program wrote 0x55.
@@ -98,7 +95,7 @@ describe('EEPROM', () => {
     it('should clear the EEPE bit and fire an interrupt when write has been completed', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
       cpu.writeData(EEDR, 0x55);
       cpu.writeData(EEARL, 15);
       cpu.writeData(EEARH, 0);
@@ -106,14 +103,12 @@ describe('EEPROM', () => {
       cpu.data[SREG] = 0x80; // SREG: I-------
       cpu.writeData(EECR, EEPE);
       cpu.cycles += 1000;
-      eeprom.tick();
       cpu.tick();
       // At this point, write shouldn't be complete yet
       expect(cpu.data[EECR] & EEPE).toEqual(EEPE);
       expect(cpu.pc).toEqual(0);
       cpu.cycles += 10000000;
       // And now, 10 million cycles later, it should.
-      eeprom.tick();
       cpu.tick();
       expect(eepromBackend.memory[15]).toEqual(0x55);
       expect(cpu.data[EECR] & EEPE).toEqual(0);
@@ -123,16 +118,14 @@ describe('EEPROM', () => {
     it('should skip the write if EEMPE is clear', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
       cpu.writeData(EEDR, 0x55);
       cpu.writeData(EEARL, 15);
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EEMPE);
       cpu.cycles = 8; // waiting for more than 4 cycles should clear EEMPE
-      eeprom.tick();
       cpu.tick();
       cpu.writeData(EECR, EEPE);
-      eeprom.tick();
       cpu.tick();
       // Ensure that nothing was written, and EEPE bit is clear
       expect(cpu.cycles).toEqual(8);
@@ -143,7 +136,7 @@ describe('EEPROM', () => {
     it('should skip the write if another write is already in progress', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
 
       // Write 0x55 to address 15
       cpu.writeData(EEDR, 0x55);
@@ -151,7 +144,6 @@ describe('EEPROM', () => {
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EEMPE);
       cpu.writeData(EECR, EEPE);
-      eeprom.tick();
       cpu.tick();
       expect(cpu.cycles).toEqual(2);
 
@@ -161,7 +153,6 @@ describe('EEPROM', () => {
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EEMPE);
       cpu.writeData(EECR, EEPE);
-      eeprom.tick();
       cpu.tick();
 
       // Ensure that second write didn't happen
@@ -173,7 +164,7 @@ describe('EEPROM', () => {
     it('should write two bytes sucessfully', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
 
       // Write 0x55 to address 15
       cpu.writeData(EEDR, 0x55);
@@ -181,13 +172,11 @@ describe('EEPROM', () => {
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EEMPE);
       cpu.writeData(EECR, EEPE);
-      eeprom.tick();
       cpu.tick();
       expect(cpu.cycles).toEqual(2);
 
       // wait long enough time for the first write to finish
       cpu.cycles += 10000000;
-      eeprom.tick();
       cpu.tick();
 
       // Write 0x66 to address 16
@@ -196,7 +185,6 @@ describe('EEPROM', () => {
       cpu.writeData(EEARH, 0);
       cpu.writeData(EECR, EEMPE);
       cpu.writeData(EECR, EEPE);
-      eeprom.tick();
       cpu.tick();
 
       // Ensure both writes took place
@@ -226,10 +214,10 @@ describe('EEPROM', () => {
 
       const cpu = new CPU(program);
       const eepromBackend = new EEPROMMemoryBackend(1024);
-      const eeprom = new AVREEPROM(cpu, eepromBackend);
+      new AVREEPROM(cpu, eepromBackend);
       eepromBackend.memory[9] = 0x22;
 
-      const runner = new TestProgramRunner(cpu, eeprom);
+      const runner = new TestProgramRunner(cpu);
       runner.runInstructions(instructionCount);
 
       expect(eepromBackend.memory[9]).toEqual(0xff);
diff --git a/src/peripherals/eeprom.ts b/src/peripherals/eeprom.ts
index bc386ca..d492aa7 100644
--- a/src/peripherals/eeprom.ts
+++ b/src/peripherals/eeprom.ts
@@ -95,7 +95,11 @@ export class AVREEPROM {
       }
 
       if (eecr & EEMPE) {
-        this.writeEnabledCycles = this.cpu.cycles + 4;
+        const eempeCycles = 4;
+        this.writeEnabledCycles = this.cpu.cycles + eempeCycles;
+        this.cpu.addClockEvent(() => {
+          this.cpu.data[EECR] &= ~EEMPE;
+        }, eempeCycles);
       }
 
       // Read
@@ -134,6 +138,11 @@ export class AVREEPROM {
         }
 
         this.cpu.data[EECR] |= EEPE;
+
+        this.cpu.addClockEvent(() => {
+          this.cpu.setInterruptFlag(this.EER);
+        }, this.writeCompleteCycles - this.cpu.cycles);
+
         // When EEPE has been set, the CPU is halted for two cycles before the
         // next instruction is executed.
         this.cpu.cycles += 2;
@@ -143,15 +152,4 @@ export class AVREEPROM {
       return false;
     };
   }
-
-  tick() {
-    const { EECR } = this.config;
-
-    if (this.writeEnabledCycles && this.cpu.cycles > this.writeEnabledCycles) {
-      this.cpu.data[EECR] &= ~EEMPE;
-    }
-    if (this.writeCompleteCycles && this.cpu.cycles > this.writeCompleteCycles) {
-      this.cpu.setInterruptFlag(this.EER);
-    }
-  }
 }
diff --git a/src/peripherals/spi.spec.ts b/src/peripherals/spi.spec.ts
index 8e11b94..635e657 100644
--- a/src/peripherals/spi.spec.ts
+++ b/src/peripherals/spi.spec.ts
@@ -160,7 +160,7 @@ describe('SPI', () => {
       return 0x5b; // we copy this byte to
     };
 
-    const runner = new TestProgramRunner(cpu, spi);
+    const runner = new TestProgramRunner(cpu);
     runner.runToBreak();
 
     // 16 cycles per clock * 8 bits = 128
@@ -172,11 +172,10 @@ describe('SPI', () => {
 
   it('should set the WCOL bit in SPSR if writing to SPDR while SPI is already transmitting', () => {
     const cpu = new CPU(new Uint16Array(1024));
-    const spi = new AVRSPI(cpu, spiConfig, FREQ_16MHZ);
+    new AVRSPI(cpu, spiConfig, FREQ_16MHZ);
 
     cpu.writeData(SPCR, SPE | MSTR);
     cpu.writeData(SPDR, 0x50);
-    spi.tick();
     cpu.tick();
     expect(cpu.readData(SPSR) & WCOL).toEqual(0);
 
@@ -186,7 +185,7 @@ describe('SPI', () => {
 
   it('should clear the SPIF bit and fire an interrupt when SPI transfer completes', () => {
     const cpu = new CPU(new Uint16Array(1024));
-    const spi = new AVRSPI(cpu, spiConfig, FREQ_16MHZ);
+    new AVRSPI(cpu, spiConfig, FREQ_16MHZ);
 
     cpu.writeData(SPCR, SPE | SPIE | MSTR);
     cpu.writeData(SPDR, 0x50);
@@ -194,13 +193,11 @@ describe('SPI', () => {
 
     // At this point, write shouldn't be complete yet
     cpu.cycles += 10;
-    spi.tick();
     cpu.tick();
     expect(cpu.pc).toEqual(0);
 
     // 100 cycles later, it should (8 bits * 8 cycles per bit = 64).
     cpu.cycles += 100;
-    spi.tick();
     cpu.tick();
     expect(cpu.data[SPSR] & SPIF).toEqual(0);
     expect(cpu.pc).toEqual(0x22); // SPI Ready interrupt
@@ -215,12 +212,10 @@ describe('SPI', () => {
     cpu.writeData(SPDR, 0x8f);
 
     cpu.cycles = 10;
-    spi.tick();
     cpu.tick();
     expect(cpu.readData(SPDR)).toEqual(0);
 
     cpu.cycles = 32; // 4 cycles per bit * 8 bits = 32
-    spi.tick();
     cpu.tick();
     expect(cpu.readData(SPDR)).toEqual(0x88);
   });
diff --git a/src/peripherals/spi.ts b/src/peripherals/spi.ts
index f67143f..7dd5d43 100644
--- a/src/peripherals/spi.ts
+++ b/src/peripherals/spi.ts
@@ -38,7 +38,7 @@ const bitsPerByte = 8;
 export class AVRSPI {
   public onTransfer: SPITransferCallback | null = null;
 
-  private transmissionCompleteCycles = 0;
+  private transmissionActive = false;
   private receivedByte: u8 = 0;
 
   // Interrupts
@@ -59,7 +59,7 @@ export class AVRSPI {
       }
 
       // Write collision
-      if (this.transmissionCompleteCycles > this.cpu.cycles) {
+      if (this.transmissionActive) {
         cpu.data[SPSR] |= SPSR_WCOL;
         return true;
       }
@@ -69,7 +69,13 @@ export class AVRSPI {
       this.cpu.clearInterrupt(this.SPI);
 
       this.receivedByte = this.onTransfer?.(value) ?? 0;
-      this.transmissionCompleteCycles = this.cpu.cycles + this.clockDivider * bitsPerByte;
+      const cyclesToComplete = this.clockDivider * bitsPerByte;
+      this.transmissionActive = true;
+      this.cpu.addClockEvent(() => {
+        this.cpu.data[SPDR] = this.receivedByte;
+        this.cpu.setInterruptFlag(this.SPI);
+        this.transmissionActive = false;
+      }, cyclesToComplete);
       return true;
     };
     cpu.writeHooks[SPSR] = (value: u8) => {
@@ -78,13 +84,9 @@ export class AVRSPI {
     };
   }
 
-  tick() {
-    if (this.transmissionCompleteCycles && this.cpu.cycles >= this.transmissionCompleteCycles) {
-      const { SPDR } = this.config;
-      this.cpu.data[SPDR] = this.receivedByte;
-      this.cpu.setInterruptFlag(this.SPI);
-      this.transmissionCompleteCycles = 0;
-    }
+  reset() {
+    this.transmissionActive = false;
+    this.receivedByte = 0;
   }
 
   get isMaster() {
diff --git a/src/peripherals/timer.spec.ts b/src/peripherals/timer.spec.ts
index 6b6a9eb..40db1f6 100644
--- a/src/peripherals/timer.spec.ts
+++ b/src/peripherals/timer.spec.ts
@@ -25,6 +25,7 @@ const TCNT0 = 0x46;
 const OCR0A = 0x47;
 const OCR0B = 0x48;
 const TIMSK0 = 0x6e;
+const TIMSK1 = 0x6f;
 
 // Timer 1 Registers
 const TIFR1 = 0x36;
@@ -66,12 +67,11 @@ const nopOpCode = '0000';
 describe('timer', () => {
   it('should update timer every tick when prescaler is 1', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(1);
@@ -79,12 +79,11 @@ describe('timer', () => {
 
   it('should update timer every 64 ticks when prescaler is 3', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCCR0B, CS01 | CS00); // Set prescaler to 64
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
     cpu.cycles = 64;
-    timer.tick();
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(1);
@@ -92,10 +91,11 @@ describe('timer', () => {
 
   it('should not update timer if it has been disabled', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCCR0B, 0); // No prescaler (timer disabled)
+    cpu.cycles = 1;
+    cpu.tick();
     cpu.cycles = 100000;
-    timer.tick();
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0); // TCNT should stay 0
@@ -103,13 +103,12 @@ describe('timer', () => {
 
   it('should set TOV if timer overflows', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0);
@@ -118,13 +117,12 @@ describe('timer', () => {
 
   it('should clear the TOV flag when writing 1 to the TOV bit, and not trigger the interrupt', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     expect(cpu.data[TIFR0]).toEqual(TOV0);
     cpu.writeData(TIFR0, TOV0);
@@ -133,15 +131,14 @@ describe('timer', () => {
 
   it('should set TOV if timer overflows in FAST PWM mode', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
     cpu.writeData(OCR0A, 0x7f);
     cpu.writeData(TCCR0A, WGM01 | WGM00); // WGM: Fast PWM
-    cpu.cycles = 1;
-    timer.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0);
@@ -150,26 +147,25 @@ describe('timer', () => {
 
   it('should generate an overflow interrupt if timer overflows and interrupts enabled', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
     cpu.data[TIMSK0] = TOIE0;
     cpu.data[SREG] = 0x80; // SREG: I-------
-    cpu.cycles = 1;
-    timer.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(2); // TCNT should be 2 (one tick above + 2 cycles for interrupt)
     expect(cpu.data[TIFR0] & TOV0).toEqual(0);
     expect(cpu.pc).toEqual(0x20);
-    expect(cpu.cycles).toEqual(3);
+    expect(cpu.cycles).toEqual(4);
   });
 
   it('should support overriding TIFR/TOV and TIMSK/TOIE bits (issue #64)', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, {
+    new AVRTimer(cpu, {
       ...timer0Config,
 
       // The following values correspond ATtiny85 config:
@@ -182,163 +178,154 @@ describe('timer', () => {
     });
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
     cpu.data[TIMSK0] = 2;
     cpu.data[SREG] = 0x80; // SREG: I-------
-    cpu.cycles = 1;
-    timer.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(2); // TCNT should be 2 (one tick above + 2 cycles for interrupt)
     expect(cpu.data[TIFR0] & 2).toEqual(0);
     expect(cpu.pc).toEqual(0x20);
-    expect(cpu.cycles).toEqual(3);
+    expect(cpu.cycles).toEqual(4);
   });
 
   it('should not generate an overflow interrupt when global interrupts disabled', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
     cpu.data[TIMSK0] = TOIE0;
     cpu.data[SREG] = 0x0; // SREG: --------
-    cpu.cycles = 1;
-    timer.tick();
+    cpu.cycles = 2;
     cpu.tick();
     expect(cpu.data[TIFR0]).toEqual(TOV0);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
   });
 
   it('should not generate an overflow interrupt when TOIE0 is clear', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0xff);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
     cpu.data[TIMSK0] = 0;
     cpu.data[SREG] = 0x80; // SREG: I-------
-    cpu.cycles = 1;
-    timer.tick();
+    cpu.cycles = 2;
     cpu.tick();
     expect(cpu.data[TIFR0]).toEqual(TOV0);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
   });
 
   it('should set OCF0A flag when timer equals OCRA', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x10);
     cpu.writeData(OCR0A, 0x11);
     cpu.writeData(TCCR0A, 0x0); // WGM: Normal
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     expect(cpu.data[TIFR0]).toEqual(OCF0A);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
   });
 
   it('should clear the timer in CTC mode if it equals to OCRA', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x10);
     cpu.writeData(OCR0A, 0x11);
     cpu.writeData(TCCR0A, WGM01); // WGM: CTC
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
   });
 
   it('should set OCF0B flag when timer equals OCRB', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x10);
     cpu.writeData(OCR0B, 0x11);
     cpu.writeData(TCCR0A, 0x0); // WGM: (Normal)
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     expect(cpu.data[TIFR0]).toEqual(OCF0B);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
   });
 
   it('should generate Timer Compare A interrupt when TCNT0 == TCNTA', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x20);
     cpu.writeData(OCR0A, 0x21);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
     cpu.writeData(TIMSK0, OCIE0A);
     cpu.writeData(95, 0x80); // SREG: I-------
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0x23); // TCNT should be 0x23 (one tick above + 2 cycles for interrupt)
     expect(cpu.data[TIFR0] & OCF0A).toEqual(0);
     expect(cpu.pc).toEqual(0x1c);
-    expect(cpu.cycles).toEqual(3);
+    expect(cpu.cycles).toEqual(4);
   });
 
   it('should not generate Timer Compare A interrupt when OCIEA is disabled', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x20);
     cpu.writeData(OCR0A, 0x21);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
     cpu.writeData(TIMSK0, 0);
     cpu.writeData(95, 0x80); // SREG: I-------
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0x21);
     expect(cpu.pc).toEqual(0);
-    expect(cpu.cycles).toEqual(1);
+    expect(cpu.cycles).toEqual(2);
   });
 
   it('should generate Timer Compare B interrupt when TCNT0 == TCNTB', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer0Config);
+    new AVRTimer(cpu, timer0Config);
     cpu.writeData(TCNT0, 0x20);
     cpu.writeData(OCR0B, 0x21);
     cpu.writeData(TCCR0B, CS00); // Set prescaler to 1
     cpu.writeData(TIMSK0, OCIE0B);
     cpu.writeData(95, 0x80); // SREG: I-------
-    timer.tick();
-    cpu.tick();
     cpu.cycles = 1;
-    timer.tick();
+    cpu.tick();
+    cpu.cycles = 2;
     cpu.tick();
     const tcnt = cpu.readData(TCNT0);
     expect(tcnt).toEqual(0x23); // TCNT should be 0x23 (0x23 + 2 cycles for interrupt)
     expect(cpu.data[TIFR0] & OCF0B).toEqual(0);
     expect(cpu.pc).toEqual(0x1e);
-    expect(cpu.cycles).toEqual(3);
+    expect(cpu.cycles).toEqual(4);
   });
 
   it('should not increment TCNT on the same cycle of TCNT write (issue #36)', () => {
@@ -353,26 +340,24 @@ describe('timer', () => {
       IN r17, 0x26    ; r17 <- TCNT
     `);
     const cpu = new CPU(program);
-    const timer = new AVRTimer(cpu, timer0Config);
-    const runner = new TestProgramRunner(cpu, timer);
+    new AVRTimer(cpu, timer0Config);
+    const runner = new TestProgramRunner(cpu);
     runner.runInstructions(instructionCount);
     expect(cpu.data[R17]).toEqual(0x31);
   });
 
   it('timer2 should count every 256 ticks when prescaler is 6 (issue #5)', () => {
     const cpu = new CPU(new Uint16Array(0x1000));
-    const timer = new AVRTimer(cpu, timer2Config);
+    new AVRTimer(cpu, timer2Config);
     cpu.writeData(TCCR2B, CS22 | CS21); // Set prescaler to 256
-    timer.tick();
+    cpu.cycles = 1;
     cpu.tick();
 
     cpu.cycles = 511;
-    timer.tick();
     cpu.tick();
     expect(cpu.readData(TCNT2)).toEqual(1);
 
     cpu.cycles = 512;
-    timer.tick();
     cpu.tick();
     expect(cpu.readData(TCNT2)).toEqual(2);
   });
@@ -387,8 +372,8 @@ describe('timer', () => {
       LDS r1, 0x46      ; r1 <- TCNT0 (2 cycles)
     `);
     const cpu = new CPU(program);
-    const timer = new AVRTimer(cpu, timer0Config);
-    const runner = new TestProgramRunner(cpu, timer);
+    new AVRTimer(cpu, timer0Config);
+    const runner = new TestProgramRunner(cpu);
     runner.runInstructions(instructionCount);
     expect(cpu.data[R1]).toEqual(2);
   });
@@ -405,8 +390,8 @@ describe('timer', () => {
       LDS r17, 0xb2   ; TCNT should equal 2 at this point
     `);
     const cpu = new CPU(program);
-    const timer = new AVRTimer(cpu, timer2Config);
-    const runner = new TestProgramRunner(cpu, timer);
+    new AVRTimer(cpu, timer2Config);
+    const runner = new TestProgramRunner(cpu);
     runner.runInstructions(instructionCount);
     expect(cpu.readData(R17)).toEqual(2);
   });
@@ -432,8 +417,8 @@ describe('timer', () => {
         IN r22, 0x26   ; TCNT0 will be 1 (end of test)
       `);
       const cpu = new CPU(program);
-      const timer = new AVRTimer(cpu, timer0Config);
-      const runner = new TestProgramRunner(cpu, timer);
+      new AVRTimer(cpu, timer0Config);
+      const runner = new TestProgramRunner(cpu);
       runner.runInstructions(instructionCount);
 
       expect(cpu.readData(R17)).toEqual(2);
@@ -463,14 +448,14 @@ describe('timer', () => {
       `);
 
       const cpu = new CPU(program);
-      const timer = new AVRTimer(cpu, timer0Config);
+      new AVRTimer(cpu, timer0Config);
 
       // Listen to Port D's internal callback
       const gpioCallback = jest.fn();
       cpu.gpioTimerHooks[PORTD] = gpioCallback;
 
       const nopCount = lines.filter((line) => line.bytes == nopOpCode).length;
-      const runner = new TestProgramRunner(cpu, timer);
+      const runner = new TestProgramRunner(cpu);
       runner.runInstructions(instructionCount - nopCount);
 
       expect(cpu.readData(TCNT0)).toEqual(0xfd);
@@ -495,7 +480,7 @@ describe('timer', () => {
   describe('16 bit timers', () => {
     it('should increment 16-bit TCNT by 1', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const timer = new AVRTimer(cpu, timer1Config);
+      new AVRTimer(cpu, timer1Config);
       cpu.writeData(TCNT1H, 0x22); // TCNT1 <- 0x2233
       cpu.writeData(TCNT1, 0x33); // ...
       const timerLow = cpu.readData(TCNT1);
@@ -503,10 +488,9 @@ describe('timer', () => {
       expect((timerHigh << 8) | timerLow).toEqual(0x2233);
       cpu.writeData(TCCR1A, 0x0); // WGM: Normal
       cpu.writeData(TCCR1B, CS10); // Set prescaler to 1
-      timer.tick();
-      cpu.tick();
       cpu.cycles = 1;
-      timer.tick();
+      cpu.tick();
+      cpu.cycles = 2;
       cpu.tick();
       cpu.readData(TCNT1);
       expect(cpu.dataView.getUint16(TCNT1, true)).toEqual(0x2234); // TCNT1 should increment
@@ -514,69 +498,68 @@ describe('timer', () => {
 
     it('should set OCF0A flag when timer equals OCRA (16 bit mode)', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const timer = new AVRTimer(cpu, timer1Config);
+      new AVRTimer(cpu, timer1Config);
       cpu.writeData(TCNT1H, 0x10); // TCNT1 <- 0x10ee
       cpu.writeData(TCNT1, 0xee); // ...
       cpu.writeData(OCR1AH, 0x10); // OCR1 <- 0x10ef
       cpu.writeData(OCR1A, 0xef); // ...
       cpu.writeData(TCCR1A, 0x0); // WGM: Normal
       cpu.writeData(TCCR1B, CS10); // Set prescaler to 1
-      timer.tick();
-      cpu.tick();
       cpu.cycles = 1;
-      timer.tick();
+      cpu.tick();
+      cpu.cycles = 2;
       cpu.tick();
       expect(cpu.data[TIFR1]).toEqual(OCF1A); // TIFR1 should have OCF1A bit on
       expect(cpu.pc).toEqual(0);
-      expect(cpu.cycles).toEqual(1);
+      expect(cpu.cycles).toEqual(2);
     });
 
     it('should generate an overflow interrupt if timer overflows and interrupts enabled', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const timer = new AVRTimer(cpu, timer1Config);
-      cpu.writeData(TCNT1H, 0x3); // TCNT1 <- 0x3ff
-      cpu.writeData(TCNT1, 0xff); // ...
+      new AVRTimer(cpu, timer1Config);
       cpu.writeData(TCCR1A, 0x3); // TCCR1A <- WGM10 | WGM11 (Fast PWM, 10-bit)
       cpu.writeData(TCCR1B, 0x9); // TCCR1B <- WGM12 | CS10
-      cpu.data[0x6f] = 0x1; // TIMSK1: TOIE1
+      cpu.writeData(TIMSK1, 0x1); // TIMSK1: TOIE1
       cpu.data[SREG] = 0x80; // SREG: I-------
-      timer.tick();
-      cpu.tick();
+      cpu.writeData(TCNT1H, 0x3); // TCNT1 <- 0x3ff
       cpu.cycles = 1;
-      timer.tick();
       cpu.tick();
+      cpu.writeData(TCNT1, 0xff); // ...
+      cpu.cycles++; // This cycle shouldn't be counted
+      cpu.tick();
+      cpu.cycles++;
+      cpu.tick(); // This is where we cause the overflow
       cpu.readData(TCNT1); // Refresh TCNT1
       expect(cpu.dataView.getUint16(TCNT1, true)).toEqual(2);
       expect(cpu.data[TIFR1] & TOV1).toEqual(0);
       expect(cpu.pc).toEqual(0x1a);
-      expect(cpu.cycles).toEqual(3);
+      expect(cpu.cycles).toEqual(5);
     });
 
     it('should reset the timer once it reaches ICR value in mode 12', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const timer = new AVRTimer(cpu, timer1Config);
+      new AVRTimer(cpu, timer1Config);
       cpu.writeData(TCNT1H, 0x50); // TCNT1 <- 0x500f
       cpu.writeData(TCNT1, 0x0f); // ...
       cpu.writeData(ICR1H, 0x50); // ICR1 <- 0x5010
       cpu.writeData(ICR1, 0x10); // ...
       cpu.writeData(TCCR1B, WGM13 | WGM12 | CS10); // Set prescaler to 1, WGM: CTC
-      timer.tick();
+      cpu.cycles = 1;
       cpu.tick();
-      cpu.cycles = 2; // 2 cycles should increment timer twice, beyond ICR1
-      timer.tick();
+      cpu.cycles = 3; // 2 cycles should increment timer twice, beyond ICR1
       cpu.tick();
       cpu.readData(TCNT1); // Refresh TCNT1
       expect(cpu.dataView.getUint16(TCNT1, true)).toEqual(0); // TCNT should be 0
       expect(cpu.data[TIFR1] & TOV1).toEqual(0);
-      expect(cpu.cycles).toEqual(2);
+      expect(cpu.cycles).toEqual(3);
     });
 
     it('should not update the high byte of TCNT if written after the low byte (issue #37)', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const timer = new AVRTimer(cpu, timer1Config);
+      new AVRTimer(cpu, timer1Config);
       cpu.writeData(TCNT1, 0x22);
       cpu.writeData(TCNT1H, 0x55);
-      timer.tick();
+      cpu.cycles = 1;
       cpu.tick();
       const timerLow = cpu.readData(TCNT1);
       const timerHigh = cpu.readData(TCNT1H);
@@ -585,14 +568,13 @@ describe('timer', () => {
 
     it('reading from TCNT1H before TCNT1L should return old value (issue #37)', () => {
       const cpu = new CPU(new Uint16Array(0x1000));
-      const timer = new AVRTimer(cpu, timer1Config);
+      new AVRTimer(cpu, timer1Config);
       cpu.writeData(TCNT1H, 0xff);
       cpu.writeData(TCNT1, 0xff);
       cpu.writeData(TCCR1B, WGM12 | CS10); // Set prescaler to 1, WGM: CTC
-      timer.tick();
-      cpu.tick();
       cpu.cycles = 1;
-      timer.tick();
+      cpu.tick();
+      cpu.cycles = 2;
       cpu.tick();
       // We read the high byte before the low byte, so the high byte should still have
       // the previous value:
@@ -622,14 +604,14 @@ describe('timer', () => {
       `);
 
       const cpu = new CPU(program);
-      const timer = new AVRTimer(cpu, timer1Config);
+      new AVRTimer(cpu, timer1Config);
 
       // Listen to Port B's internal callback
       const gpioCallback = jest.fn();
       cpu.gpioTimerHooks[PORTB] = gpioCallback;
 
       const nopCount = lines.filter((line) => line.bytes == nopOpCode).length;
-      const runner = new TestProgramRunner(cpu, timer);
+      const runner = new TestProgramRunner(cpu);
       runner.runInstructions(instructionCount - nopCount);
 
       expect(cpu.readData(TCNT1)).toEqual(0x49);
diff --git a/src/peripherals/timer.ts b/src/peripherals/timer.ts
index 81c1e47..b8a6668 100644
--- a/src/peripherals/timer.ts
+++ b/src/peripherals/timer.ts
@@ -245,6 +245,7 @@ export class AVRTimer {
   private timerMode: TimerMode;
   private topValue: TimerTopValue;
   private tcnt: u16 = 0;
+  private tcntNext: u16 = 0;
   private compA: CompBitsValue;
   private compB: CompBitsValue;
   private tcntUpdated = false;
@@ -280,7 +281,7 @@ export class AVRTimer {
   constructor(private cpu: CPU, private config: AVRTimerConfig) {
     this.updateWGMConfig();
     this.cpu.readHooks[config.TCNT] = (addr: u8) => {
-      this.tick();
+      this.count(false);
       if (this.config.bits === 16) {
         this.cpu.data[addr + 1] = this.tcnt >> 8;
       }
@@ -288,9 +289,10 @@ export class AVRTimer {
     };
 
     this.cpu.writeHooks[config.TCNT] = (value: u8) => {
-      this.tcnt = (this.highByteTemp << 8) | value;
+      this.tcntNext = (this.highByteTemp << 8) | value;
       this.countingUp = true;
       this.tcntUpdated = true;
+      this.cpu.updateClockEvent(this.count, 0);
       this.timerUpdated();
     };
     this.cpu.writeHooks[config.OCRA] = (value: u8) => {
@@ -324,8 +326,10 @@ export class AVRTimer {
     };
     cpu.writeHooks[config.TCCRB] = (value) => {
       this.cpu.data[config.TCCRB] = value;
-      this.tcntUpdated = true;
       this.divider = this.config.dividers[this.CS];
+      this.reschedule();
+      this.tcntUpdated = true;
+      this.cpu.updateClockEvent(this.count, 0);
       this.updateWGMConfig();
       return true;
     };
@@ -348,6 +352,11 @@ export class AVRTimer {
     this.lastCycle = 0;
     this.ocrA = 0;
     this.ocrB = 0;
+    this.icr = 0;
+    this.tcnt = 0;
+    this.tcntNext = 0;
+    this.tcntUpdated = false;
+    this.countingUp = false;
   }
 
   get TCCRA() {
@@ -389,7 +398,7 @@ export class AVRTimer {
     this.topValue = topValue;
   }
 
-  tick() {
+  count = (reschedule = true) => {
     const { divider, lastCycle } = this;
     const delta = this.cpu.cycles - lastCycle;
     if (divider && delta >= divider) {
@@ -411,7 +420,18 @@ export class AVRTimer {
         this.cpu.setInterruptFlag(this.OVF);
       }
     }
-    this.tcntUpdated = false;
+    if (this.tcntUpdated) {
+      this.tcnt = this.tcntNext;
+      this.tcntUpdated = false;
+    }
+    if (reschedule) {
+      this.cpu.addClockEvent(this.count, this.lastCycle + divider - this.cpu.cycles);
+    }
+  };
+
+  private reschedule() {
+    this.cpu.clearClockEvent(this.count);
+    this.cpu.addClockEvent(this.count, this.lastCycle + this.divider - this.cpu.cycles);
   }
 
   private phasePwmCount(value: u16, delta: u8) {
diff --git a/src/peripherals/twi.spec.ts b/src/peripherals/twi.spec.ts
index e43ae38..2628b57 100644
--- a/src/peripherals/twi.spec.ts
+++ b/src/peripherals/twi.spec.ts
@@ -51,7 +51,6 @@ describe('TWI', () => {
     cpu.writeData(TWCR, TWIE);
     cpu.data[SREG] = 0x80; // SREG: I-------
     twi.completeStart(); // This will set the TWINT flag
-    twi.tick();
     cpu.tick();
     expect(cpu.pc).toEqual(0x30); // 2-wire Serial Interface Vector
     expect(cpu.cycles).toEqual(2);
@@ -64,7 +63,7 @@ describe('TWI', () => {
       const twi = new AVRTWI(cpu, twiConfig, FREQ_16MHZ);
       jest.spyOn(twi.eventHandler, 'start');
       cpu.writeData(TWCR, TWINT | TWSTA | TWEN);
-      twi.tick();
+      cpu.cycles++;
       cpu.tick();
       expect(twi.eventHandler.start).toHaveBeenCalledWith(false);
     });
@@ -173,7 +172,7 @@ describe('TWI', () => {
       `);
       const cpu = new CPU(program);
       const twi = new AVRTWI(cpu, twiConfig, FREQ_16MHZ);
-      const runner = new TestProgramRunner(cpu, twi, onTestBreak);
+      const runner = new TestProgramRunner(cpu, onTestBreak);
       twi.eventHandler = {
         start: jest.fn(),
         stop: jest.fn(),
@@ -342,7 +341,7 @@ describe('TWI', () => {
       `);
       const cpu = new CPU(program);
       const twi = new AVRTWI(cpu, twiConfig, FREQ_16MHZ);
-      const runner = new TestProgramRunner(cpu, twi, onTestBreak);
+      const runner = new TestProgramRunner(cpu, onTestBreak);
       twi.eventHandler = {
         start: jest.fn(),
         stop: jest.fn(),
diff --git a/src/peripherals/twi.ts b/src/peripherals/twi.ts
index 914b67f..000bd2a 100644
--- a/src/peripherals/twi.ts
+++ b/src/peripherals/twi.ts
@@ -95,8 +95,6 @@ export class NoopTWIEventHandler implements TWIEventHandler {
 export class AVRTWI {
   public eventHandler: TWIEventHandler = new NoopTWIEventHandler(this);
 
-  private nextTick: (() => void) | null = null;
-
   // Interrupts
   private TWI: AVRInterruptConfig = {
     address: this.config.twiInterrupt,
@@ -116,7 +114,7 @@ export class AVRTWI {
       const { status } = this;
       if (clearInt && value & TWCR_TWEN) {
         const twdrValue = this.cpu.data[this.config.TWDR];
-        this.nextTick = () => {
+        this.cpu.addClockEvent(() => {
           if (value & TWCR_TWSTA) {
             this.eventHandler.start(status !== STATUS_TWI_IDLE);
           } else if (value & TWCR_TWSTO) {
@@ -129,19 +127,12 @@ export class AVRTWI {
             const ack = !!(value & TWCR_TWEA);
             this.eventHandler.readByte(ack);
           }
-        };
+        }, 0);
         return true;
       }
     };
   }
 
-  tick() {
-    if (this.nextTick) {
-      this.nextTick();
-      this.nextTick = null;
-    }
-  }
-
   get prescaler() {
     switch (this.cpu.data[this.config.TWSR] & TWSR_TWPS_MASK) {
       case 0:
diff --git a/src/peripherals/usart.spec.ts b/src/peripherals/usart.spec.ts
index c1e1570..fb56967 100644
--- a/src/peripherals/usart.spec.ts
+++ b/src/peripherals/usart.spec.ts
@@ -146,10 +146,9 @@ describe('USART', () => {
   describe('tick()', () => {
     it('should trigger data register empty interrupt if UDRE is set', () => {
       const cpu = new CPU(new Uint16Array(1024));
-      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
       cpu.writeData(UCSR0B, UDRIE | TXEN);
       cpu.data[SREG] = 0x80; // SREG: I-------
-      usart.tick();
       cpu.tick();
       expect(cpu.pc).toEqual(PC_INT_UDRE);
       expect(cpu.cycles).toEqual(2);
@@ -158,12 +157,11 @@ describe('USART', () => {
 
     it('should trigger data TX Complete interrupt if TXCIE is set', () => {
       const cpu = new CPU(new Uint16Array(1024));
-      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
       cpu.writeData(UCSR0B, TXCIE | TXEN);
       cpu.writeData(UDR0, 0x61);
       cpu.data[SREG] = 0x80; // SREG: I-------
       cpu.cycles = 1e6;
-      usart.tick();
       cpu.tick();
       expect(cpu.pc).toEqual(PC_INT_TXC);
       expect(cpu.cycles).toEqual(1e6 + 2);
@@ -172,10 +170,9 @@ describe('USART', () => {
 
     it('should not trigger data TX Complete interrupt if UDR was not written to', () => {
       const cpu = new CPU(new Uint16Array(1024));
-      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
       cpu.writeData(UCSR0B, TXCIE | TXEN);
       cpu.data[SREG] = 0x80; // SREG: I-------
-      usart.tick();
       cpu.tick();
       expect(cpu.pc).toEqual(0);
       expect(cpu.cycles).toEqual(0);
@@ -183,12 +180,11 @@ describe('USART', () => {
 
     it('should not trigger any interrupt if interrupts are disabled', () => {
       const cpu = new CPU(new Uint16Array(1024));
-      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
       cpu.writeData(UCSR0B, UDRIE | TXEN);
       cpu.writeData(UDR0, 0x61);
       cpu.data[SREG] = 0; // SREG: 0 (disable interrupts)
       cpu.cycles = 1e6;
-      usart.tick();
       cpu.tick();
       expect(cpu.pc).toEqual(0);
       expect(cpu.cycles).toEqual(1e6);
@@ -242,16 +238,14 @@ describe('USART', () => {
   describe('integration', () => {
     it('should set the TXC bit after ~1.04mS when baud rate set to 9600', () => {
       const cpu = new CPU(new Uint16Array(1024));
-      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
       cpu.writeData(UCSR0B, TXEN);
       cpu.writeData(UBRR0L, 103); // baud: 9600
       cpu.writeData(UDR0, 0x48); // 'H'
       cpu.cycles += 16000; // 1ms
-      usart.tick();
       cpu.tick();
       expect(cpu.data[UCSR0A] & TXC).toEqual(0);
       cpu.cycles += 800; // 0.05ms
-      usart.tick();
       cpu.tick();
       expect(cpu.data[UCSR0A] & TXC).toEqual(TXC);
     });
diff --git a/src/peripherals/usart.ts b/src/peripherals/usart.ts
index c1441ad..c7dcbd4 100644
--- a/src/peripherals/usart.ts
+++ b/src/peripherals/usart.ts
@@ -87,8 +87,6 @@ export class AVRUSART {
     enableMask: UCSRB_TXCIE,
   };
 
-  private txCompleteCycles = 0;
-
   constructor(private cpu: CPU, private config: USARTConfig, private freqMHz: number) {
     this.reset();
     this.cpu.writeHooks[config.UCSRA] = (value) => {
@@ -119,7 +117,11 @@ export class AVRUSART {
         }
       }
       const symbolsPerChar = 1 + this.bitsPerChar + this.stopBits + (this.parityEnabled ? 1 : 0);
-      this.txCompleteCycles = this.cpu.cycles + (this.UBRR * this.multiplier + 1) * symbolsPerChar;
+      const cyclesToComplete = (this.UBRR * this.multiplier + 1) * symbolsPerChar;
+      this.cpu.addClockEvent(() => {
+        cpu.setInterruptFlag(this.UDRE);
+        cpu.setInterruptFlag(this.TXC);
+      }, cyclesToComplete);
       this.cpu.clearInterrupt(this.TXC);
       this.cpu.clearInterrupt(this.UDRE);
     };
@@ -131,15 +133,6 @@ export class AVRUSART {
     this.cpu.data[this.config.UCSRC] = UCSRC_UCSZ1 | UCSRC_UCSZ0; // default: 8 bits per byte
   }
 
-  tick() {
-    const { txCompleteCycles, cpu } = this;
-    if (txCompleteCycles && cpu.cycles >= txCompleteCycles) {
-      cpu.setInterruptFlag(this.UDRE);
-      cpu.setInterruptFlag(this.TXC);
-      this.txCompleteCycles = 0;
-    }
-  }
-
   private get UBRR() {
     return (this.cpu.data[this.config.UBRRH] << 8) | this.cpu.data[this.config.UBRRL];
   }
diff --git a/src/utils/test-utils.ts b/src/utils/test-utils.ts
index 2bec178..4b2efff 100644
--- a/src/utils/test-utils.ts
+++ b/src/utils/test-utils.ts
@@ -13,33 +13,29 @@ export function asmProgram(source: string) {
 }
 
 export class TestProgramRunner {
-  constructor(
-    private readonly cpu: CPU,
-    private readonly peripheral: { tick: () => void },
-    private readonly onBreak?: (cpu: CPU) => void
-  ) {}
+  constructor(private readonly cpu: CPU, private readonly onBreak?: (cpu: CPU) => void) {}
 
   runInstructions(count: number) {
-    const { cpu, peripheral, onBreak } = this;
+    const { cpu, onBreak } = this;
     for (let i = 0; i < count; i++) {
       if (cpu.progMem[cpu.pc] === BREAK_OPCODE) {
         onBreak?.(cpu);
         throw new Error('BREAK instruction encountered');
       }
       avrInstruction(cpu);
-      peripheral.tick();
+      cpu.tick();
     }
   }
 
   runToBreak(maxIterations = 5000) {
-    const { cpu, peripheral, onBreak } = this;
+    const { cpu, onBreak } = this;
     for (let i = 0; i < maxIterations; i++) {
       if (cpu.progMem[cpu.pc] === BREAK_OPCODE) {
         onBreak?.(cpu);
         return;
       }
       avrInstruction(cpu);
-      peripheral.tick();
+      cpu.tick();
     }
     throw new Error('Program ran for too long without a BREAK instruction');
   }