2 files changed, 29 insertions, 3 deletions
diff --git a/src/peripherals/timer.spec.ts b/src/peripherals/timer.spec.ts
index 27ef7fb..9330168 100644
--- a/src/peripherals/timer.spec.ts
+++ b/src/peripherals/timer.spec.ts
@@ -1245,6 +1245,17 @@ describe('timer', () => {
       expect(cpu.readData(R19)).toEqual(0x5);
       expect(cpu.readData(R20)).toEqual(0x3);
     });
+
+    it('should mask the unused bits of OCR1A when using fixed top values', () => {
+      const cpu = new CPU(new Uint16Array(0x1000));
+      new AVRTimer(cpu, timer1Config);
+      cpu.writeData(TCCR1A, WGM10 | WGM11); // WGM: FastPWM, top 0x3ff
+      cpu.writeData(TCCR1B, WGM12);
+      cpu.writeData(OCR1AH, 0xff);
+      cpu.writeData(OCR1A, 0xff);
+      expect(cpu.readData(OCR1A)).toEqual(0xff);
+      expect(cpu.readData(OCR1AH)).toEqual(0x03);
+    });
   });
 
   describe('External clock', () => {
diff --git a/src/peripherals/timer.ts b/src/peripherals/timer.ts
index 53777e2..6167394 100644
--- a/src/peripherals/timer.ts
+++ b/src/peripherals/timer.ts
@@ -388,11 +388,16 @@ export class AVRTimer {
       const updateTempRegister = (value: u8) => {
         this.highByteTemp = value;
       };
+      const updateOCRHighRegister = (value: u8, old: u8, addr: u16) => {
+        this.highByteTemp = value & (this.ocrMask >> 8);
+        cpu.data[addr] = this.highByteTemp;
+        return true;
+      };
       this.cpu.writeHooks[config.TCNT + 1] = updateTempRegister;
-      this.cpu.writeHooks[config.OCRA + 1] = updateTempRegister;
-      this.cpu.writeHooks[config.OCRB + 1] = updateTempRegister;
+      this.cpu.writeHooks[config.OCRA + 1] = updateOCRHighRegister;
+      this.cpu.writeHooks[config.OCRB + 1] = updateOCRHighRegister;
       if (this.hasOCRC) {
-        this.cpu.writeHooks[config.OCRC + 1] = updateTempRegister;
+        this.cpu.writeHooks[config.OCRC + 1] = updateOCRHighRegister;
       }
       this.cpu.writeHooks[config.ICR + 1] = updateTempRegister;
     }
@@ -481,6 +486,16 @@ export class AVRTimer {
     }
   }
 
+  get ocrMask() {
+    switch (this.topValue) {
+      case TopOCRA:
+      case TopICR:
+        return 0xffff;
+      default:
+        return this.topValue;
+    }
+  }
+
   private updateWGMConfig() {
     const { config, WGM } = this;
     const wgmModes = config.bits === 16 ? wgmModes16Bit : wgmModes8Bit;