3 files changed, 97 insertions, 6 deletions
diff --git a/src/peripherals/eeprom.ts b/src/peripherals/eeprom.ts
index d492aa7..8056847 100644
--- a/src/peripherals/eeprom.ts
+++ b/src/peripherals/eeprom.ts
@@ -78,6 +78,7 @@ export class AVREEPROM {
     enableRegister: this.config.EECR,
     enableMask: EERIE,
     constant: true,
+    inverseFlag: true,
   };
 
   constructor(
diff --git a/src/peripherals/usart.spec.ts b/src/peripherals/usart.spec.ts
index fb56967..f92ea00 100644
--- a/src/peripherals/usart.spec.ts
+++ b/src/peripherals/usart.spec.ts
@@ -18,8 +18,10 @@ const UDR0 = 0xc6;
 // Register bit names
 const U2X0 = 2;
 const TXEN = 8;
+const RXEN = 16;
 const UDRIE = 0x20;
 const TXCIE = 0x40;
+const RXC = 0x80;
 const TXC = 0x40;
 const UDRE = 0x20;
 const USBS = 0x08;
@@ -235,7 +237,20 @@ describe('USART', () => {
     });
   });
 
-  describe('integration', () => {
+  describe('writeByte', () => {
+    it('should return false if called when RX is busy', () => {
+      const cpu = new CPU(new Uint16Array(1024));
+      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      cpu.writeData(UCSR0B, RXEN);
+      cpu.writeData(UBRR0L, 103); // baud: 9600
+      expect(usart.writeByte(10)).toEqual(true);
+      expect(usart.writeByte(10)).toEqual(false);
+      cpu.tick();
+      expect(usart.writeByte(10)).toEqual(false);
+    });
+  });
+
+  describe('Integration tests', () => {
     it('should set the TXC bit after ~1.04mS when baud rate set to 9600', () => {
       const cpu = new CPU(new Uint16Array(1024));
       new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
@@ -249,5 +264,27 @@ describe('USART', () => {
       cpu.tick();
       expect(cpu.data[UCSR0A] & TXC).toEqual(TXC);
     });
+
+    it('should be ready to recieve the next byte after ~1.04ms when baudrate set to 9600', () => {
+      const cpu = new CPU(new Uint16Array(1024));
+      const usart = new AVRUSART(cpu, usart0Config, FREQ_16MHZ);
+      const rxCompleteCallback = jest.fn();
+      usart.onRxComplete = rxCompleteCallback;
+      cpu.writeData(UCSR0B, RXEN);
+      cpu.writeData(UBRR0L, 103); // baud: 9600
+      expect(usart.writeByte(0x42)).toBe(true);
+      cpu.cycles += 16000; // 1ms
+      cpu.tick();
+      expect(cpu.data[UCSR0A] & RXC).toEqual(0); // byte not received yet
+      expect(usart.rxBusy).toBe(true);
+      expect(rxCompleteCallback).not.toHaveBeenCalled();
+      cpu.cycles += 800; // 0.05ms
+      cpu.tick();
+      expect(cpu.data[UCSR0A] & RXC).toEqual(RXC);
+      expect(usart.rxBusy).toBe(false);
+      expect(rxCompleteCallback).toHaveBeenCalled();
+      expect(cpu.readData(UDR0)).toEqual(0x42);
+      expect(cpu.readData(UDR0)).toEqual(0);
+    });
   });
 });
diff --git a/src/peripherals/usart.ts b/src/peripherals/usart.ts
index f9b0f87..f55fef3 100644
--- a/src/peripherals/usart.ts
+++ b/src/peripherals/usart.ts
@@ -65,13 +65,31 @@ const UCSRC_UCSZ0 = 0x2; // Character Size 0
 const UCSRC_UCPOL = 0x1; // Clock Polarity
 /* eslint-enable @typescript-eslint/no-unused-vars */
 
+const rxMasks = {
+  5: 0x1f,
+  6: 0x3f,
+  7: 0x7f,
+  8: 0xff,
+  9: 0xff,
+};
 export class AVRUSART {
   public onByteTransmit: USARTTransmitCallback | null = null;
   public onLineTransmit: USARTLineTransmitCallback | null = null;
+  public onRxComplete: (() => void) | null = null;
 
+  private rxBusyValue = false;
+  private rxByte = 0;
   private lineBuffer = '';
 
   // Interrupts
+  private RXC: AVRInterruptConfig = {
+    address: this.config.rxCompleteInterrupt,
+    flagRegister: this.config.UCSRA,
+    flagMask: UCSRA_RXC,
+    enableRegister: this.config.UCSRB,
+    enableMask: UCSRB_RXCIE,
+    constant: true,
+  };
   private UDRE: AVRInterruptConfig = {
     address: this.config.dataRegisterEmptyInterrupt,
     flagRegister: this.config.UCSRA,
@@ -90,19 +108,29 @@ export class AVRUSART {
   constructor(private cpu: CPU, private config: USARTConfig, private freqHz: number) {
     this.reset();
     this.cpu.writeHooks[config.UCSRA] = (value) => {
-      cpu.data[config.UCSRA] = value;
-      cpu.clearInterruptByFlag(this.UDRE, value);
+      cpu.data[config.UCSRA] = value & (UCSRA_MPCM | UCSRA_U2X);
       cpu.clearInterruptByFlag(this.TXC, value);
       return true;
     };
     this.cpu.writeHooks[config.UCSRB] = (value, oldValue) => {
+      cpu.updateInterruptEnable(this.RXC, value);
       cpu.updateInterruptEnable(this.UDRE, value);
       cpu.updateInterruptEnable(this.TXC, value);
+      if (value & UCSRB_RXEN && oldValue & UCSRB_RXEN) {
+        cpu.clearInterrupt(this.RXC);
+      }
       if (value & UCSRB_TXEN && !(oldValue & UCSRB_TXEN)) {
         // Enabling the transmission - mark UDR as empty
         cpu.setInterruptFlag(this.UDRE);
       }
     };
+    this.cpu.readHooks[config.UDR] = () => {
+      const mask = rxMasks[this.bitsPerChar] ?? 0xff;
+      const result = this.rxByte & mask;
+      this.rxByte = 0;
+      this.cpu.clearInterrupt(this.RXC);
+      return result;
+    };
     this.cpu.writeHooks[config.UDR] = (value) => {
       if (this.onByteTransmit) {
         this.onByteTransmit(value);
@@ -116,12 +144,10 @@ export class AVRUSART {
           this.lineBuffer += ch;
         }
       }
-      const symbolsPerChar = 1 + this.bitsPerChar + this.stopBits + (this.parityEnabled ? 1 : 0);
-      const cyclesToComplete = (this.UBRR * this.multiplier + 1) * symbolsPerChar;
       this.cpu.addClockEvent(() => {
         cpu.setInterruptFlag(this.UDRE);
         cpu.setInterruptFlag(this.TXC);
-      }, cyclesToComplete);
+      }, this.cyclesPerChar);
       this.cpu.clearInterrupt(this.TXC);
       this.cpu.clearInterrupt(this.UDRE);
     };
@@ -131,6 +157,33 @@ export class AVRUSART {
     this.cpu.data[this.config.UCSRA] = UCSRA_UDRE;
     this.cpu.data[this.config.UCSRB] = 0;
     this.cpu.data[this.config.UCSRC] = UCSRC_UCSZ1 | UCSRC_UCSZ0; // default: 8 bits per byte
+    this.rxBusyValue = false;
+    this.rxByte = 0;
+    this.lineBuffer = '';
+  }
+
+  get rxBusy() {
+    return this.rxBusyValue;
+  }
+
+  writeByte(value: number) {
+    const { cpu, config } = this;
+    if (this.rxBusyValue || !(cpu.data[config.UCSRB] & UCSRB_RXEN)) {
+      return false;
+    }
+    this.rxBusyValue = true;
+    cpu.addClockEvent(() => {
+      this.rxByte = value;
+      this.rxBusyValue = false;
+      cpu.setInterruptFlag(this.RXC);
+      this.onRxComplete?.();
+    }, this.cyclesPerChar);
+    return true;
+  }
+
+  private get cyclesPerChar() {
+    const symbolsPerChar = 1 + this.bitsPerChar + this.stopBits + (this.parityEnabled ? 1 : 0);
+    return (this.UBRR * this.multiplier + 1) * symbolsPerChar;
   }
 
   private get UBRR() {