3 files changed, 130 insertions, 2 deletions
diff --git a/src/index.ts b/src/index.ts
index c1089aa..1429db5 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -60,3 +60,4 @@ export * from './peripherals/twi';
 export { spiConfig, SPIConfig, SPITransferCallback, AVRSPI } from './peripherals/spi';
 export { AVRClock, AVRClockConfig, clockConfig } from './peripherals/clock';
 export { AVRWatchdog, watchdogConfig, WatchdogConfig } from './peripherals/watchdog';
+export { AVRUSI } from './peripherals/usi';
diff --git a/src/peripherals/gpio.ts b/src/peripherals/gpio.ts
index 07667d9..9d84307 100644
--- a/src/peripherals/gpio.ts
+++ b/src/peripherals/gpio.ts
@@ -210,6 +210,7 @@ export class AVRIOPort {
   private lastValue: u8 = 0;
   private lastDdr: u8 = 0;
   private lastPin: u8 = 0;
+  openCollector: u8 = 0;
 
   constructor(private cpu: CPU, readonly portConfig: Readonly<AVRPortConfig>) {
     cpu.gpioPorts.add(this);
@@ -317,10 +318,12 @@ export class AVRIOPort {
     const ddr = this.cpu.data[this.portConfig.DDR];
     const port = this.cpu.data[this.portConfig.PORT];
     const bitMask = 1 << index;
+    const openState = port & bitMask ? PinState.InputPullUp : PinState.Input;
+    const highValue = this.openCollector & bitMask ? openState : PinState.High;
     if (ddr & bitMask) {
-      return this.lastValue & bitMask ? PinState.High : PinState.Low;
+      return this.lastValue & bitMask ? highValue : PinState.Low;
     } else {
-      return port & bitMask ? PinState.InputPullUp : PinState.Input;
+      return openState;
     }
   }
 
diff --git a/src/peripherals/usi.ts b/src/peripherals/usi.ts
new file mode 100644
index 0000000..c9e09e7
--- /dev/null
+++ b/src/peripherals/usi.ts
@@ -0,0 +1,124 @@
+import { AVRInterruptConfig, CPU } from '../cpu/cpu';
+import { AVRIOPort } from './gpio';
+
+const USICR = 0x2d;
+const USISR = 0x2e;
+const USIDR = 0x2f;
+const USIBR = 0x30;
+
+// USISR bits
+const USICNT_MASK = 0xf;
+const USIDC = 1 << 4;
+const USIPF = 1 << 5;
+const USIOIF = 1 << 6;
+const USISIF = 1 << 7;
+
+// USICR bits
+const USITC = 1 << 0;
+const USICLK = 1 << 1;
+const USICS0 = 1 << 2;
+const USICS1 = 1 << 3;
+const USIWM0 = 1 << 4;
+const USIWM1 = 1 << 5;
+const USIOIE = 1 << 6;
+const USISIE = 1 << 7;
+
+export class AVRUSI {
+  // Interrupts
+  private START: AVRInterruptConfig = {
+    address: 0xd,
+    flagRegister: USISR,
+    flagMask: USISIF,
+    enableRegister: USICR,
+    enableMask: USISIE,
+  };
+
+  private OVF: AVRInterruptConfig = {
+    address: 0xe,
+    flagRegister: USISR,
+    flagMask: USIOIF,
+    enableRegister: USICR,
+    enableMask: USIOIE,
+  };
+
+  constructor(cpu: CPU, port: AVRIOPort, portPin: number, dataPin: number, clockPin: number) {
+    const PIN = portPin;
+    const PORT = PIN + 2;
+    port.addListener((value) => {
+      const twoWire = (cpu.data[USICR] & USIWM1) === USIWM1;
+      if (twoWire) {
+        if (value & (1 << clockPin) && !(value & (1 << dataPin))) {
+          // Start condition detected
+          cpu.setInterruptFlag(this.START);
+        }
+        if (value & (1 << clockPin) && value & (1 << dataPin)) {
+          // Stop condition detected
+          cpu.data[USISR] |= USIPF;
+        }
+      }
+    });
+    const updateOutput = () => {
+      const oldValue = cpu.data[PORT];
+      const newValue =
+        cpu.data[USIDR] & 0x80 ? oldValue | (1 << dataPin) : oldValue & ~(1 << dataPin);
+      cpu.writeHooks[PORT](newValue, oldValue, PORT, 0xff);
+      if (newValue & 0x80 && !(cpu.data[PIN] & 0x80)) {
+        cpu.data[USISR] |= USIDC; // Shout output HIGH (pulled-up), but input is LOW
+      } else {
+        cpu.data[USISR] &= ~USIDC;
+      }
+    };
+    const count = () => {
+      const counter = (cpu.data[USISR] + 1) & USICNT_MASK;
+      cpu.data[USISR] = (cpu.data[USISR] & ~USICNT_MASK) | counter;
+      if (!counter) {
+        cpu.data[USIBR] = cpu.data[USIDR];
+        cpu.setInterruptFlag(this.OVF);
+      }
+    };
+    const shift = (inputValue: number) => {
+      cpu.data[USIDR] = (cpu.data[USIDR] << 1) | inputValue;
+      updateOutput();
+    };
+    cpu.writeHooks[USIDR] = (value: number) => {
+      cpu.data[USIDR] = value;
+      updateOutput();
+      return true;
+    };
+    cpu.writeHooks[USISR] = (value: number) => {
+      const writeClearMask = USISIF | USIOIF | USIPF;
+      cpu.data[USISR] = (cpu.data[USISR] & writeClearMask & ~value) | (value & 0xf);
+      cpu.clearInterruptByFlag(this.START, value);
+      cpu.clearInterruptByFlag(this.OVF, value);
+      return true;
+    };
+    cpu.writeHooks[USICR] = (value: number) => {
+      cpu.data[USICR] = value & ~(USICLK | USITC);
+      cpu.updateInterruptEnable(this.START, value);
+      cpu.updateInterruptEnable(this.OVF, value);
+      const clockSrc = value & ((USICS1 | USICS0) >> 2);
+      const mode = value & ((USIWM1 | USIWM0) >> 4);
+      const usiClk = value & USICLK;
+      port.openCollector = mode >= 2 ? 1 << dataPin : 0;
+      const inputValue = cpu.data[PIN] & (1 << dataPin) ? 1 : 0;
+      if (usiClk && !clockSrc) {
+        shift(inputValue);
+        count();
+      }
+      if (value & USITC) {
+        cpu.writeHooks[PIN](1 << clockPin, cpu.data[PIN], PIN, 0xff);
+        const newValue = cpu.data[PIN] & (1 << clockPin);
+        if (usiClk && (clockSrc === 2 || clockSrc === 3)) {
+          if (clockSrc === 2 && newValue) {
+            shift(inputValue);
+          }
+          if (clockSrc === 3 && !newValue) {
+            shift(inputValue);
+          }
+          count();
+        }
+        return true;
+      }
+    };
+  }
+}