diff --git a/README.md b/README.md
index 6018633..6a258c9 100644
--- a/README.md
+++ b/README.md
@@ -14,4 +14,5 @@ This repository specifies the control protocol implemented by [gex-core](https:/
 - [UNIT_DI.md](UNIT_DI.md) - Digital Input
 - [UNIT_NEOPIXEL.md](UNIT_NEOPIXEL.md) - NeoPixel driver
 - [UNIT_SIPO.md](UNIT_SIPO.md) - Multi-cast Shift Register Driver
+- [UNIT_I2C.md](UNIT_I2C.md) - I2C master
 
diff --git a/UNIT_I2C.md b/UNIT_I2C.md
new file mode 100644
index 0000000..4e53de0
--- /dev/null
+++ b/UNIT_I2C.md
@@ -0,0 +1,46 @@
+# I2C master
+
+## Commands
+
+### WRITE (0x00)
+
+*Request:*
+- u16 - slave address
+- u8[] - bytes to write
+
+### READ (0x01)
+
+*Request:*
+- u16 - slave address
+- u16 - number of bytes to read
+
+*Response:*
+- u8[] - received bytes
+
+### READ_REG (0x02)
+
+Read a register value. First writes the register number, then reads a number of bytes.
+For devices implementing auto-increment, the register width field can be used to read
+multiple registers at once.
+
+*Request:*
+- u16 - address
+- u8 - register number
+- u16 - register width (number of bytes to read)
+
+*Response:*
+- u8[] - received bytes
+
+### WRITE_REG (0x03)
+
+Write a register; first writes the register number, then (in the same transaction)
+the data. If the device supports it, can write multiple registers at once.
+
+*Request:*
+- u16 - address
+- u8 - register number
+- u8[] - bytes to write
+
+## Events
+
+*No events defined for this unit type.*