diff --git a/README.md b/README.md
index 6a258c9..6187a0c 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,7 @@ This repository specifies the control protocol implemented by [gex-core](https:/
 - [UNIT_DO.md](UNIT_DO.md) - Digital Output
 - [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_SIPO.md](UNIT_SIPO.md) - Shift Register Driver with multicast support
 - [UNIT_I2C.md](UNIT_I2C.md) - I2C master
+- [UNIT_SPI.md](UNIT_SPI.md) - SPI master up to 16 slaves and multicast support
 
diff --git a/UNIT_SPI.md b/UNIT_SPI.md
new file mode 100644
index 0000000..c76cf57
--- /dev/null
+++ b/UNIT_SPI.md
@@ -0,0 +1,40 @@
+# SPI master
+
+- Can drive shift registers or communicate with any SPI based devices.
+- Configurable speed, polarity and phase
+- Up to 16 slave select signals
+- Supports multicast (write to multiple slaves at once)
+
+*NOTE:* To use Multicast with bi-directionally connected devices, the MISO pins should be
+connected through protection resistors to prevent a short circuit on signal collision.
+
+## Commands
+
+### QUERY (0x00)
+
+Write and read some bytes.
+
+The write and read sections can overlap if needed; some devices use this
+to report a status word while the command is being written. 0x00 is output on MOSI while collecting a response.
+
+If the overlap is not desired (first write, then read), set the number of discarded
+bytes equal to the number of written bytes.
+
+*Request:*
+- u8 - slave number 0-16
+- u16 - number of discarded MISO bytes before collecting the response
+- u16 - response length (bytes)
+- u8[] - bytes to write
+
+*Response:*
+- u8[] - received bytes
+
+### MULTICAST (0x01)
+
+*Request:*
+- u16 - slaves (packed)
+- u8[] - bytes to write
+
+## Events
+
+*No events defined for this unit type.*