diff --git a/README.md b/README.md
index 6e5fed2..8867af4 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,91 @@
 # step32
 
 ESP32 development board with stepper motor driver
+
+# Pin assignment
+
+## A4988 Stepper Driver
+
+| GPIO | Signal  |
+| ---- | ------  |
+| 12   | ~RESET  |
+| 13   | ~ENABLE |
+| 14   | ~SLEEP  |
+| 16   | MS1     |
+| 17   | MS2     |
+| 18   | MS3     |
+| 19   | DIR     |
+| 23   | STEP    |
+| 25   | DAC     |
+
+### Microstepping
+
+| Resolution     | MS1 | MS2 | MS3 |
+| ----------     | --- | --- | --- |
+| Full Step      | 0   | 0   | 0   |
+| Half Step      | 1   | 0   | 0   |
+| Quarter Step   | 0   | 1   | 0   |
+| Eigth Step     | 1   | 1   | 0   |
+| Sixteenth Step | 1   | 1   | 1   |
+
+### Current limiting
+
+The DAC pin is used to set the current limit.
+
+The current limit is 15/23 of the DAC voltage.
+
+| Current limit | DAC voltage |
+| ------------- | ----------- |
+| 1A            | 1.533V      |
+| 1.5A          | 2.300V      |
+| 2A            | 3.066V      |
+
+The DAC output is not optional. It must be set correctly for the stepper driver to function.
+
+
+## I2C Expansion
+
+| GPIO | Signal |
+| ---- | ------ |
+| 21   | SDA    |
+| 22   | SCL    |
+
+The I2C bus is terminated on this board. If not desired, remove R5 and R6, next to the I2C connectors.
+
+## End stops
+
+| GPIO | Signal |
+| ---- | ------ |
+| 34   | END1   |
+| 35   | END2   |
+
+Note that these pins can be configured either as analogue or as digital inputs, depending on the type of end stops required.
+
+## LED
+
+| GPIO | Signal |
+| ---- | ------ |
+| 2    | LED    |
+
+# Motor connection
+
+There are two connectors for the stepper motor. Only one should be used at a time. They correspond to the two most common ways of connecting bipolar stepper motor coils to a 4-pin header. The coil assignment is marked on the silkscreen by each connector for convenience.
+
+# Heat sink
+
+The A4988 can operate at low currents without a heat sink. At higher currents (greater than 1 amp) a heat sink may be required.
+
+# Dimensions and mounting
+
+The PCB is 55mm wide and 64mm high, excluding the antenna. The antenna sticks out by around 6mm.
+
+The mounting holes are 47mm apart horizontally and 53mm apart vertically. They are inset 4mm from the sides and from the bottom edge. They are sized to accept M3 screws.
+
+# Power supply
+
+This board can be powered either from an external 12 volt supply (2.5mm barrel jack), in which case a buck converter generates 3.3 volts for the ESP32, or directly from USB, in which case a linear regulator generates the 3.3 volt supply.
+
+The motor can not be powered from USB. Only the low voltage circuit will be energised when powered by USB.
+
+While the motor power is labelled as 12 volts, it should operate anywhere between 7 and 20 volts. Do not exceed 20 volts.
+