Servo 1.0 protocol

Introduction

The protocol for the SERVO boards will be explained on this page.

This page describes both the SPI and the I2C version. See SPI versus I2C protocols for the explanation about how the protocols work in general.

The default address of the servo board is 0x86.

Write ports

The ports on the servo board just set a single byte-value. So writing more than one byte to such a port is redundant.

The spi_servo board defines several ports.

port	function
0x20-0x26	8 bit register. Set servo 0-6 position. The pulse width changes from about 1ms for 0 to about 2ms for 0xff. To "overdrive" your servos you need to use 0x28-0x2e.
0x28-0x2e	(*) 16 bit register. Set servo 0-6 position in microseconds. Don't set values larger than 2500. (if you do, funny things will happen if the total of all servos exceeds 20 miliseconds. Don't set the value too low. I expect you'll get pulses that are 20 miliseconds too long if you do.
0x30-0x36	(*) 8 bit register. Set the default value for this servo. This value is used when the board is powered up. This value is stored in non volatile memory.
0x38-0x3e	(*) 8 bit register. Set the timeout value in tenths of a second. (5 = 0.5 seconds, 35=3.5 seconds). If no update of this servo is received within the timeout time, the servo will revert to the default value.
0xf0	Change address. Requires a write to 0xf1 and 0xf2 first.
0xf1	Write 0x55 here to start unlocking the change address register.
0xf2	Write 0xaa here to unlock the change address register.

(*) From version 1.1 and up.

Read ports

The spi_servo board supports two read ports:

port	function
0x01	identification string. (terminated with 0).
0x20-0x3e	read the corresponding write register.

Examples

For SPI in the examples below, "data sent" means the data on the MOSI line, while "data received" means the data on the MISO line. when MISO reads "xx" you should ignore the data. When MOSI reads "xx" it doesn't matter what you send.

For I2C in the examples below, you should first initiate a "write" transaction with the data in the "data sent column". Don't send the "xx" bytes. Then you initiate a "read" transaction, and you will get the data in the "data received" column (and again not the "xx" bytes).

Read identification

read the identification string of the board. ('spi_servo 1.0').

data sent	data recieved	explanation
0x87	xx	select destination with address 0x82 for READ.
0x01	xx	identify
xx	0x73	's'
xx	0x70	'p'
xx	0x69	'i'
xx	...	etc.