Servo 1.0 protocol

The addresses on the SPI bus are 7 bits wide. The lower bit specifies if the transaction is to be a read or a write. Write transactions have the lower bit cleared (0), read transactions have the lower bit set (1).

Each transaction on the SPI bus starts with the address of the board. The spi_servo board will ignore any transactions on the SPI bus that do not start with its own address.

After the address a single byte indicates the "port" on the board that the data is written to. The software can thus define 256 ports on each board.

The default address of the servo board is 0x86.

write ports

Some ports just set a single value. So writing more than one byte to such a port is redundant. Other ports are logically a stream of bytes. So writing more than one byte is encouraged.

The spi_servo board defines several ports.

port	function
0x20	Set servo 0 position
0x21	Set servo 1 position
0x22	Set servo 2 position
0x23	Set servo 3 position
0x24	Set servo 4 position
0x25	Set servo 5 position
0x26	Set servo 6 position
0xf0	change address.

read ports

The spi_servo board supports two read ports:

port	function
0x01	identification string. (terminated with 0).
0x20	read servo 0 position
0x21	read servo 1 position
0x22	read servo 2 position
0x23	read servo 3 position
0x24	read servo 4 position
0x25	read servo 5 position
0x26	read servo 6 position

examples

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.