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.
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.
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
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. |