Servo 1.0 protocol

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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
0x10-0x11 8 bit register. Reserved for debug. Do not write to these registers.
0x12 8 bit register. Each bit controls the enable of an output. If disabled (0) the output drops to zero. Some servos stop wiggling, become back-drivable and consume less power in this state.
0x20-0x26 8 bit register. Set servo 0-6 position. The pulse width changes from about 1.00ms for 0 to about 2.00ms for 0xff. To "overdrive" your servos you need to use 0x28-0x2e or adjust the offset and scaling using 0x58/0x59.
0x28-0x2e (*) 16 bit register. Set servo 0-6 timing 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.
0x50 (#) set servo values as an array. Write 1-7 servo position bytes (always starting at servo 0) in one transaction.
0x51 (#) set servo values as an array. Write 1-7 servo timing values (16 bits/servo) in one transaction.
0x58 (#) 8bit servo positions are converted to 16 bit timing values using tv = base + pos*mul; default base = 988 giving the normal 1000-2000 microsecond timing value range. This register is 16 bits and modifies the base.
0x59 (#) this changes the multiplication factor. See 0x58.
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.

(#) From version 2.1 and up. (0x50 from 2.0)

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.


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 0x86 for READ.
0x01 xx identify
xx 0x73 's'
xx 0x70 'p'
xx 0x69 'i'
xx ... etc.