Pi Servo Hat Hookup Guide

Contributors: SFUptownMaker
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Software - Python

We'll go over in some detail here how to access and use the pi servo hat in Python. Full example code is available in the product GitHub repository.

Set Up Access to SMBus Resources

First point: in most OS level interactions, the I2C bus is referred to as SMBus. Thus we get our first lines of code. This imports the smbus module, creates an object of type SMBus, and attaches it to bus "1" of the Pi's various SMBuses.

import smbus
bus = smbus.SMBus(1)

We have to tell the program the part's address. By default, it is 0x40, so set a variable to that for later use.

addr = 0x40

Next, we want to enable the PWM chip and tell it to automatically increment addresses after a write (that lets us do single-operation multi-byte writes).

bus.write_byte_data(addr, 0, 0x20)
bus.write_byte_data(addr, 0xfe, 0x1e)

Write Values to the PWM Registers

That's all the setup that needs to be done. From here on out, we can write data to the PWM chip and expect to have it respond. Here's an example.

bus.write_word_data(addr, 0x06, 0)
bus.write_word_data(addr, 0x08, 1250)

The first write is to the "start time" register for channel 0. By default, the PWM frequency of the chip is 200Hz, or one pulse every 5ms. The start time register determines when the pulse goes high in the 5ms cycle. All channels are synchronized to that cycle. Generally, this should be written to 0.

The second write is to the "stop time" register, and it controls when the pulse should go low. The range for this value is from 0 to 4095, and each count represents one slice of that 5ms period (5ms/4095), or about 1.2us. Thus, the value of 1250 written above represents about 1.5ms of high time per 5ms period.

Servo motors get their control signal from that pulse width. Generally speaking, a pulse width of 1.5ms yields a "neutral" position, halfway between the extremes of the motor's range. 1.0ms yields approximately 90 degrees off center, and 2.0ms yields -90 degrees off center. In practice, those values may be slightly more or less than 90 degrees, and the motor may be capable of slightly more or less than 90 degrees of motion in either direction.

To address other channels, simply increase the address of the two registers above by 4. Thus, start time for channel 1 is 0x0A, for channel 2 is 0x0E, channel 3 is 0x12, etc. and stop time address for channel 1 is 0x0C, for channel 2 is 0x10, channel 3 is 0x14, etc. See the table below.

Channel # Start Address Stop Address
Ch 0 0x06 0x08
Ch 1 0x0A 0x0C
Ch 2 0x0E 0x10
Ch 3 0x12 0x14
Ch 4 0x16 0x18
Ch 5 0x1A 0x1C
Ch 6 0x1E 0x20
Ch 7 0x22 0x24
Ch 8 0x26 0x28
Ch 9 0x2A 0x2C
Ch 10 0x2E 0x30
Ch 11 0x32 0x34
Ch 12 0x36 0x38
Ch 13 0x3A 0x3C
Ch 14 0x3E 0x40
Ch 15 0x42 0x44

If you write a 0 to the start address, every degree of offset from 90 degrees requires 4.6 counts written to the stop address. In other words, multiply the number of degrees offset from neutral you wish to achieve by 4.6, then either add or subtract that result from 1250, depending on the direction of motion you wish. For example, a 45 degree offset from center would be 207 (45x4.6) counts either more or less than 1250, depending upon the direction you desire the motion to be in.