TI_INA226_micropython

This library provides support for the TI INA226 power measurement IC with micropython firmware. Datasheet and other information on the IC: https://www.ti.com/product/INA226

This library is derived from https://github.com/robert-hh/INA219
with the friendly support of the community at https://forum.micropython.org/

Motivation

I needed a micropython library for the INA226 devices for a small power meter project I was working on. I did find libraries for the Raspberry Pi, but none for micropython. Thus, I had to modify an existing library for the INA219 devices.

Basics

To use the device, it has to be configured at startup. In it's default configuration, the calibration register is not set and thus the current and power cannot be directly read out.
By default, this library configures the device to a maximum current of 3.6 A and 36V bus voltage. Resistance of the shunt is assumed as 0.1 Ohm.

Calculations

The following values need to be calculated in order to set the configuration and calibration register values:

• calibration register value
• power LSB value
• current LSB value
• configuration register value Configuration register value is derived from the values of the corresponding bits.

Default configuration register is as follows:

BitNr	D15	D14	D13	D12	D11	D10	D09	D08	D07	D06	D05	D04	D03	D02	D01	D00
Name	RST	N/A	N/A	N/A	AVG2	AVG1	AVG0	VBUSCT2	VBUSCT1	VBUSCT0	VSHCT2	VSHCT1	VSHCT0	MODE3	MODE2	MODE1
Value	0	1	0	0	0	0	0	1	0	0	1	0	0	1	1	1

Default configuration according to the datasheet:

• Averaging mode: 1 sample
• Bus voltage conversion time: 1.1ms
• Shunt voltage conversion time: 1.1ms
• Operating mode: Shunt and Bus voltage, continuous

Possible values for the configuration registers can be found in the library.

Calculating the current_LSB

As example, a maximum expected current of 3.6A is assumed.

current_LSB = max_expected_I / (2^15)
current_LSB = 3.6 A / (2^15)
current_LSB = 0.0001098632813 = 0.00011 = 0.0001 -> 100uA/bit

Calculating the calibration register

Cal_value = 0.00512 / (current_LSB * Rshunt)
Cal_value = 0.00512 / (0.0001 * 0.1)
Cal_value = 512

Calculating the power_LSB

power_LSB = 25 * current_LSB
power_LSB = 25 * 0.0001 = 0.0025 -> 2.5mW/bit

Usage information

In order to be able to set calibration and configuration to custom values, some work needs to be done by the user
In the library, a method "set_calibration_custom" exists, which expects the calibration register value and the
configuration register value as arguments. If no arguments are given, it uses default values.
For easier calculation, a Google spreadsheet is available at this Google spreadsheet
With ina_calc_config.py I've provided a crude, menu guided configuration calculator for Python3. It's a bit rough around the edges but works.

Default values

cal_value = 512
current_lsb = 0.0001
power_lsb = 0.0025
Rshunt = 0.1
Averaging mode = 512 samples
Bus voltage conversion time = 588us
Shunt voltage conversion time = 588us
Operating mode = Shunt and Bus Voltage, continuous

Example code

This code was written to test (i.e. just see if it works without errors) the library on an ESP01S module with 1MB flash

import ina226
from time import sleep
from machine import Pin, I2C
# i2c
i2c = I2C(scl=Pin(2), sda=Pin(0))
# ina226
ina = ina226.INA226(i2c, 0x40)
# default configuration and calibration value
ina.set_calibration()
print(ina.bus_voltage)
print(ina.shunt_voltage)
print(ina.current)
print(ina.power)