Operating the AD5272 with Python 3 and smbus2 on Raspberry Pi 3 B+

Question

I have a problem operating the AD5272 using Python 3 on Raspberry Pi 3 B+. AD5272 is the digitally controlled rheostat with I2C interface (my part has resistance 20 kOhm).

The problem is following:

The resistance between A and W terminals doesn't change whatever position I'm trying to set-up and stays around 10KOhm. (By default when AD5272 turns on, is sets the half of full resistance there). When I'm reading the resistance from RDAC - is reads zero resistance.

Here is my code:

#!/usr/bin/python3
import smbus2
import time, os

class AD527x ():
    # command bits which are dependant on I2C address of device
    def __init__ (self, bus=1, address=0x2E, resistance = 20000, positions = 1023, reset = False):
        self.bus = smbus2.SMBus(bus)
        self.positions = positions
        self.address = address
        self.resistance = resistance

    def write_position (self, position):
        # sets the rheostat to certain position

        # if wiper position is higher than maximal
        if position > self.positions:
            position = self.positions

        # approximate resistance of rheostat
        resistance = self.resistance*position/self.positions

        # for AD5274 needs to be shifted for 2 digits left
        if self.positions == 256:
            position = position << 2

        print ("Binary representation of position for RDAC : " + bin (position))

        # Writing position is sneding 2 bytes one by one

        # MSB Data: 0 0 C3 C2 C1 C0 D9 D8
        # For writing command bytes: C3 = 0; C2 = 0; C1 = 0; C0 = 1 
        # 0 0  0  0  0  1  ?  ?
        # ? ? - MSB of 10-digit binary representation of wiper position between 0 and  1023
        # two first positions : D9_8 = (position & 0b1100000000) >> 8

        MSB = (1 << 3)  | ((position & 0b1100000000) >> 8)
        print ("MSB : " + bin(MSB))
        # take last 8 bits
        LSB = position & 0b11111111
        print ("LSB : " + bin(LSB))
        print ("All Bytes : " + bin((MSB << 8) +LSB))
        print ("Value : " + str(resistance))

        self.bus.write_i2c_block_data(self.address, 0, [MSB, LSB])
        self.read_position()

    def read_position (self):
        # reads current position of rheostat

        # prepare the circuit to send data
        # MSB Data: 0 0 C3 C2 C1 C0 D9 D8
        # For reading command bytes: C3 = 0; C2 = 0; C1 = 1; C0 = 0
        # 0 0  0  0  1  0  ?  ?
        # ? ? - Doen't matter - Just use zeros
        # LSB - doesn't matter, just using 0b00000000

        MSB = 0b00001000
        LSB = 0b00000000
        self.bus.write_i2c_block_data(self.address, 0, [MSB, LSB])

        #read 2 bytes from RDAC
        a = self.bus.read_byte(self.address)
        time.sleep (0.005)
        b = self.bus.read_byte(self.address)
        value = ((a << 8) | b )
        print (value)
        # take 10 lats bits only
        value = value & 0b1111111111
        print (value)

def main():
    device = AD527x()
    device.write_position(1023)

if __name__ == "__main__":
    main()

The output is:

Binary representation of position for RDAC : 0b1111111111
MSB : 0b1011
LSB : 0b11111111
All Bytes : 0b101111111111
Value : 20000.0
4096
0

Computer : Rapberry Pi 3, Model B+

OS : Raspbian 9

Python Version: 3.5.3

I2C package : smbus2

Part Number: AD5272BRMZ-20

Datasheet: http://www.farnell.com/datasheets/1706490.pdf

Product link: http://www.newark.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-1&urlRequestType=Base&partNumber=52R8114&storeId=10194

What am I doing wrong and how to fix it?

The wiring of part was triple checked. The reads from RDAC is always the same, whatever value I'm trying to write there. I tried both commands:

self.bus.write_i2c_block_data(self.address, 0, [MSB, LSB])

and

self.bus.write_byte (self.address, MSB)
self.bus.write_byte (self.address, LSB)

Result was always the same: Resistance doesn't change. Resistance was checked with external Ohmmeter device.

Please, help!

Answer 1

I just hit this same issue. I haven't looked at interfacing with Python yet but this may help nonetheless.

In my circuit, the AD5272 I2C is addressed with GND and I have terminal A connected to VDD (5V) and W in series with a 100K resistor to GND.

First I ensured the device was operational through the Linux driver, the modules should already be builtin. As root:

$ echo ad5272 0x2f > /sys/bus/i2c/devices/i2c-1/new_device
$ dmesg | tail -n 2
[  711.195428] i2c i2c-1: new_device: Instantiated device ad5272 at 0x2f
[  711.242537] ad_dpot 1-002f: ad5272 1024-Position Digital Potentiometer registered
$ cat /sys/bus/i2c/devices/1-002f/rdac0 
511
$ echo 0 > /sys/bus/i2c/devices/1-002f/rdac0
$ cat /sys/bus/i2c/devices/1-002f/rdac0
0
$ echo 1023 > /sys/bus/i2c/devices/1-002f/rdac0
$ cat /sys/bus/i2c/devices/1-002f/rdac0
1023

Measuring the voltage from W to GND gave ~5V at 0 and ~2.5V at 1023.

I then repeated with a logic analyser connected to the I2C bus. The commands for writes were:

0x5e 0x1c 0x03 0x5e 0x04 0x00
0x5e 0x1c 0x03 0x5e 0x07 0xff

This should hopefully provide a hint, re-read the sections in the datasheet on "write protection" and "control register description". In my setup I only want to use the digital interface and not stored memory for control, so I replicated from user space with a slight alteration:

sudo i2cset 1 0x2f 0x1c 0x02 b
sudo i2cset 1 0x2f 0x04 0x00 b
sudo i2cset 1 0x2f 0x07 0xff b