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Digital potentiometer DS1803 is an I2C-controlled device that can digitally control the potentiometer.
Opposite to the physical potentiometers, there are no movable parts.
DS1803 has two digital potentiometers controlled independently. We use just one with the lower cardinal number (index 0). In our example, it is a 100k spread between GND and VCC, and its output is connected to the ADC (analogue to digital converter) input of the ESP32 MCU. This way, the potentiometer's wiper is controlled remotely via the I2C bus.
The device's I2C address is 0x28, and the ADC input GPIO pin is 7.
The digital potentiometer in our laboratory node forms then a loopback device: it can be set (also read) via I2C, and the resulting voltage can be measured on the separate PIN (ADC) 1. This way, it is possible, e.g. to draw a relation between the potentiometer setting and ADC readings to check whether it is linear or forms some other curve.
Reading of the ADC is possible using the regular analogRead(pin)
function.
To implement this scenario, it is advised to get familiar with at least one of the following scenarios first:
They enable you to present the data on the display (i.e. readings).
To handle communication with the DS1803 digital potentiometer, we use bare I2C programming. For this reason, we need to include only the I2C protocol library:
#include <Wire.h>
Below, we present a sample control library that you need to include in your code:
#define POT_ADC 7 //GPIO 7 in ESP32 #define DS1803_ADDRESS 0x28 //I2C Address enum POT_LIST {POT_1 = 0xA9, POT_2=0xAA, POT_ALL=0xAF}; //We have only POT_1 connected typedef enum POT_LIST POT_ID; //Prototypes void setPotentiometer(TwoWire& I2CPipe, byte potValue, POT_ID potNumber); byte readPotentiometer(TwoWire& I2CPipe, POT_ID potNumber); //Implementation void setPotentiometer(TwoWire& I2CPipe, byte potValue, POT_ID potNumber) { I2CPipe.beginTransmission(DS1803_ADDRESS); I2CPipe.write(potNumber); I2CPipe.write(potValue); I2CPipe.endTransmission(true); }; byte readPotentiometer(TwoWire& I2CPipe, POT_ID potNumber) //reads selected potentiometer { byte buffer[2]; I2CPipe.requestFrom(DS1803_ADDRESS,2); buffer[0]=I2CPipe.read(); buffer[1]=I2CPipe.read(); return (potNumber==POT_1?buffer[0]:buffer[1]); };
readPotentiometer(…)
function does not read the voltage over ADC. It returns a set value (the same as provided by setPotentiometer(…)
), which is on the digital side of the DS1803 device. Analogue input (a resulting voltage converted to the digital representation) can be obtained simply using regular analogRead(pin)
that reads from the ADC.
Iterate over the potentiometer settings, read related voltage readings via ADC, and present them in graphical form (as a plot). As the maximum resolution is 256, you can use a plot of 256 points or any other lower value covering all ranges. Present graph (plot) on either ePaper or OLED display, and while doing the readings, you should present data in the LCD (upper row for a set value, lower for a reading of the ADC).
Check if you can see all the displays. Remember to use potentiometer 1 (index 0) because only this one is connected to the ADC input of the ESP32 MCU. In these steps, we present only how to handle communication with a digital potentiometer and how to read the ADC input of the MCU. Methods for displaying the measurements and plotting the graph are present in other scenarios. Remember to include the functions above in your code unless you want to integrate them with your solution.
Below, we assume that you have embedded functions handling operations on the digital potentiometer as defined above in your source file.
…
Describe activities done in Step n.
A relation between the potentiometer set value and ADC reading should be almost linear from 0V up to about 3V. It becomes horizontal because the ESP32 chip limits the ADC range to 3V, so going beyond 3V (and due to the electronic construction as in figure ##REF:xxx## it may go to about 3.3V) gives no further increase but rather a reading of the 4096 value (which means the input voltage is over the limit). For this reason, your plot may be finished suddenly with a horizontal instead of linearity decreasing function. It is by design. ADC input of the ESP32 can tolerate values between 3V and 3.3V. The linear correlation mentioned above is never perfect, either because of the devices' implementation imperfection (ESP32's ADC input and digital potentiometer output) or because of the electromagnetic noise. There are many devices in our lab room.
The ADC readings are changing slightly, but I have not changed the potentiometer value. What is going on?: The ADC in ESP32 is quite noisy, mainly when using WiFi parallelly. Refer to the Coursebook and ESP32 documentation on how to increase measurement time that will make internally many readings and return to you an average. Use the analogSetCycles(cycles)
function to increase the number of readings for the averaging algorithm. The default is 8, but you can increase it up to 255. Note that the higher the cycles
parameter value, the longer the reading takes, so tune your main loop accordingly, particularly when using an asynchronous approach (timer-based). Eventually, you can implement low-pass filters yourself (in the software).