SPI Digital Potentiometer not working on ESP32

Question

I have an MCP4151 digital potentiometer chip. I have managed to get it to work using and Arduino Uno using the code below:

#include <SPI.h>
byte address = 0x00;
int CS= 5;
void setup()
{
 pinMode (CS, OUTPUT);
 SPI.begin();
}
void loop()
{
 for (int i = 0; i <= 128; i++)
 {
 digitalPotWrite(i);
 delay(10);
 }
 delay(500);
 for (int i = 128; i >= 0; i--)
 {
 digitalPotWrite(i);
 delay(10);
 }
}
int digitalPotWrite(int value)
{
 digitalWrite(CS, LOW);
 SPI.transfer(address);
 SPI.transfer(value);
 digitalWrite(CS, HIGH);
}

But when I try to get it to work using an ESP32 programmed using Arduino language it doesn't. I have connected like that

• SCK -> 18
• MOSI -> 23
• SS -> 5

I also used the potentiometer inside like a voltage divider for test purposes. GND on one leg, 3.3v on the other and an LED on the wiper. Are there other settings that I have to implement in order the ESP32 SPI to work like an Arduino SPI? Did I miss something?

• Chris_hro: If I am using the SPI defaults (Serial.begin()) and print the variables on the serial monitor Serial.println(MOSI); Serial.println(SCK); Serial.println(SS); I got these defaults MOSI 23, SCLK 18 and SS 5 (VSPI).

Answer 1

I managed to get it to work in a strange way. I opened the example included in the ESP32 SPI Library and copied some points from there.

I first initialise a pointer to the SPI class, then I call begin() at that pointer and inside the digitalPotWrite function I call beginTransaction(10000,MSBFIRST,SPI_MODE0) and end the transaction before digitalPotWrite finishes. Full code here:

#include <SPI.h>
static const int spiClk = 1000000;
byte address = 0x00;
SPIClass * hspi = NULL;
void setup()
{
 pinMode(15, OUTPUT);
 hspi = new SPIClass(HSPI);
 hspi->begin(); 
}
void loop()
{
 for (int i = 0; i <= 128; i++)
 {
 digitalPotWrite(i);
 delay(10);
 }
 delay(500);
 for (int i = 128; i >= 0; i--)
 {
 digitalPotWrite(i);
 delay(10);
 }
}
int digitalPotWrite(int value)
{
 hspi->beginTransaction(SPISettings(spiClk, MSBFIRST, SPI_MODE0));
 digitalWrite(15, LOW);
 hspi->transfer(address);
 hspi->transfer(value);
 digitalWrite(15, HIGH);
 hspi->endTransaction();
}

Answer 2

The ESP32 SPI library has defaults for the HSPI pins (14, 12, 13, 15). For VSPI pins, you must specify the pins in begin().

SPI.begin(18, 19, 23, 5);

• Christos Mitsis: I both tried to use the HSPI pins with SPI.begin(); and those pins using SPI.begin(18, 19, 23, 5); but nothing worked. Maybe ESP32 used an incompatible frequency for SPI?

Answer 3

Change the int for a void in the function... That's it. When you call that function is never going back, still waiting for a return.

int digitalPotWrite(int value)
{
 digitalWrite(CS, LOW);
 SPI.transfer(address);
 SPI.transfer(value);
 digitalWrite(CS, HIGH);
}