7 years, 10 months ago.

How to read MPR121 status registers from inside the interrupt ISR

I have been trying to work this problem out for several hours now with no luck. I am using a Nucleo F767ZI board and an MPR121 sensor for capacitive touch sensing. As a test program to ensure that the sensor worked, I had the micro controller continuously read the status register 0x00 (over i2c) in an infinite while loop and when one of the first three electrodes were touched, a corresponding LED would light up. This worked perfectly.

I am now attempting to modify the code to use InterruptIn so that the micro controller isn't continuously checking the status register. The sensor has an IRQ out pin pulled high until a change in the status of one of the electrodes is detected in which case it is pulled down until either register 0 or register 1 (the electrode status registers) is read from. When I run this code, the callback function is successfully called but when register 0 is read, it always seems to return 0, as if none of the electrodes were touched, yet it pulls the interrupt pin low correctly when touched, and resets when I attempt to read. I don't know if this would be a problem related to the sensor, or if the Nucleo board isn't reading the values correctly.

#include "mbed.h"
#include "MPR121.h"

//Serial pc(USBTX, USBRX);
DigitalOut red(PF_13);
DigitalOut green(PE_9);
DigitalOut blue(PE_11);

///if defined TARGET_LPC1768 || TARGET_LPC11U24
  I2C i2c(PB_9, PB_8);
  InterruptIn irq(PF_12);
  MPR121 touch_pad(i2c, irq, MPR121::ADDR_VSS);

void checkPads()
{   
    uint8_t value = touch_pad.readRegister(MPR121::ELE0_7_STAT);
        
        if(value % 2 < 1)
            red = 0;
        else
            red = 1;
            
        if(value % 4 < 2)
            green = 0;
        else
            green = 1;
        
        if(value % 8 < 4)
            blue = 0;
        else
            blue = 1;
}

void fallInterrupt()
{
    checkPads();
}

int main() 
{       
    touch_pad.init();
    touch_pad.enable();
    irq.fall(&fallInterrupt);
    
    while (1) {}
}

Essentially, if the 'checkPads()' function is moved to the while loop, it performs as expected, but moving it to the interrupt function causes it to not work properly, even though the fallInterrupt function is being called when expected. Any insight on the problem would be appreciated.

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