Want to create a capacitance proximity/touch sensor with a RP2040 Pico board using PIO

Hello, I'm a hardware gal and can't make heads or tails of that library. I'm not sure why you need 2 GPIO pins. I assume the author is trying to measure the charge time of the capacitance seen on a GPIO pin. So something like this,

So presumably, the GPIO pin can output low to reset the effective capacitance on the pin. Then go Hi-Z and measure the time taken for the capacitance to charge to a VIH threshold.

I'm not sure I know the PIO module well enough to write a how-to guide, but I am happy to answer questions to the best of my knowledge, because I think the PIO module is pretty neat.

Thank you so much for opening this discussion, it has helped me quite a bit too! I gave the single IO pin with pullup resistor a try and I think it works okay:

# PIO Capsense experiment
# -scottiebabe 2022
import time
from machine import Pin
import rp2

machine.freq(125_000_000)

# Ensure pin pull-ups and pull-downs are disabled
# So pin is Hi-Z when set as input
Pin(28,Pin.IN)

@rp2.asm_pio(set_init=[rp2.PIO.OUT_LOW])
def capsense():
    set(pindirs, 1) # set GPIO as output
    set(pins, 0)
    set(x,24)
    label('dischargeloop') #wait (24+1)*5 = 125 cycles (1 us)
    jmp( x_dec, 'dischargeloop') [4]
    mov(isr,null) # clear ISR, not sure if this is required
    set(x,1) #  x = 1
    in_(x,1) # shift x into isr
    in_(null,12) # shift 12 0's into isr
    mov(x,isr)   # set x = 1<<12 (4096)
    set(pindirs, 0)     # set GPIO as input
    label('timing')
    jmp(pin,'done')     # breakout of timing loop when pin reads high
    jmp(x_dec,'timing') # decrement x until timeout
    label('done')
    mov(isr,x)          # load ISR with count value in x
    push(noblock)       # push ISR into RX fifo

# Create the StateMachine with the capsense program, using pin28.
sm = rp2.StateMachine(0, capsense, freq=125_000_000, set_base=Pin(28),jmp_pin=Pin(28))


# Start the StateMachine.
sm.active(1)

while True:
    print(sm.get())
    time.sleep(0.05)

Happy to explain or clarify or be corrected (I don't consider myself a PIO expert yet LOL) 

Brilliant. Well done. I'm going to try and convert that to C and see how I can get it to work.

It just so happens that I just discovered another capacitive sensor library that does things slightly differently by using AnalogRead (in Arduino) via analog pins to calculate capacitive values. At least this one works straight out the box with the Pico board.

https://github.com/Nyanyan/FastCapacitiveSensor

In fact I came up with my own circuit, which still gave me pretty good non-contact sensitivity as I didn't have any resistors greater than 1M ohm.

It will be interesting to compare this test with the other library (once got working with PIO) and maybe also convert this new library to PIO if possible to see if any improved performance.

Still lots to learn...

Kudos to you for taking on the PIO, It feels pretty awesome when you get first statemachine program to work.

Very neat, I haven't seen a capsense implementation like that before. I'm also impressed the ADC can sample such a high-impedance source so reliably, very cool! 

Hmm pushing to a full fifo results in the push being discarded, it doesn't overwrite values in the fifo. So the sm.get() isn't the most recent result. 

I had to make a small modification to the fast capacitive sensing library as it is targeting Arduino UNO (5V and 10bit ADC resolution).

So this line was changed: double INPUTTHRESHOLD = VOLTAGE * 4096 / 3.3 * 0.9;

Then I applied quite a bit of sampling by using min and max values to generate my psuedo capacitive values.

Here's my Pico Arduino code (first hack version):

/* Arduino Code of Raspberry Pi Pico board
 * MIT License
 * 
 * Copyright (c) C Gerrish (BigG) - relates to this example
 * 
 * Example is based on a modified version of this library:
 * https://github.com/Nyanyan/FastCapacitiveSensor
 * 
 * The following was changed in the library:
 * double INPUTTHRESHOLD = VOLTAGE * 4096 / 3.3 * 0.9;
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *  
 */
 
#include <FastCapacitiveSensor.h>
FastCapacitiveSensor sensor1;

const byte MAXSAMPLES = 10;

void setup() {
  pinMode(A1, OUTPUT);
  pinMode(A0, INPUT);
  Serial.begin(115200);
  sensor1.begin(A0, A1, 3.3, 10, 10, 0.2);
}

int cntr = 0;
long tmax = 0;
long tmin = 0xffffffff;

int smplCntr = 0;
long samples[MAXSAMPLES];

bool sampleRdy = false;

void loop() {
  long t = long(sensor1.touch());
  if (cntr == MAXSAMPLES) {
    if (smplCntr == MAXSAMPLES) {
      if (!sampleRdy) sampleRdy = true;
      smplCntr = 0;
    }
    if (sampleRdy) {
      long totval = 0;
      for (byte i = 0; i < MAXSAMPLES; i++) {
        totval += samples[i];
      }
      // This is for the Serial Plotter and to make sure 0 is minimum value on y-axis
      Serial.print(0); Serial.print(",");
      Serial.println(totval/MAXSAMPLES);
    }
    samples[smplCntr] = tmax - tmin;
    smplCntr++;
    cntr = 0;
    tmax = 0;
    tmin = 0xffffffff;        
  }
  if (t < tmin) tmin = t;
  if (t > tmax) tmax = t;
  cntr++;
}

Here's a short video (no audio) showing the Arduino Serial Plotter output:

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No doubt this could be improved upon. It's a start at least.

I digressed a little from my PIO development plans, only because I decided for some reason to cut an inch off the wire. This totally changed detection sensitivity and now it was only able to detect touch rather than proximity... basically this is a classic case of hardware changes overruling any software methodology. I am going to have to replace or modify again to return to my original objective of proximity detection.

Anyway, I modified my code to determine state change. Works pretty well.

/* Arduino Code of Raspberry Pi Pico board
 *  This code determines state change of a capacitive touch sensor
 *  
 * MIT License applies
 * 
 * Copyright (c) C Gerrish (BigG) - relates to this example
 * 
 * Example is based on a modified version of this library:
 * https://github.com/Nyanyan/FastCapacitiveSensor
 * 
 * The following was changed in the library:
 * double INPUTTHRESHOLD = VOLTAGE * 4096 / 3.3 * 0.9;
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *  
 */
 
#include <FastCapacitiveSensor.h>


#define LEDPIN    20

// Number of samples taken
const byte MAXSAMPLES = 10;

// Number of values used to calibrate
// Using 30 samples to calibrate base level in order to normalise to zero for no touch
const byte CALIBSAMPLES = 30;

// 20,000 seemed to be best fit to get my touch easily recognised (adjust to suit your setup)
// if too sensitive then raise value or if not sensitive enough then reduce value
const int MAXCAPVALUE = 20000;

FastCapacitiveSensor sensor1;

void setup() {
  pinMode(A1, OUTPUT);
  pinMode(A0, INPUT);
  pinMode(LEDPIN,OUTPUT);
  
  Serial.begin(115200);
  sensor1.begin(A0, A1, 3.3, 10, 10, 0.2);
}

int cntr = 0;
long tmax = 0;
long tmin = 0xffffffff;

int smplCntr = 0;
long samples[MAXSAMPLES];

int caliCntr = 0;
long calibrate = 0;

bool sampleRdy = false;
bool caliRdy = false;

bool CapDect = false;
bool PrevDect = false;

void loop() {
  long t = long(sensor1.touch());
  if (cntr == MAXSAMPLES) {
    if (smplCntr == MAXSAMPLES) {
      if (!sampleRdy) sampleRdy = true;
      smplCntr = 0;
    }
    if (sampleRdy) {
      long totval = 0;
      for (byte i = 0; i < MAXSAMPLES; i++) {
        totval += samples[i];
      }
      if (caliCntr < CALIBSAMPLES) {
        calibrate += totval/MAXSAMPLES;
        caliCntr++;
      }
      else {
        if (!caliRdy) {
          caliRdy = true;
          calibrate /= CALIBSAMPLES;
        }
        // Using constrain to limit min and max values to 0 and MAXCAPVALUE    
        totval = constrain(totval/MAXSAMPLES - calibrate, 0, MAXCAPVALUE);
        // Using map as a way to show a 0 to 100 representation
        // So basically if >70% then taken as touch otherwise sensor not touched
        CapDect = map(totval,0,MAXCAPVALUE,0,100)>70?true:false;
        
        // Showing change of state on Serial Monitor
        if (CapDect != PrevDect) {
          if (CapDect) {
            Serial.println("ON");
            digitalWrite(LEDPIN, HIGH);
          }
          else {
            Serial.println("OFF");
            digitalWrite(LEDPIN, LOW);
          }
        }
        
        PrevDect = CapDect;
        
      }
    }
    samples[smplCntr] = tmax - tmin;
    smplCntr++;
    cntr = 0;
    tmax = 0;
    tmin = 0xffffffff;        
  }
  if (t < tmin) tmin = t;
  if (t > tmax) tmax = t;
  cntr++;
}

Here's a short no audio video demo (note camera buffer seemed to create a lag behind serial output for some reason - as such LED seems out of sync in video):

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I digressed a little from my PIO development plans, only because I decided for some reason to cut an inch off the wire. This totally changed detection sensitivity and now it was only able to detect touch rather than proximity... basically this is a classic case of hardware changes overruling any software methodology. I am going to have to replace or modify again to return to my original objective of proximity detection.

Anyway, I modified my code to determine state change. Works pretty well.

/* Arduino Code of Raspberry Pi Pico board
 *  This code determines state change of a capacitive touch sensor
 *  
 * MIT License applies
 * 
 * Copyright (c) C Gerrish (BigG) - relates to this example
 * 
 * Example is based on a modified version of this library:
 * https://github.com/Nyanyan/FastCapacitiveSensor
 * 
 * The following was changed in the library:
 * double INPUTTHRESHOLD = VOLTAGE * 4096 / 3.3 * 0.9;
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *  
 */
 
#include <FastCapacitiveSensor.h>


#define LEDPIN    20

// Number of samples taken
const byte MAXSAMPLES = 10;

// Number of values used to calibrate
// Using 30 samples to calibrate base level in order to normalise to zero for no touch
const byte CALIBSAMPLES = 30;

// 20,000 seemed to be best fit to get my touch easily recognised (adjust to suit your setup)
// if too sensitive then raise value or if not sensitive enough then reduce value
const int MAXCAPVALUE = 20000;

FastCapacitiveSensor sensor1;

void setup() {
  pinMode(A1, OUTPUT);
  pinMode(A0, INPUT);
  pinMode(LEDPIN,OUTPUT);
  
  Serial.begin(115200);
  sensor1.begin(A0, A1, 3.3, 10, 10, 0.2);
}

int cntr = 0;
long tmax = 0;
long tmin = 0xffffffff;

int smplCntr = 0;
long samples[MAXSAMPLES];

int caliCntr = 0;
long calibrate = 0;

bool sampleRdy = false;
bool caliRdy = false;

bool CapDect = false;
bool PrevDect = false;

void loop() {
  long t = long(sensor1.touch());
  if (cntr == MAXSAMPLES) {
    if (smplCntr == MAXSAMPLES) {
      if (!sampleRdy) sampleRdy = true;
      smplCntr = 0;
    }
    if (sampleRdy) {
      long totval = 0;
      for (byte i = 0; i < MAXSAMPLES; i++) {
        totval += samples[i];
      }
      if (caliCntr < CALIBSAMPLES) {
        calibrate += totval/MAXSAMPLES;
        caliCntr++;
      }
      else {
        if (!caliRdy) {
          caliRdy = true;
          calibrate /= CALIBSAMPLES;
        }
        // Using constrain to limit min and max values to 0 and MAXCAPVALUE    
        totval = constrain(totval/MAXSAMPLES - calibrate, 0, MAXCAPVALUE);
        // Using map as a way to show a 0 to 100 representation
        // So basically if >70% then taken as touch otherwise sensor not touched
        CapDect = map(totval,0,MAXCAPVALUE,0,100)>70?true:false;
        
        // Showing change of state on Serial Monitor
        if (CapDect != PrevDect) {
          if (CapDect) {
            Serial.println("ON");
            digitalWrite(LEDPIN, HIGH);
          }
          else {
            Serial.println("OFF");
            digitalWrite(LEDPIN, LOW);
          }
        }
        
        PrevDect = CapDect;
        
      }
    }
    samples[smplCntr] = tmax - tmin;
    smplCntr++;
    cntr = 0;
    tmax = 0;
    tmin = 0xffffffff;        
  }
  if (t < tmin) tmin = t;
  if (t > tmax) tmax = t;
  cntr++;
}

Here's a short no audio video demo (note camera buffer seemed to create a lag behind serial output for some reason - as such LED seems out of sync in video):

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