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Raspberry Pi
Raspberry Pi Forum Want to create a capacitance proximity/touch sensor with a RP2040 Pico board using PIO
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Forum Thread Details
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  • proximity_sensors
  • rp2040
  • PIO
  • capacitive
Related

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

BigG
BigG over 3 years ago

There is nothing quite like solving a problem to help develop new skills. In this case I have found a library solution to what I want to achieve but it is not RP2040 compatible and of course it does not use PIO, which is the skill I am trying to acquire.

Having seen other members deliver some great PIO related projects, I thought this would be a great opportunity to build a solution together while helping me, and hopefully others, grasp how to develop PIO code from scratch.

Basically I want to create a capacitance touch or proximity sensor using a Raspberry Pi Pico (RP2040) board.

There is an Arduino library that can do this: https://github.com/PaulStoffregen/CapacitiveSensor

But it is not RP2040 compatible: https://github.com/PaulStoffregen/CapacitiveSensor/issues/42

Having reviewed this library, it appears on face value to be well suited to use PIO. Only, I don't know how to start.

As far as I can make out, there is only one function to apply PIO... but maybe others see things differently. I'm keen to learn more.

// Private Methods /////////////////////////////////////////////////////////////
// Functions only available to other functions in this library

int CapacitiveSensor::SenseOneCycle(void)
{
    noInterrupts();
	DIRECT_WRITE_LOW(sReg, sBit);	// sendPin Register low
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to input (pullups are off)
	DIRECT_MODE_OUTPUT(rReg, rBit); // receivePin to OUTPUT
	DIRECT_WRITE_LOW(rReg, rBit);	// pin is now LOW AND OUTPUT
	delayMicroseconds(10);
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to input (pullups are off)
	DIRECT_WRITE_HIGH(sReg, sBit);	// sendPin High
    interrupts();

	while ( !DIRECT_READ(rReg, rBit) && (total < CS_Timeout_Millis) ) {  // while receive pin is LOW AND total is positive value
		total++;
	}
	//Serial.print("SenseOneCycle(1): ");
	//Serial.println(total);

	if (total > CS_Timeout_Millis) {
		return -2;         //  total variable over timeout
	}

	// set receive pin HIGH briefly to charge up fully - because the while loop above will exit when pin is ~ 2.5V
    noInterrupts();
	DIRECT_WRITE_HIGH(rReg, rBit);
	DIRECT_MODE_OUTPUT(rReg, rBit);  // receivePin to OUTPUT - pin is now HIGH AND OUTPUT
	DIRECT_WRITE_HIGH(rReg, rBit);
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to INPUT (pullup is off)
	DIRECT_WRITE_LOW(sReg, sBit);	// sendPin LOW
    interrupts();

#ifdef FIVE_VOLT_TOLERANCE_WORKAROUND
	DIRECT_MODE_OUTPUT(rReg, rBit);
	DIRECT_WRITE_LOW(rReg, rBit);
	delayMicroseconds(10);
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to INPUT (pullup is off)
#else
	while ( DIRECT_READ(rReg, rBit) && (total < CS_Timeout_Millis) ) {  // while receive pin is HIGH  AND total is less than timeout
		total++;
	}
#endif
	//Serial.print("SenseOneCycle(2): ");
	//Serial.println(total);

	if (total >= CS_Timeout_Millis) {
		return -2;     // total variable over timeout
	} else {
		return 1;
	}
}

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  • scottiebabe
    0 scottiebabe over 3 years ago

    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,

    image image

    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.

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  • BigG
    0 BigG over 3 years ago in reply to scottiebabe

    Thanks for the comment.

    scottiebabe said:
    I'm not sure why you need 2 GPIO pins.

    I believe one GPIO is used as an output pin (rather than use VDD as per your diagram) and the other GPIO is used to measure the charge time... I see you figured that out anyway.

    I think parts of the SenseOneCycle function, which I am trying to change to PIO can be easy to convert, like setting pins to input or output and then setting high or low.

    Parts I'm not sure about are things like...

    How to covert delayMicroseconds(10); to PIO code if CPU clock speed is unknown. Maybe in this application 10 microseconds is not that critical and if so what should I have in PIO.

    Then I am assuming that this part of the code probably won't work that well in PIO as involves Math. If so then probably the PIO code part might need to be split into two or more parts etc.

    while ( !DIRECT_READ(rReg, rBit) && (total < CS_Timeout_Millis) ) {  // while receive pin is LOW AND total is positive value
            total++;
    }

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  • BigG
    0 BigG over 3 years ago in reply to scottiebabe

    Thanks for the comment.

    scottiebabe said:
    I'm not sure why you need 2 GPIO pins.

    I believe one GPIO is used as an output pin (rather than use VDD as per your diagram) and the other GPIO is used to measure the charge time... I see you figured that out anyway.

    I think parts of the SenseOneCycle function, which I am trying to change to PIO can be easy to convert, like setting pins to input or output and then setting high or low.

    Parts I'm not sure about are things like...

    How to covert delayMicroseconds(10); to PIO code if CPU clock speed is unknown. Maybe in this application 10 microseconds is not that critical and if so what should I have in PIO.

    Then I am assuming that this part of the code probably won't work that well in PIO as involves Math. If so then probably the PIO code part might need to be split into two or more parts etc.

    while ( !DIRECT_READ(rReg, rBit) && (total < CS_Timeout_Millis) ) {  // while receive pin is LOW AND total is positive value
            total++;
    }

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  • scottiebabe
    0 scottiebabe over 3 years ago in reply to BigG

    In python I'm used to just specify the frequency I want and letting the libraries set the statemachine clock divider values for me.

    smHsync = rp2.StateMachine(0, hsyncpio, freq=25_000_000,  set_base=Pin(16))
    smVsync = rp2.StateMachine(1, vsyncpio, freq=25_000_000,  set_base=Pin(17))

    By default the statemachine should run at the Sysclock frequency and I don't believe the config function alters the clock divider

    image 

    So you would need a busy loop that takes 10us * sysclock (typ 125MHz) = 1250 cycles.

    I have been thinking about how to make a state machine to scan a 4x4 keypad, and will agree trying to do math isn't easy...

    Since there is no add instruction, to count we would have to decrement a scratch register which could be initialized the the timeout count.

    And later convert to an equivalent positive count. If we omit the timeout and begin counting down from -1 (0xFFFF...) then the positive equivalent is just the bitwise complement.

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  • BigG
    0 BigG over 3 years ago in reply to BigG
    BigG said:
    I think parts of the SenseOneCycle function, which I am trying to change to PIO can be easy to convert, like setting pins to input or output and then setting high or low.

    LOL.

    Even this part is proving quite tricky. From what I've gathered you have to use the shift register to set multiple pins as input or output and that this is referenced based on the pin you select.

    What is not clear at all is whether an input pin can be changed to an output pin and this back to input again.

    Would love to know...

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  • scottiebabe
    0 scottiebabe over 3 years ago in reply to BigG

    This I do have answer with confidence, yes you can absolutely go back and forth between inputs and outputs, I used that feature to tri-state multiplex my fairy lights AngelSparkles. Writing a 1 to the pindirs register sets the pin to be an output.

    set(pindirs, 0b011)

    If you are planning to use 2 gpio pins, you could configure the tx pin to be a sideset pin and then assign its output state at every instruction with the 

    .side(val) then you don't have to try and bit twiddle and an input and output pin together.

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  • BigG
    0 BigG over 3 years ago in reply to scottiebabe

    Ah brilliant. Thanks for this explanation.

    I have just discovered a very useful video on YouTube which is helping a bit too.

    You don't have permission to edit metadata of this video.
    Edit media
    x
    image
    Upload Preview
    image

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  • scottiebabe
    0 scottiebabe over 3 years ago in reply to BigG

    That video looks incredible! Great find!

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