Capacitance meter

If you go to the TIDesigns page: http://www.ti.com/tidesigns and search for “Capacitive” or “Capacitive Touch” download one (or more) of the projects and study the documentation.

Thanks for your reply. I've found some nice samples. 

But it looks like a lot is based (or applied) for (human) touch. I just want to measure capacitance in a very plain and simple way. Measure capacitance of (wet) soil.

I found the following article and i've created it on my MSP430. 

http://wordpress.codewrite.co.uk/pic/2014/01/21/cap-meter-with-arduino-uno/

...And it looks like, its working! :)

My question basically was/is; (1) is this a good approach? Do I need a timer? (2) Can I directly connect such capacitor on my MSP430 pin (max 2mA)?

I hope you get the right documents how are describing the several ways to measure a capacitance.

The “Capacitive Touch” is in principle the same as your Arduino project. The “Capacitive Touch” measures the capacitance of the PCB dielectric –which changes when an object (finger) nears the PCB- mostly with the RC method. The Arduino project creates a voltage divider.

The accuracy of the measurements depends on the accuracy of the used reference.

The Arduino way is a possible way, only the used voltage needs to be in the range of your MSP (3.3V) then you also will not reach the maximum allowed current of your port pin.

Hi,

Thanks for your reply. I'm still struggling with my capacitance meter (for weeks now! :) ). Maybe someone could help me out.

I have the following setup, read ADC10 on P1.1 after setting P1.4 high.

[P1.4] ------------>|| CT

[P1.1] <------------|| 20pF ------------ GND

I would like to measure CT using the following formula: "(adcValue * 20.0f) / (1023 - adcValue)".

But my readings doesn't make any sense! I tried with 100pf, 50pf, 10pf. But the readings are stuck around 950 raw value with sometimes random peaks.

ADC10 Init code:

ADC10CTL1 = INCH_1;
ADC10CTL0 = ADC10ON + ADC10IE;
ADC10AE0 |= PinCapacitance;

ADC10 read code:

ADC10CTL0 &= ~ENC;
while (ADC10CTL1 & BUSY) {  }
int adcValue = ADC10MEM;
ADC10CTL0 |= ENC + ADC10SC;

Any ideas? Thanks!

This is roughly the equivalent of your measurement;

Your Read code looks a little bit strange to me. You disable conversion, wait, read ADC result and start conversion. I think the order should be: enable and start conversion, wait till ready, read ADC result and disable conversion.

Hi Leo,

Thanks for your quick reply.

Yes, the scheme looks like something I work with. The only thing is; I set P1.4 to HIGH, and Cref is only connected to ground. I'm using a "msp430g2131"

P1.4 --------> || CT --------[P1.1]--------|| Cref -----------GND

Oops, you are right about the code. I was moving code lines/part to find the issue. It looks like it didn't make any (visual) difference.

For instance when I remove CT (and P1.4 becomes floating) I get almost the same ADC reading, around 920. How is that possible? Is it not sensitive enough?

Do you have a code sample? Any suggestions?

...or is there any better approach?

Thanks a lot.

Hoi!

Thanks again for your detailed reply and your time. I really appreciate your help.

I tried to create the setup you suggested. See image!

I use the following ports:

• P1.1 for Measurement
• P1.2 for Discharge (adc)
• P1.4 for Charge

This code for ADC setup:

ADC10CTL1 = INCH_1;             // A1
ADC10CTL0 = ADC10ON + ADC10IE;	// ADC10 On + Interrupt
ADC10AE0 |= PinCapacitance;     // P1.1 ADC10 option select

The following code for reading (in a while(1)-loop):

// 1) Prepare for measure (discharge)
P1DIR &= ~PinCapacitance;
P1OUT &= ~PinDischarge;		// Make Discharge pin low
P1OUT &= ~PinCharge;		// Make Charge pin low
ADC10AE0 &= ~PinDischarge;	// Disable AE
sleep(10000);

// 2) Measure
ADC10CTL0 |= ENC + ADC10SC;	// Enable ADC
ADC10AE0 |= PinDischarge;	// Enable AE
P1OUT |= PinCharge;		// Make Charge pin high
while (ADC10CTL1 & BUSY) { }    // Wait for ADC
adcValue = ADC10MEM;		// Read results
ADC10CTL0 &= ~ENC;		// Disable conversion

// 3) Clean
P1OUT &= ~PinCharge;		// Make Charge pin low
P1OUT &= ~PinDischarge;		// Make Discharge pin low
ADC10AE0 &= ~PinDischarge;	// Disable AE

The results:

CT = *nothing*, CRef = 20pF, ADC = 916
CT = 10pF, CRef = 20pF, ADC = 921
CT = 20pF, CRef = 20pF, ADC = 928
CT = 40pF, CRef = 20pF, ADC = 935
CT = 100pF, CRef = 20pF, ADC = 955
CT = 1nF, CRef = 20pF, ADC = 1020

CT = 100pF, CRef = 10pF, ADC = 954
CT = 40pF, CRef = 100pF, ADC = 961

My questions:

1) First of all, did I miss something in my setup or code?

2) How I can stretch the values more? From 0..1023 (instead of 916..1021)?

3) Is it possible to create a function out of these readings?

4) Touching the CT capacitor doen't make any (visible) difference in the readings. Is that because of the limited reading-range?

4) I'm measuring the ADC value in the CCS debugger (after measurement), does it influence the outcome?

5) Currently i'm working with ceramic disc capacitors. 

Thanks!!

Quick reply;

I've not pasted all my code, but I have an interrupt:

#pragma vector = ADC10_VECTOR
__interrupt void ADC10_ISR(void)
{
  __bic_SR_register_on_exit(CPUOFF);
}

I'll try that test; i'll keep ya posted!

Analyzing your measurement values it looks like you have enabled the internal Pull-Up on P1.1 or P1.2!

Pull-up looks like disabled: P1REN = 0x00

According to my multi-meter it's around 3.55V (USB powered launchpad)

Just to make sure: When I directly connect my P1.1 to GND it read the expected ~0, and when connected to VCC it reads ~1023.

So, I have the feeling it's some stupid setting I can't find! :)

My GPIO setup:
P1DIR = 0xFF;
P1OUT = 0;
P2DIR = 0xFF;
P2OUT = 0;

P1DIR &= ~PinCapacitance;
P1DIR |= PinCharge;
P1DIR &= ~PinDischarge;

P1OUT &= ~PinCapacitance;
P1OUT &= ~PinCharge;
P1OUT &= ~PinDischarge;

Hehe, yes I can imagine. My current code looks like this:

#include <msp430.h> 

//Defines
#define PinLedGreen 	BIT6				// 1.6
#define PinLedRed 		BIT0				// 1.0
#define PinCapacitance	BIT1				// 1.1
#define PinDischarge		BIT2 				// 1.2
#define PinCharge 		BIT4 				// 1.4

int adcValue = 0;

int main(void)
{
         // Initialize
	 WDTCTL = WDTPW | WDTHOLD;	// Stop watchdog timer

	 P1DIR = 0xFF;                             // All P1.x outputs
	 P1OUT = 0;                                	// All P1.x reset
	 P2DIR = 0xFF;                             // All P2.x outputs
	 P2OUT = 0;                                	// All P2.x reset

	 //Gpio's
	 P1DIR |= PinLedGreen + PinLedRed; 	// Output
	 P1DIR &= ~PinCapacitance;
	 P1DIR |= PinCharge;
	 P1DIR &= ~PinDischarge;

	 P1OUT &= ~PinLedGreen;
	 P1OUT &= ~PinCapacitance;
	 P1OUT &= ~PinLedRed;
	 P1OUT &= ~PinCharge;
	 P1OUT &= ~PinDischarge;

	 //Analog
	 ADC10CTL1 = INCH_1;               	                // A1
	 ADC10CTL0 = ADC10ON + ADC10IE;		// ADC10 On + Interrupt
	 ADC10AE0 |= PinCapacitance;                      // P1.1 ADC10 option select

    while (1)
    {
    	P1OUT &= ~PinLedRed;
    	P1OUT &= ~PinLedGreen;

    	// 1) Prepare for measure (discharge)
    	P1OUT &= ~PinDischarge;						// Make Discharge pin low
    	P1OUT &= ~PinCharge;						// Make Charge pin low
    	ADC10AE0 &= ~PinDischarge;					// Disable AE
    	__delay_cycles(100000);
    	// 2) Measure
    	ADC10CTL0 |= ENC + ADC10SC;					// Enable ADC
    	ADC10AE0 |= PinDischarge;					// Enable AE
    	P1OUT |= PinCharge;							// Make Charge pin high
    	P1OUT |= PinDischarge;						// Make Discharge pin high
	while (ADC10CTL1 & BUSY) { }    			// Wait for ADC
	adcValue = ADC10MEM;						// Read results
	ADC10CTL0 &= ~ENC;							// Disable conversion

    	// 3) Clean
      	P1OUT &= ~PinCharge;		// Make Charge pin low
      	P1OUT &= ~PinDischarge;		// Make Discharge pin low
      	ADC10AE0 &= ~PinDischarge;	// Disable AE

	// 4) Result
	if (adcValue < 975) //I Measured the ADC value here!
	{
	P1OUT |= PinLedRed;
	}
	else
	{
	P1OUT |= PinLedGreen;
	}
	__delay_cycles(1000000);
    }
}

//////////////////////////////////////////////////////////////////////////////////////////////
#pragma vector = ADC10_VECTOR
__interrupt void ADC10_ISR(void)
{

}


#pragma vector=TIMER0_A0_VECTOR
__interrupt void TIMER0_A0_ISR(void)
{

}

Cool! You are a hero! I should buy you a beer! ;)

btw: how did the TxD/RxD jumpers help me here? What did they block?

I'm going to play around with this (difference caps sizes / times). Thanks a lot!