Hi all,
I am using a BLE nano 2, which is based on the nRF52832. I am trying to measure small capacities of a few pF with this method: wordpress.codewrite.co.uk/.../ And the BLE nano 2: www.kickstarter.com/.../description
So the code that I am using is:
const int OUT_PIN = A3; //P0_2
const int IN_PIN = A5; //P0_4
//Capacitance between IN_PIN and Ground
//Stray capacitance is always present. Extra capacitance can be added to
//allow higher capacitance to be measured.
const float IN_STRAY_CAP_TO_GND = 24.48; //initially this was 30.00
const float IN_EXTRA_CAP_TO_GND = 0.0;
const float IN_CAP_TO_GND = IN_STRAY_CAP_TO_GND + IN_EXTRA_CAP_TO_GND;
const int MAX_ADC_VALUE = 1023;
void setup()
{
pinMode(OUT_PIN, OUTPUT);
//digitalWrite(OUT_PIN, LOW); //This is the default state for outputs
pinMode(IN_PIN, OUTPUT);
//digitalWrite(IN_PIN, LOW);
Serial.begin(115200);
}
void loop()
{
//Capacitor under test between OUT_PIN and IN_PIN
//Rising high edge on OUT_PIN
pinMode(IN_PIN, INPUT);
digitalWrite(OUT_PIN, HIGH);
int val = analogRead(IN_PIN);
//Clear everything for next measurement
digitalWrite(OUT_PIN, LOW);
pinMode(IN_PIN, OUTPUT);
//Calculate and print result
float capacitance = (float)val * IN_CAP_TO_GND / (float)(MAX_ADC_VALUE - val);
Serial.print(F("Capacitance Value = "));
Serial.print(capacitance, 3);
Serial.print(F(" pF ("));
Serial.print(val);
Serial.println(F(") "));
while (millis() % 500 != 0)
;
}
When using an Arduino Uno (10 Bit ADC), everything is fine and I have about 1 pF noise. But when using the nRF52832-based chip, I have a strong noise. The readings strongly depend on the Pins which I am using (AX). With some it does not work at all, e.g. P0_28 (A2). I had the best results, when using P0_2 and P0_4 (A3, A5). The noise in this configuration still is about +-10 pF, no matter if using 10 Bit or 12 Bit resolution. I have had similar issues, when using an Arduino Due, which is also based on a Cortex M4 chip....
Do you have any idea, why I have these issues with the changed hardware?
I would appreciate any hint.
Best regards, Christian
