The nRF52 family includes an adjustable 'successive-approximation ADC' which can be configured to convert data with up to 14-bit resolution (0..16383), and the reference voltage can be adjusted up to 3.6V internally.
The default values for the ADC are 10-bit resolution (0..1023) with a 3.6V reference voltage, meaning every digit returned from the ADC = 3600mV/1024 = 3.515625mV.
Analog Reference Voltage
The internal reference voltage is 0.6V with a variable gain setting, and can be adjust via the analogReference(...) function, providing one of the following values:
- AR_INTERNAL (0.6V Ref * 6 = 0..3.6V) <-- DEFAULT
- AR_INTERNAL_3_0 (0.6V Ref * 5 = 0..3.0V)
- AR_INTERNAL_2_4 (0.6V Ref * 4 = 0..2.4V)
- AR_INTERNAL_1_8 (0.6V Ref * 3 = 0..1.8V)
- AR_INTERNAL_1_2 (0.6V Ref * 2 = 0..1.6V)
- AR_VDD4 (VDD/4 REF * 4 = 0..VDD)
For example:
// Set the analog reference to 3.0V (default = 3.6V) analogReference(AR_INTERNAL_3_0);
Analog Resolution
The ADC resolution can be set to 8, 10, 12 or 14 bits using the analogReadResolution(...) function, with the default value being 10-bit:
// Set the resolution to 12-bit (0..4095) analogReadResolution(12); // Can be 8, 10, 12 or 14
Default ADC Example (10-bit, 3.6V Reference)
The original source for this code is included in the nRF52 BSP and can be viewed online here.
#include <Arduino.h>
#include <Adafruit_TinyUSB.h> // for Serial
int adcin = A5;
int adcvalue = 0;
float mv_per_lsb = 3600.0F/1024.0F; // 10-bit ADC with 3.6V input range
void setup() {
Serial.begin(115200);
while ( !Serial ) delay(10); // for nrf52840 with native usb
}
void loop() {
// Get a fresh ADC value
adcvalue = analogRead(adcin);
// Display the results
Serial.print(adcvalue);
Serial.print(" [");
Serial.print((float)adcvalue * mv_per_lsb);
Serial.println(" mV]");
delay(100);
}
Advanced Example (12-bit, 3.0V Reference)
The original source for this code is included in the nRF52 BSP and can be viewed online here.
#include <Arduino.h>
#include <Adafruit_TinyUSB.h> // for Serial
#if defined ARDUINO_NRF52840_CIRCUITPLAY
#define PIN_VBAT A8 // this is just a mock read, we'll use the light sensor, so we can run the test
#endif
uint32_t vbat_pin = PIN_VBAT; // A7 for feather nRF52832, A6 for nRF52840
#define VBAT_MV_PER_LSB (0.73242188F) // 3.0V ADC range and 12-bit ADC resolution = 3000mV/4096
#ifdef NRF52840_XXAA
#define VBAT_DIVIDER (0.5F) // 150K + 150K voltage divider on VBAT
#define VBAT_DIVIDER_COMP (2.0F) // Compensation factor for the VBAT divider
#else
#define VBAT_DIVIDER (0.71275837F) // 2M + 0.806M voltage divider on VBAT = (2M / (0.806M + 2M))
#define VBAT_DIVIDER_COMP (1.403F) // Compensation factor for the VBAT divider
#endif
#define REAL_VBAT_MV_PER_LSB (VBAT_DIVIDER_COMP * VBAT_MV_PER_LSB)
float readVBAT(void) {
float raw;
// Set the analog reference to 3.0V (default = 3.6V)
analogReference(AR_INTERNAL_3_0);
// Set the resolution to 12-bit (0..4095)
analogReadResolution(12); // Can be 8, 10, 12 or 14
// Let the ADC settle
delay(1);
// Get the raw 12-bit, 0..3000mV ADC value
raw = analogRead(vbat_pin);
// Set the ADC back to the default settings
analogReference(AR_DEFAULT);
analogReadResolution(10);
// Convert the raw value to compensated mv, taking the resistor-
// divider into account (providing the actual LIPO voltage)
// ADC range is 0..3000mV and resolution is 12-bit (0..4095)
return raw * REAL_VBAT_MV_PER_LSB;
}
uint8_t mvToPercent(float mvolts) {
if(mvolts<3300)
return 0;
if(mvolts <3600) {
mvolts -= 3300;
return mvolts/30;
}
mvolts -= 3600;
return 10 + (mvolts * 0.15F ); // thats mvolts /6.66666666
}
void setup() {
Serial.begin(115200);
while ( !Serial ) delay(10); // for nrf52840 with native usb
// Get a single ADC sample and throw it away
readVBAT();
}
void loop() {
// Get a raw ADC reading
float vbat_mv = readVBAT();
// Convert from raw mv to percentage (based on LIPO chemistry)
uint8_t vbat_per = mvToPercent(vbat_mv);
// Display the results
Serial.print("LIPO = ");
Serial.print(vbat_mv);
Serial.print(" mV (");
Serial.print(vbat_per);
Serial.println("%)");
delay(1000);
}
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