really improve stability by turning on PGA capacitor!

Author: ladyada

Adafruit_NAU7802.cpp

@@ -63,13 +63,34 @@ bool Adafruit_NAU7802::begin(TwoWire *theWire) {
 
   if (!reset())
     return false;
+  if (!enable(true))
+    return false;
   if (!setLDO(NAU7802_3V0))
     return false;
   if (!setGain(NAU7802_GAIN_128))
     return false;
   if (!setRate(NAU7802_RATE_10SPS))
     return false;
-  if (!enable(true))
+
+  // disable ADC chopper clock
+  Adafruit_I2CRegister adc_reg = Adafruit_I2CRegister(i2c_dev, NAU7802_ADC);
+  Adafruit_I2CRegisterBits chop =
+      Adafruit_I2CRegisterBits(&adc_reg, 2, 4); // # bits, bit_shift
+  if (!chop.write(0x3))
+    return false;
+
+  // use low ESR caps
+  Adafruit_I2CRegister pga_reg = Adafruit_I2CRegister(i2c_dev, NAU7802_PGA);
+  Adafruit_I2CRegisterBits ldomode =
+      Adafruit_I2CRegisterBits(&pga_reg, 1, 6); // # bits, bit_shift
+  if (!ldomode.write(0))
+    return false;
+
+  // PGA stabilizer cap on output
+  Adafruit_I2CRegister pwr_reg = Adafruit_I2CRegister(i2c_dev, NAU7802_POWER);
+  Adafruit_I2CRegisterBits capen =
+      Adafruit_I2CRegisterBits(&pwr_reg, 1, 7); // # bits, bit_shift
+  if (!capen.write(1))
     return false;
 
   return true;
@@ -286,3 +307,31 @@ NAU7802_SampleRate Adafruit_NAU7802::getRate(void) {
 
   return (NAU7802_SampleRate)rate_select.read();
 }
+
+/**************************************************************************/
+/*!
+    @brief  Perform the internal calibration procedure
+    @param mode The calibration mode to perform: NAU7802_CALMOD_INTERNAL,
+    NAU7802_CALMOD_OFFSET or NAU7802_CALMOD_GAIN
+    @returns True on calibrations success
+*/
+/**************************************************************************/
+bool Adafruit_NAU7802::calibrate(NAU7802_Calibration mode) {
+  Adafruit_I2CRegister ctrl2_reg = Adafruit_I2CRegister(i2c_dev, NAU7802_CTRL2);
+  Adafruit_I2CRegisterBits cal_start =
+      Adafruit_I2CRegisterBits(&ctrl2_reg, 1, 2); // # bits, bit_shift
+  Adafruit_I2CRegisterBits cal_err =
+      Adafruit_I2CRegisterBits(&ctrl2_reg, 1, 3); // # bits, bit_shift
+  Adafruit_I2CRegisterBits cal_mod =
+      Adafruit_I2CRegisterBits(&ctrl2_reg, 2, 0); // # bits, bit_shift
+
+  if (!cal_mod.write(mode))
+    return false;
+  if (!cal_start.write(true))
+    return false;
+  while (!cal_start.read()) {
+    delay(10);
+  }
+
+  return !cal_err.read();
+}

Adafruit_NAU7802.h

@@ -25,6 +25,9 @@
 #define NAU7802_CTRL1 0x01           ///< Control/config register #1
 #define NAU7802_CTRL2 0x02           ///< Control/config register #2
 #define NAU7802_ADCO_B2 0x12         ///< ADC ouput LSB
+#define NAU7802_ADC 0x15             ///< ADC / chopper control
+#define NAU7802_PGA 0x1B             ///< PGA control
+#define NAU7802_POWER 0x1C           ///< power control
 #define NAU7802_REVISION_ID 0x1F     ///< Chip revision ID
 
 /*! The possible LDO voltages */
@@ -61,6 +64,13 @@ typedef enum _sample_rates {
   NAU7802_RATE_320SPS = 7,
 } NAU7802_SampleRate;
 
+/*! The possible calibration modes */
+typedef enum _calib_mode {
+  NAU7802_CALMOD_INTERNAL = 0,
+  NAU7802_CALMOD_OFFSET = 2,
+  NAU7802_CALMOD_GAIN = 3,
+} NAU7802_Calibration;
+
 /**************************************************************************/
 /*!
     @brief  NAU7802 driver.
@@ -81,6 +91,7 @@ class Adafruit_NAU7802 {
   NAU7802_Gain getGain(void);
   bool setRate(NAU7802_SampleRate gain);
   NAU7802_SampleRate getRate(void);
+  bool calibrate(NAU7802_Calibration mode);
 
 private:
   Adafruit_I2CRegister *_pu_ctrl_reg = NULL;

examples/nau7802_test/nau7802_test.ino

@@ -46,14 +46,30 @@ void setup() {
     case NAU7802_RATE_80SPS:  Serial.println("80 SPS"); break;
     case NAU7802_RATE_320SPS:  Serial.println("320 SPS"); break;
   }
+
+  // Take 10 readings to flush out readings
+  for (uint8_t i=0; i<10; i++) {
+    while (! nau.available()) delay(1);
+    nau.read();
+  }
+
+  while (! nau.calibrate(NAU7802_CALMOD_INTERNAL)) {
+    Serial.println("Failed to calibrate internal offset, retrying!");
+    delay(1000);
+  }
+  Serial.println("Calibrated internal offset");
+
+  while (! nau.calibrate(NAU7802_CALMOD_OFFSET)) {
+    Serial.println("Failed to calibrate system offset, retrying!");
+    delay(1000);
+  }
+  Serial.println("Calibrated system offset");
 }
 
 void loop() {
   while (! nau.available()) {
     delay(1);
   }
-  Serial.println("Ready!");
   int32_t val = nau.read();
   Serial.print("Read "); Serial.println(val);
-  delay(1000);
 }