Subversion Repositories FlightCtrl

// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
// + Copyright (c) 04.2007 Holger Buss
// + Nur für den privaten Gebrauch
// + www.MikroKopter.com
// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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// ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>

#include "analog.h"
#include "attitude.h"
#include "sensors.h"
#include "printf_P.h"

// for Delay functions
#include "timer0.h"

// For debugOut
#include "uart0.h"

// For reading and writing acc. meter offsets.
#include "eeprom.h"

// For debugOut.digital
#include "output.h"

// set ADC enable & ADC Start Conversion & ADC Interrupt Enable bit
#define startADC() (ADCSRA |= (1<<ADEN)|(1<<ADSC)|(1<<ADIE))

const char* recal = ", recalibration needed.";

/*
 * For each A/D conversion cycle, each analog channel is sampled a number of times
 * (see array channelsForStates), and the results for each channel are summed.
 * Here are those for the gyros and the acc. meters. They are not zero-offset.
 * They are exported in the analog.h file - but please do not use them! The only
 * reason for the export is that the ENC-03_FC1.3 modules needs them for calibrating
 * the offsets with the DAC.
 */
volatile uint16_t sensorInputs[8];
int16_t acc[3];
int16_t filteredAcc[3] = { 0,0,0 };

/*
 * These 4 exported variables are zero-offset. The "PID" ones are used
 * in the attitude control as rotation rates. The "ATT" ones are for
 * integration to angles.
 */
int16_t gyro_PID[2];
int16_t gyro_ATT[2];
int16_t gyroD[2];
int16_t yawGyro;

/*
 * Offset values. These are the raw gyro and acc. meter sums when the copter is
 * standing still. They are used for adjusting the gyro and acc. meter values
 * to be centered on zero.
 */

sensorOffset_t gyroOffset;
sensorOffset_t accOffset;
sensorOffset_t gyroAmplifierOffset;

/*
 * In the MK coordinate system, nose-down is positive and left-roll is positive.
 * If a sensor is used in an orientation where one but not both of the axes has
 * an opposite sign, PR_ORIENTATION_REVERSED is set to 1 (true).
 * Transform:
 * pitch <- pp*pitch + pr*roll
 * roll  <- rp*pitch + rr*roll
 * Not reversed, GYRO_QUADRANT:
 * 0: pp=1, pr=0, rp=0, rr=1  // 0    degrees
 * 1: pp=1, pr=-1,rp=1, rr=1  // +45  degrees
 * 2: pp=0, pr=-1,rp=1, rr=0  // +90  degrees
 * 3: pp=-1,pr=-1,rp=1, rr=1  // +135 degrees
 * 4: pp=-1,pr=0, rp=0, rr=-1 // +180 degrees
 * 5: pp=-1,pr=1, rp=-1,rr=-1 // +225 degrees
 * 6: pp=0, pr=1, rp=-1,rr=0  // +270 degrees
 * 7: pp=1, pr=1, rp=-1,rr=1  // +315 degrees
 * Reversed, GYRO_QUADRANT:
 * 0: pp=-1,pr=0, rp=0, rr=1  // 0    degrees with pitch reversed
 * 1: pp=-1,pr=-1,rp=-1,rr=1  // +45  degrees with pitch reversed
 * 2: pp=0, pr=-1,rp=-1,rr=0  // +90  degrees with pitch reversed
 * 3: pp=1, pr=-1,rp=-1,rr=1  // +135 degrees with pitch reversed
 * 4: pp=1, pr=0, rp=0, rr=-1 // +180 degrees with pitch reversed
 * 5: pp=1, pr=1, rp=1, rr=-1 // +225 degrees with pitch reversed
 * 6: pp=0, pr=1, rp=1, rr=0  // +270 degrees with pitch reversed
 * 7: pp=-1,pr=1, rp=1, rr=1  // +315 degrees with pitch reversed
 */

void rotate(int16_t* result, uint8_t quadrant, uint8_t reverse) {}

/*
void rotate(int16_t* result, uint8_t quadrant, uint8_t reverse) {
  static const int8_t rotationTab[] = {1,1,0,-1,-1,-1,0,1};
  // Pitch to Pitch part
  int8_t xx = (reverse & 1) ? rotationTab[(quadrant+4)%8] : rotationTab[quadrant];
  // Roll to Pitch part
  int8_t xy = rotationTab[(quadrant+2)%8];
  // Pitch to Roll part
  int8_t yx = reverse ? rotationTab[(quadrant+2)%8] : rotationTab[(quadrant+6)%8];
  // Roll to Roll part
  int8_t yy = rotationTab[quadrant];

  int16_t xIn = result[0];
  result[0] = xx*result[0] + xy*result[1];
  result[1] = yx*xIn + yy*result[1];
 
  if (quadrant & 1) {
        // A rotation was used above, where the factors were too large by sqrt(2).
        // So, we multiply by 2^n/sqt(2) and right shift n bits, as to divide by sqrt(2).
        // A suitable value for n: Sample is 11 bits. After transformation it is the sum
        // of 2 11 bit numbers, so 12 bits. We have 4 bits left...
        result[0] = (result[0]*11) >> 4;
        result[1] = (result[1]*11) >> 4;
  }
}
*/
/*
 * Air pressure
 */
volatile uint8_t rangewidth = 105;

// Direct from sensor, irrespective of range.
// volatile uint16_t rawAirPressure;

// Value of 2 samples, with range.
uint16_t simpleAirPressure;

// Value of AIRPRESSURE_SUMMATION_FACTOR samples, with range, filtered.
int32_t filteredAirPressure;

// Partial sum of AIRPRESSURE_SUMMATION_FACTOR samples.
int32_t airPressureSum;

// The number of samples summed into airPressureSum so far.
uint8_t pressureMeasurementCount;

/*
 * Battery voltage, in units of: 1k/11k / 3V * 1024 = 31.03 per volt.
 * That is divided by 3 below, for a final 10.34 per volt.
 * So the initial value of 100 is for 9.7 volts.
 */
int16_t UBat = 100;

/*
 * Control and status.
 */
volatile uint16_t ADCycleCount = 0;
volatile uint8_t analogDataReady = 1;

/*
 * Experiment: Measuring vibration-induced sensor noise.
 */
uint16_t gyroNoisePeak[3];
uint16_t accNoisePeak[3];

volatile uint8_t adState;
volatile uint8_t adChannel;

// ADC channels
#define AD_GYRO_YAW       0
#define AD_GYRO_ROLL      1
#define AD_GYRO_PITCH     2
#define AD_AIRPRESSURE    3
#define AD_UBAT           4
#define AD_ACC_Z          5
#define AD_ACC_ROLL       6
#define AD_ACC_PITCH      7

/*
 * Table of AD converter inputs for each state.
 * The number of samples summed for each channel is equal to
 * the number of times the channel appears in the array.
 * The max. number of samples that can be taken in 2 ms is:
 * 20e6 / 128 / 13 / (1/2e-3) = 24. Since the main control
 * loop needs a little time between reading AD values and
 * re-enabling ADC, the real limit is (how much?) lower.
 * The acc. sensor is sampled even if not used - or installed
 * at all. The cost is not significant.
 */

const uint8_t channelsForStates[] PROGMEM = {
  AD_GYRO_PITCH, AD_GYRO_ROLL, AD_GYRO_YAW,
  AD_ACC_PITCH, AD_ACC_ROLL, AD_AIRPRESSURE,

  AD_GYRO_PITCH, AD_GYRO_ROLL, AD_ACC_Z, // at 8, measure Z acc.
  AD_GYRO_PITCH, AD_GYRO_ROLL, AD_GYRO_YAW, // at 11, finish yaw gyro
 
  AD_ACC_PITCH,   // at 12, finish pitch axis acc.
  AD_ACC_ROLL,    // at 13, finish roll axis acc.
  AD_AIRPRESSURE, // at 14, finish air pressure.
 
  AD_GYRO_PITCH,  // at 15, finish pitch gyro
  AD_GYRO_ROLL,   // at 16, finish roll gyro
  AD_UBAT         // at 17, measure battery.
};

// Feature removed. Could be reintroduced later - but should work for all gyro types then.
// uint8_t GyroDefectPitch = 0, GyroDefectRoll = 0, GyroDefectYaw = 0;

void analog_init(void) {
        uint8_t sreg = SREG;
        // disable all interrupts before reconfiguration
        cli();

        //ADC0 ... ADC7 is connected to PortA pin 0 ... 7
        DDRA = 0x00;
        PORTA = 0x00;
        // Digital Input Disable Register 0
        // Disable digital input buffer for analog adc_channel pins
        DIDR0 = 0xFF;
        // external reference, adjust data to the right
        ADMUX &= ~((1<<REFS1)|(1<<REFS0)|(1<<ADLAR));
        // set muxer to ADC adc_channel 0 (0 to 7 is a valid choice)
        ADMUX = (ADMUX & 0xE0);
        //Set ADC Control and Status Register A
        //Auto Trigger Enable, Prescaler Select Bits to Division Factor 128, i.e. ADC clock = SYSCKL/128 = 156.25 kHz
        ADCSRA = (1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
        //Set ADC Control and Status Register B
        //Trigger Source to Free Running Mode
        ADCSRB &= ~((1<<ADTS2)|(1<<ADTS1)|(1<<ADTS0));

        startAnalogConversionCycle();

        // restore global interrupt flags
        SREG = sreg;
}

uint16_t rawGyroValue(uint8_t axis) {
        return sensorInputs[AD_GYRO_PITCH-axis];
}

uint16_t rawAccValue(uint8_t axis) {
        return sensorInputs[AD_ACC_PITCH-axis];
}

void measureNoise(const int16_t sensor,
                volatile uint16_t* const noiseMeasurement, const uint8_t damping) {
        if (sensor > (int16_t) (*noiseMeasurement)) {
                *noiseMeasurement = sensor;
        } else if (-sensor > (int16_t) (*noiseMeasurement)) {
                *noiseMeasurement = -sensor;
        } else if (*noiseMeasurement > damping) {
                *noiseMeasurement -= damping;
        } else {
                *noiseMeasurement = 0;
        }
}

/*
 * Min.: 0
 * Max: About 106 * 240 + 2047 = 27487; it is OK with just a 16 bit type.
 */
uint16_t getSimplePressure(int advalue) {
        return (uint16_t) OCR0A * (uint16_t) rangewidth + advalue;
}

void startAnalogConversionCycle(void) {
  analogDataReady = 0;

  // Stop the sampling. Cycle is over.
  for (uint8_t i = 0; i < 8; i++) {
    sensorInputs[i] = 0;
  }
  adState = 0;
  adChannel = AD_GYRO_PITCH;
  ADMUX = (ADMUX & 0xE0) | adChannel;
  startADC();
}

/*****************************************************
 * Interrupt Service Routine for ADC
 * Runs at 312.5 kHz or 3.2 �s. When all states are
 * processed further conversions are stopped.
 *****************************************************/
ISR(ADC_vect) {
  sensorInputs[adChannel] += ADC;
  // set up for next state.
  adState++;
  if (adState < sizeof(channelsForStates)) {
    adChannel = pgm_read_byte(&channelsForStates[adState]);
    // set adc muxer to next adChannel
    ADMUX = (ADMUX & 0xE0) | adChannel;
    // after full cycle stop further interrupts
    startADC();
  } else {
    ADCycleCount++;
    analogDataReady = 1;
    // do not restart ADC converter.
  }
}

void analog_updateGyros(void) {
  // for various filters...
  int16_t tempOffsetGyro[2], tempGyro;
 
  debugOut.digital[0] &= ~DEBUG_SENSORLIMIT;
  for (uint8_t axis=0; axis<2; axis++) {
    tempGyro = rawGyroValue(axis);
    /*
     * Process the gyro data for the PID controller.
     */
    // 1) Extrapolate: Near the ends of the range, we boost the input significantly. This simulates a
    //    gyro with a wider range, and helps counter saturation at full control.
   
    if (staticParams.bitConfig & CFG_GYRO_SATURATION_PREVENTION) {
      if (tempGyro < SENSOR_MIN_PITCHROLL) {
                debugOut.digital[0] |= DEBUG_SENSORLIMIT;
                tempGyro = tempGyro * EXTRAPOLATION_SLOPE - EXTRAPOLATION_LIMIT;
      } else if (tempGyro > SENSOR_MAX_PITCHROLL) {
                debugOut.digital[0] |= DEBUG_SENSORLIMIT;
                tempGyro = (tempGyro - SENSOR_MAX_PITCHROLL) * EXTRAPOLATION_SLOPE + SENSOR_MAX_PITCHROLL;
      }
    }

    // 2) Apply sign and offset, scale before filtering.
    tempOffsetGyro[axis] = (tempGyro - gyroOffset.offsets[axis]) * GYRO_FACTOR_PITCHROLL;
  }

  // 2.1: Transform axes.
  rotate(tempOffsetGyro, staticParams.gyroQuadrant, staticParams.imuReversedFlags & 1);

  for (uint8_t axis=0; axis<2; axis++) {
        // 3) Filter.
    tempOffsetGyro[axis] = (gyro_PID[axis] * (staticParams.gyroPIDFilterConstant - 1) + tempOffsetGyro[axis]) / staticParams.gyroPIDFilterConstant;

    // 4) Measure noise.
    measureNoise(tempOffsetGyro[axis], &gyroNoisePeak[axis], GYRO_NOISE_MEASUREMENT_DAMPING);

    // 5) Differential measurement.
    gyroD[axis] = (gyroD[axis] * (staticParams.gyroDFilterConstant - 1) + (tempOffsetGyro[axis] - gyro_PID[axis])) / staticParams.gyroDFilterConstant;

    // 6) Done.
    gyro_PID[axis] = tempOffsetGyro[axis];

    // Prepare tempOffsetGyro for next calculation below...
    tempOffsetGyro[axis] = (rawGyroValue(axis) - gyroOffset.offsets[axis]) * GYRO_FACTOR_PITCHROLL;
  }
 
  /*
   * Now process the data for attitude angles.
   */
   rotate(tempOffsetGyro, staticParams.gyroQuadrant, staticParams.imuReversedFlags & 1);

   gyro_ATT[PITCH] = tempOffsetGyro[PITCH];
   gyro_ATT[ROLL] = tempOffsetGyro[ROLL];

   debugOut.analog[22 + 0] = gyro_PID[0];
   debugOut.analog[22 + 1] = gyro_PID[1];

   debugOut.analog[24 + 0] = gyro_ATT[0];
   debugOut.analog[24 + 1] = gyro_ATT[1];

  // 2) Filter. This should really be quite unnecessary. The integration should gobble up any noise anyway and the values are not used for anything else.
  // gyro_ATT[PITCH] = (gyro_ATT[PITCH] * (staticParams.attitudeGyroFilter - 1) + tempOffsetGyro[PITCH]) / staticParams.attitudeGyroFilter;
  // gyro_ATT[ROLL]  = (gyro_ATT[ROLL]  * (staticParams.attitudeGyroFilter - 1) + tempOffsetGyro[ROLL])  / staticParams.attitudeGyroFilter;

  // Yaw gyro.
  if (staticParams.imuReversedFlags & 2)
    yawGyro = gyroOffset.offsets[YAW] - sensorInputs[AD_GYRO_YAW];
  else
    yawGyro = sensorInputs[AD_GYRO_YAW] - gyroOffset.offsets[YAW];
}

void analog_updateAccelerometers(void) {
  // Pitch and roll axis accelerations.
  for (uint8_t axis=0; axis<2; axis++) {
    acc[axis] = rawAccValue(axis) - accOffset.offsets[axis];
  }

  rotate(acc, staticParams.accQuadrant, staticParams.imuReversedFlags & 4);

  for(uint8_t axis=0; axis<3; axis++) {
    filteredAcc[axis] = (filteredAcc[axis] * (staticParams.accFilterConstant - 1) + acc[axis]) / staticParams.accFilterConstant;
    measureNoise(acc[axis], &accNoisePeak[axis], 1);
  }

  // Z acc.
  if (staticParams.imuReversedFlags & 8)
    acc[Z] = accOffset.offsets[Z] - sensorInputs[AD_ACC_Z];
  else
    acc[Z] = sensorInputs[AD_ACC_Z] - accOffset.offsets[Z];
}

void analog_updateAirPressure(void) {
  static uint16_t pressureAutorangingWait = 25;
  uint16_t rawAirPressure;
  int16_t newrange;
  // air pressure
  if (pressureAutorangingWait) {
    //A range switch was done recently. Wait for steadying.
    pressureAutorangingWait--;
    debugOut.analog[27] = (uint16_t) OCR0A;
    debugOut.analog[31] = simpleAirPressure;
  } else {
    rawAirPressure = sensorInputs[AD_AIRPRESSURE];
    if (rawAirPressure < MIN_RAWPRESSURE) {
      // value is too low, so decrease voltage on the op amp minus input, making the value higher.
      newrange = OCR0A - (MAX_RAWPRESSURE - MIN_RAWPRESSURE) / (rangewidth * 4); // 4; // (MAX_RAWPRESSURE - rawAirPressure) / (rangewidth * 2) + 1;
      if (newrange > MIN_RANGES_EXTRAPOLATION) {
        pressureAutorangingWait = (OCR0A - newrange) * AUTORANGE_WAIT_FACTOR; // = OCRA0 - OCRA0 +
        OCR0A = newrange;
      } else {
        if (OCR0A) {
          OCR0A--;
          pressureAutorangingWait = AUTORANGE_WAIT_FACTOR;
        }
      }
    } else if (rawAirPressure > MAX_RAWPRESSURE) {
      // value is too high, so increase voltage on the op amp minus input, making the value lower.
      // If near the end, make a limited increase
      newrange = OCR0A + (MAX_RAWPRESSURE - MIN_RAWPRESSURE) / (rangewidth * 4); // 4;  // (rawAirPressure - MIN_RAWPRESSURE) / (rangewidth * 2) - 1;
      if (newrange < MAX_RANGES_EXTRAPOLATION) {
        pressureAutorangingWait = (newrange - OCR0A) * AUTORANGE_WAIT_FACTOR;
        OCR0A = newrange;
      } else {
        if (OCR0A < 254) {
          OCR0A++;
          pressureAutorangingWait = AUTORANGE_WAIT_FACTOR;
        }
      }
    }
   
    // Even if the sample is off-range, use it.
    simpleAirPressure = getSimplePressure(rawAirPressure);
    debugOut.analog[27] = (uint16_t) OCR0A;
    debugOut.analog[31] = simpleAirPressure;
   
    if (simpleAirPressure < MIN_RANGES_EXTRAPOLATION * rangewidth) {
      // Danger: pressure near lower end of range. If the measurement saturates, the
      // copter may climb uncontrolledly... Simulate a drastic reduction in pressure.
      debugOut.digital[1] |= DEBUG_SENSORLIMIT;
      airPressureSum += (int16_t) MIN_RANGES_EXTRAPOLATION * rangewidth
        + (simpleAirPressure - (int16_t) MIN_RANGES_EXTRAPOLATION
           * rangewidth) * PRESSURE_EXTRAPOLATION_COEFF;
    } else if (simpleAirPressure > MAX_RANGES_EXTRAPOLATION * rangewidth) {
      // Danger: pressure near upper end of range. If the measurement saturates, the
      // copter may descend uncontrolledly... Simulate a drastic increase in pressure.
      debugOut.digital[1] |= DEBUG_SENSORLIMIT;
      airPressureSum += (int16_t) MAX_RANGES_EXTRAPOLATION * rangewidth
        + (simpleAirPressure - (int16_t) MAX_RANGES_EXTRAPOLATION
           * rangewidth) * PRESSURE_EXTRAPOLATION_COEFF;
    } else {
      // normal case.
      // If AIRPRESSURE_SUMMATION_FACTOR is an odd number we only want to add half the double sample.
      // The 2 cases above (end of range) are ignored for this.
      debugOut.digital[1] &= ~DEBUG_SENSORLIMIT;
      if (pressureMeasurementCount == AIRPRESSURE_SUMMATION_FACTOR - 1)
        airPressureSum += simpleAirPressure / 2;
      else
        airPressureSum += simpleAirPressure;
    }
   
    // 2 samples were added.
    pressureMeasurementCount += 2;
    if (pressureMeasurementCount >= AIRPRESSURE_SUMMATION_FACTOR) {
      filteredAirPressure = (filteredAirPressure * (AIRPRESSURE_FILTER - 1)
                             + airPressureSum + AIRPRESSURE_FILTER / 2) / AIRPRESSURE_FILTER;
      pressureMeasurementCount = airPressureSum = 0;
    }
  }
}

void analog_updateBatteryVoltage(void) {
  // Battery. The measured value is: (V * 1k/11k)/3v * 1024 = 31.03 counts per volt (max. measurable is 33v).
  // This is divided by 3 --> 10.34 counts per volt.
  UBat = (3 * UBat + sensorInputs[AD_UBAT] / 3) / 4;
  debugOut.analog[11] = UBat;
  debugOut.analog[21] = sensorInputs[AD_UBAT];
}

void analog_update(void) {
  analog_updateGyros();
  analog_updateAccelerometers();
  analog_updateAirPressure();
  analog_updateBatteryVoltage();
}

void analog_setNeutral() {
  gyro_init();
 
  if (gyroOffset_readFromEEProm()) {
    printf("gyro offsets invalid%s",recal);
    gyroOffset.offsets[PITCH] = gyroOffset.offsets[ROLL] = 512 * GYRO_SUMMATION_FACTOR_PITCHROLL;
    gyroOffset.offsets[YAW] = 512 * GYRO_SUMMATION_FACTOR_YAW;
  }
 
  if (accOffset_readFromEEProm()) {
    printf("acc. meter offsets invalid%s",recal);
    accOffset.offsets[PITCH] = accOffset.offsets[ROLL] = 512 * ACC_SUMMATION_FACTOR_PITCHROLL;
    accOffset.offsets[Z] = 717 * ACC_SUMMATION_FACTOR_Z;
  }

  // Noise is relative to offset. So, reset noise measurements when changing offsets.
  gyroNoisePeak[PITCH] = gyroNoisePeak[ROLL] = 0;
  accNoisePeak[PITCH] = accNoisePeak[ROLL] = 0;
 
  // Setting offset values has an influence in the analog.c ISR
  // Therefore run measurement for 100ms to achive stable readings
  delay_ms_with_adc_measurement(100, 0);

  // Rough estimate. Hmm no nothing happens at calibration anyway.
  // airPressureSum = simpleAirPressure * (AIRPRESSURE_SUMMATION_FACTOR/2);
  // pressureMeasurementCount = 0;
}

void analog_calibrateGyros(void) {
#define GYRO_OFFSET_CYCLES 32
  uint8_t i, axis;
  int32_t offsets[3] = { 0, 0, 0 };
  gyro_calibrate();
 
  // determine gyro bias by averaging (requires that the copter does not rotate around any axis!)
  for (i = 0; i < GYRO_OFFSET_CYCLES; i++) {
    delay_ms_with_adc_measurement(10, 1);
    for (axis = PITCH; axis <= YAW; axis++) {
      offsets[axis] += rawGyroValue(axis);
    }
  }
 
  for (axis = PITCH; axis <= YAW; axis++) {
    gyroOffset.offsets[axis] = (offsets[axis] + GYRO_OFFSET_CYCLES / 2) / GYRO_OFFSET_CYCLES;

    int16_t min = (512-200) * (axis==YAW) ? GYRO_SUMMATION_FACTOR_YAW : GYRO_SUMMATION_FACTOR_PITCHROLL;
    int16_t max = (512+200) * (axis==YAW) ? GYRO_SUMMATION_FACTOR_YAW : GYRO_SUMMATION_FACTOR_PITCHROLL;
    if(gyroOffset.offsets[axis] < min || gyroOffset.offsets[axis] > max)
      versionInfo.hardwareErrors[0] |= FC_ERROR0_GYRO_PITCH << axis;
  }

  gyroOffset_writeToEEProm();  
  startAnalogConversionCycle();
}

/*
 * Find acc. offsets for a neutral reading, and write them to EEPROM.
 * Does not (!} update the local variables. This must be done with a
 * call to analog_calibrate() - this always (?) is done by the caller
 * anyway. There would be nothing wrong with updating the variables
 * directly from here, though.
 */
void analog_calibrateAcc(void) {
#define ACC_OFFSET_CYCLES 32
  uint8_t i, axis;
  int32_t offsets[3] = { 0, 0, 0 };

  for (i = 0; i < ACC_OFFSET_CYCLES; i++) {
    delay_ms_with_adc_measurement(10, 1);
    for (axis = PITCH; axis <= YAW; axis++) {
      offsets[axis] += rawAccValue(axis);
    }
  }

  for (axis = PITCH; axis <= YAW; axis++) {
    accOffset.offsets[axis] = (offsets[axis] + ACC_OFFSET_CYCLES / 2) / ACC_OFFSET_CYCLES;
    int16_t min,max;
    if (axis==Z) {
        if (staticParams.imuReversedFlags & 8) {
        // TODO: This assumes a sensitivity of +/- 2g.
                min = (256-200) * ACC_SUMMATION_FACTOR_Z;
                        max = (256+200) * ACC_SUMMATION_FACTOR_Z;
        } else {
        // TODO: This assumes a sensitivity of +/- 2g.
                min = (768-200) * ACC_SUMMATION_FACTOR_Z;
                        max = (768+200) * ACC_SUMMATION_FACTOR_Z;
        }
    } else {
        min = (512-200) * ACC_SUMMATION_FACTOR_PITCHROLL;
        max = (512+200) * ACC_SUMMATION_FACTOR_PITCHROLL;
    }
    if(gyroOffset.offsets[axis] < min || gyroOffset.offsets[axis] > max) {
      versionInfo.hardwareErrors[0] |= FC_ERROR0_ACC_X << axis;
    }
  }

  accOffset_writeToEEProm();
  startAnalogConversionCycle();
}