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ADC: randomise the sampling rate

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this is based on suggestions from Kari and Mike, and should reduce the
aliasing effects we get from the simple averaging
This commit is contained in:
Andrew Tridgell 2011-09-17 12:30:19 +10:00
parent 465c10a31f
commit a828d00125
1 changed files with 17 additions and 4 deletions
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21
libraries/AP_ADC/AP_ADC_ADS7844.cpp
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@ -65,6 +65,14 @@ static volatile uint16_t _count[8];
static uint32_t last_ch6_micros; static uint32_t last_ch6_micros;
// TCNT2 values for various interrupt rates,
// assuming 256 prescaler. Note that these values
// assume a zero-time ISR. The actual rate will be a
// bit lower than this
#define TCNT2_781_HZ (256-80)
#define TCNT2_1008_HZ (256-62)
#define TCNT2_1302_HZ (256-48)
static inline unsigned char ADC_SPI_transfer(unsigned char data) static inline unsigned char ADC_SPI_transfer(unsigned char data)
{ {
/* Put data into buffer, sends the data */ /* Put data into buffer, sends the data */
@ -76,9 +84,10 @@ static inline unsigned char ADC_SPI_transfer(unsigned char data)
} }
ISR (TIMER2_OVF_vect) ISR (TIMER2_OVF_vect)
{ {
uint8_t ch; uint8_t ch;
static uint8_t timer_offset;
bit_clear(PORTC, 4); // Enable Chip Select (PIN PC4) bit_clear(PORTC, 4); // Enable Chip Select (PIN PC4)
ADC_SPI_transfer(adc_cmd[0]); // Command to read the first channel ADC_SPI_transfer(adc_cmd[0]); // Command to read the first channel
@ -110,7 +119,11 @@ ISR (TIMER2_OVF_vect)
} }
bit_set(PORTC, 4); // Disable Chip Select (PIN PC4) bit_set(PORTC, 4); // Disable Chip Select (PIN PC4)
TCNT2 = 200;
// this gives us a sample rate between 781Hz and 1302Hz. We
// randomise it to try to minimise aliasing effects
timer_offset = (timer_offset + 49) % 32;
TCNT2 = TCNT2_781_HZ + timer_offset;
} }
@ -151,10 +164,10 @@ void AP_ADC_ADS7844::Init(void)
// Enable Timer2 Overflow interrupt to capture ADC data // Enable Timer2 Overflow interrupt to capture ADC data
TIMSK2 = 0; // Disable interrupts TIMSK2 = 0; // Disable interrupts
TCCR2A = 0; // normal counting mode TCCR2A = 0; // normal counting mode
TCCR2B = _BV(CS21) | _BV(CS22); // Set prescaler of 256 TCCR2B = _BV(CS21) | _BV(CS22); // Set prescaler of clk/256
TCNT2 = 0; TCNT2 = 0;
TIFR2 = _BV(TOV2); // clear pending interrupts; TIFR2 = _BV(TOV2); // clear pending interrupts;
TIMSK2 = _BV(TOIE2) ; // enable the overflow interrupt TIMSK2 = _BV(TOIE2); // enable the overflow interrupt
} }
// Read one channel value // Read one channel value