SDK: nRF5 SDK for Thread and Zigbee 1.0.0
I have code for the SAADC based on the Low Power SAADC example here.
I have modified it to sample 4 inputs, and changed the settings as follows:
#define UART_PRINTING_ENABLED //Enable to see SAADC output on UART. Comment out for low power operation.
#define RTC_FREQUENCY 32 //Determines the RTC frequency and prescaler
#define RTC_CC_VALUE 32 //Determines the RTC interrupt frequency and thereby the SAADC sampling frequency
#define SAADC_CALIBRATION_INTERVAL 10 //Determines how often the SAADC should be calibrated relative to NRF_DRV_SAADC_EVT_DONE event. E.g. value 5 will make the SAADC calibrate every fifth time the NRF_DRV_SAADC_EVT_DONE is received.
#define SAADC_SAMPLES_IN_BUFFER 1 //Number of SAADC samples in RAM before returning a SAADC event. For low power SAADC set this constant to 1. Otherwise the EasyDMA will be enabled for an extended time which consumes high current.
#define SAADC_OVERSAMPLE NRF_SAADC_OVERSAMPLE_DISABLED //Oversampling setting for the SAADC. Setting oversample to 4x
// This will make the SAADC output a single averaged value when the SAMPLE task is triggered 4 times.
// Enable BURST mode to make the SAADC sample 4 times when triggering SAMPLE task once.
// Do not use oversampling if more than one channel is active
#define SAADC_BURST_MODE 0 //Set to 1 to enable BURST mode, otherwise set to 0.
I further modified the code to calibrate when Button 0 is pressed, though eventually calibration will be done as needed based on the measured temperature.
I find that after calibration, the data in the buffer is shifted by one channel. That is the data at channel[0] has the values I would expect for channel[3], the data at channel[1] has the value I would expect for channel[0], etc
Like this:
Before calibration:
{0xFFFA, 0x0804, 0x0A20, 0x0705}
After calibration:
{0x0705, 0xFFFA, 0x0804, 0x0A20}
The SAADC is initialized as follows:
void saadc_init(void)
{
int i;
ret_code_t err_code;
nrf_drv_saadc_config_t saadc_config;
nrf_saadc_channel_config_t channel_config[NUM_ADC_CHANNELS];
//Configure SAADC
saadc_config.low_power_mode = true; //Enable low power mode.
saadc_config.resolution = NRF_SAADC_RESOLUTION_12BIT; //Set SAADC resolution to 12-bit. This will make the SAADC output values from 0 (when input voltage is 0V) to 2^12=2048 (when input voltage is 3.6V for channel gain setting of 1/6).
saadc_config.oversample = NRF_SAADC_OVERSAMPLE_DISABLED; //Disable oversample. Can't oversample with more than one channel active.
saadc_config.interrupt_priority = APP_IRQ_PRIORITY_LOW; //Set SAADC interrupt to low priority.
//Initialize SAADC
err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback); //Initialize the SAADC with configuration and callback function. The application must then implement the saadc_callback function, which will be called when SAADC interrupt is triggered
APP_ERROR_CHECK(err_code);
for (i=0; i<NUM_ADC_CHANNELS; i++)
{
//Configure SAADC channel
channel_config[i].reference = NRF_SAADC_REFERENCE_INTERNAL; //Set internal reference of fixed 0.6 volts
channel_config[i].gain = NRF_SAADC_GAIN1; //Set input gain to 1. The maximum SAADC input voltage is then 0.6V/(1)=0.6V. The single ended input range is then 0V-0.6V
channel_config[i].acq_time = NRF_SAADC_ACQTIME_10US; //Set acquisition time. Set low acquisition time to enable maximum sampling frequency of 200kHz. Set high acquisition time to allow maximum source resistance up to 800 kohm, see the SAADC electrical specification in the PS.
channel_config[i].mode = NRF_SAADC_MODE_SINGLE_ENDED; //Set SAADC as single ended. This means it will only have the positive pin as input, and the negative pin is shorted to ground (0V) internally.
// Select the input pin for the channel
switch(i)
{
case 0: channel_config[i].pin_p = NRF_SAADC_INPUT_AIN0; break; // LOOP: AIN0 pin maps to physical pin P0.02.
case 1: channel_config[i].pin_p = NRF_SAADC_INPUT_AIN1; break; // VOLT: AIN1 pin maps to phsyical pin P0.03
case 2: channel_config[i].pin_p = NRF_SAADC_INPUT_AIN4; channel_config[i].gain = NRF_SAADC_GAIN1_2; break; // VMON: AIN4 pin maps to physical pin P0.28
case 3: channel_config[i].pin_p = NRF_SAADC_INPUT_AIN5; channel_config[i].gain = NRF_SAADC_GAIN1_2; break; // TEMP: AIN5 pin maps to physical pin P0.29
}
channel_config[i].pin_n = NRF_SAADC_INPUT_DISABLED; //Since the SAADC is single ended, the negative pin is disabled. The negative pin is shorted to ground internally.
channel_config[i].resistor_p = NRF_SAADC_RESISTOR_DISABLED; //Disable pullup resistor on the input pin
channel_config[i].resistor_n = NRF_SAADC_RESISTOR_DISABLED; //Disable pulldown resistor on the input pin
//Initialize SAADC channel
err_code = nrf_drv_saadc_channel_init(i, &channel_config[i]); //Initialize SAADC channel 0 with the channel configuration
APP_ERROR_CHECK(err_code);
if(SAADC_BURST_MODE)
{
// NRF_SAADC->CH[i].CONFIG |= 0x01000000; //Configure burst mode for channel 0. Burst is useful together with oversampling. When triggering the SAMPLE task in burst mode, the SAADC will sample "Oversample" number of times as fast as it can and then output a single averaged value to the RAM buffer. If burst mode is not enabled, the SAMPLE task needs to be triggered "Oversample" number of times to output a single averaged value to the RAM buffer.
}
}
err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0],SAADC_SAMPLES_IN_BUFFER * NUM_ADC_CHANNELS); //Set SAADC buffer 1. The SAADC will start to write to this buffer
APP_ERROR_CHECK(err_code);
err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1],SAADC_SAMPLES_IN_BUFFER * NUM_ADC_CHANNELS); //Set SAADC buffer 2. The SAADC will write to this buffer when buffer 1 is full. This will give the applicaiton time to process data in buffer 1.
APP_ERROR_CHECK(err_code);
}
Is there a work around? or fix for this?
Mary
