Slow ADC

Question

Hello,

I am using the ADC on my custom board, based on nRF9160.

I am getting correct results when measuring a known voltage. However, I noticed that the execution is very slow.

I am using the following code. Using a scope, I can see that it takes ~3.8ms.

#define ADC_BIT_RANGE			14
#define ADC_NB_SAMPLES_TAKEN	8

static const struct device *adc_dev = DEVICE_DT_GET(DT_INST(0, nordic_nrf_saadc));

static int16_t m_sample_buffer[ADC_NB_SAMPLES_TAKEN];



int ADC_MeasureMillivots_Blocking(uint8_t ch)
{
	uint8_t i ;
	int val_mv = -1 ; 
	bool success = false ;

	struct adc_sequence_options sequenceOption ={
		.callback = NULL,
		.extra_samplings = ADC_NB_SAMPLES_TAKEN-1 ,
		.interval_us = 0 ,
		.user_data = NULL 
	};

	struct adc_sequence sequence = {
			.options	 = &sequenceOption,
			.channels    = BIT(ch),	
			.buffer      = &m_sample_buffer,
			.buffer_size = sizeof(m_sample_buffer),
			.resolution  = ADC_BIT_RANGE,
			.oversampling = 0,
			.calibrate	 = false };

	
	struct adc_channel_cfg channelConfig = {
		.gain = ADC_GAIN_1_6,
		.reference = ADC_REF_INTERNAL,
		.acquisition_time = ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 5),
		.channel_id = 1,							// Will be updated just a few line below
		.input_positive = 0 };	// Will be updated just a few line below


	// There are only 8 channels: AN0 to AN7
	if(ch <= 7)
	{
		// Exit low power mode
		pm_device_action_run(adc_dev, PM_DEVICE_ACTION_RESUME);		

		// Update the configuration of the ADC channel to read, according to what is requested in argument
		channelConfig.channel_id = ch ;
		channelConfig.input_positive = (1 + ch) ; // Channel are offet by 1 : CH0 is input 1, CH1 is input 2, ...
		
		// Update setup
		if(!adc_channel_setup(adc_dev, &channelConfig) )
		{
			// Read result
			if( !adc_read(adc_dev, &sequence) )
			{				
				success = true ;				
			}
		}
	}

	// Enter ADC low power
	pm_device_action_run(adc_dev, PM_DEVICE_ACTION_SUSPEND);

	// Convert the result
	if(success)
	{
		// Make an average of the samples
		val_mv = 0 ;
		for(i=0;i<ADC_NB_SAMPLES_TAKEN;i++){
			val_mv += m_sample_buffer[i] ;			
		}
		val_mv /= ADC_NB_SAMPLES_TAKEN ;

		// Convert to millivolts
		adc_raw_to_millivolts(adc_ref_internal(adc_dev), channelConfig.gain, sequence.resolution, &val_mv) ;
	}
	else
	{
	    // Error !
		val_mv = -1 ;
	}

	return val_mv ;
}

Device tree is as simple as :

&adc {
	status = "okay";
};

Removing the adc_read() reduces the function execution to ~10µs. It's obviously useless, as I don't get any results. But it shows that the other lines of code in this function have a very small impact on execution time.

I tried several things, but none of these made a significant difference:

- Remove the PM_DEVICE_ACTION_SUSPEND/RESUME

- Reduce the number of samples

- Reduce the resolution

- Change the acquisition time to 5, 10 or 20µs

On this topic, I notices that using the macro seems to provide strange results. File zephyr/drivers/adc.h states that range is 0-16383, but I see the following values :

> ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 5) returns 16389
> ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 10) returns 16394
> ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 20) returns 16404

I am currently using NCS v2.5.2

Any hint on what can be wrong / how to improve speed ?

Thanks.