Zephyr adc sequence read returns not consistent values in periodic reads

Question

Hi all,

I'm quite new in Zephyr. Just to let you know ;P

I'm developing a code based on the nRF52832 in a custom board and I'd like to measure the voltage of two separate batteries.

These batteries are connected to a resistor voltage divider which is enabled or disabled by a digital switch. Here is the schema

* packEn: 1 --> enables the two voltage dividers, 0--> disables the connection to the batteries

* Vpack1 --> Connected to AIN4

* Vpack2 --> Connected to AIN5

I'm using the nRF Connect SDK v2.4.1 with VS code.

In the main loop I read periodically the Vpack1 and Vpack2, but the values I get are random in most of the cases.

I'm sure that it's a configuration issue in the ADC, but I cannot find it, so I hope you can give a hand to find out what's the problem. Thank you.

This is my code:

#define HW_MAP_BAT_ENABLE_PIN				    NRF_GPIO_PIN_MAP(0, 30)
#define HW_MAP_BAT_R1_DIVIDER_VALUE		        150000.0f								//	Value in ohm
#define HW_MAP_BAT_R2_DIVIDER_VALUE	        	100000.0f								//	Value in ohm


#define	DRV_BAT_PACKS							2


typedef struct
{
	bool		valid;
	
	struct
	{
		bool	present;
		int16_t voltage;
	}pack[DRV_BAT_PACKS];
	
}drvBat_st;

static const struct device *adcDev =  DEVICE_DT_GET(DT_NODELABEL(adc));


#define	DRV_BAT_ADC_RESOLUTION						12
#define	DRV_BAT_ADC_GAIN							ADC_GAIN_1_3
#define	DRV_BAT_ADC_REFERENCE						ADC_REF_INTERNAL
#define	DRV_BAT_ADC_ADQUISITION_TIME				ADC_ACQ_TIME(ADC_ACQ_TIME_MICROSECONDS, 40)

#define DRV_BAT_ADC_PRIMARY_CHANNEL_ID 				4
#define DRV_BAT_ADC_PRIMRAY_CHANNEL_INPUT 			NRF_SAADC_INPUT_AIN4
#define DRV_BAT_ADC_SECONDARY_CHANNEL_ID 			5
#define DRV_BAT_ADC_SECONDARY_CHANNEL_INPUT 		NRF_SAADC_INPUT_AIN5


static const struct adc_channel_cfg primary = 	{	
													.gain 				= DRV_BAT_ADC_GAIN,
													.reference 			= DRV_BAT_ADC_REFERENCE,
													.acquisition_time 	= DRV_BAT_ADC_ADQUISITION_TIME,
													.channel_id 		= DRV_BAT_ADC_PRIMARY_CHANNEL_ID,
													.differential		= 0,
													#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
													.input_positive 	= DRV_BAT_ADC_PRIMRAY_CHANNEL_INPUT,
													#endif
												};

static const struct adc_channel_cfg secondary =	{
													.gain 				= DRV_BAT_ADC_GAIN,
													.reference 			= DRV_BAT_ADC_REFERENCE,
													.acquisition_time 	= DRV_BAT_ADC_ADQUISITION_TIME,
													.channel_id 		= DRV_BAT_ADC_SECONDARY_CHANNEL_ID,
													.differential		= 0,
													#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
													.input_positive 	= DRV_BAT_ADC_SECONDARY_CHANNEL_INPUT,
													#endif
												};

static drvBat_st bat;


rc_t drvBat_Init (void)
{
	//
	//  Init the ADC channel configuration needed
	//

	static bool init = false;

	if ( true == init )
	{
		return RC_SUCCESS;
	}

	//	Configure adc channels used to measure battery voltage

	if ( 0 != adc_channel_setup(adcDev, &primary) )
	{
		return RC_ERROR;
	}

	if ( 0 != adc_channel_setup(adcDev, &secondary) )
	{
		return RC_ERROR;
	}

	//	Init battery power control
	
	nrfx_gpiote_out_config_t config = NRFX_GPIOTE_CONFIG_OUT_SIMPLE(false);
	nrfx_gpiote_out_init (HW_MAP_BAT_ENABLE_PIN, &config);											//	Configure battery control pin
	
	init = true;
		
	return RC_SUCCESS;
}


rc_t drvBat_Measure (void)
{
	//
	//
	//

	int16_t samples[2];

	const struct adc_sequence sequence = 	{
												.channels 		= BIT(DRV_BAT_ADC_PRIMARY_CHANNEL_ID) | BIT(DRV_BAT_ADC_SECONDARY_CHANNEL_ID) ,
												.buffer 		= samples,
												.buffer_size 	= sizeof(samples),
												.resolution 	= DRV_BAT_ADC_RESOLUTION,
												.oversampling 	= 0,					// don't oversample
												.calibrate 		= false					// calibrate												
											};

	nrfx_gpiote_out_set(HW_MAP_BAT_ENABLE_PIN);											//	Enable the voltage divider

	k_msleep(DRV_BAT_ENABLE_TIMEOUT);													//	Little wait to ensure that voltage is stable before performing an adc read

	if ( 0 != adc_read(adcDev, &sequence) )
	{
		return RC_ERROR;
	}

	nrfx_gpiote_out_clear(HW_MAP_BAT_ENABLE_PIN);										//	Disable the voltage divider

	for (uint8_t i=0; i<DRV_BAT_PACKS; i++)
	{
		int32_t raw = (int32_t)samples[i];

		if ( 0 != adc_raw_to_millivolts(DRV_BAT_ADC_REFERENCE, DRV_BAT_ADC_GAIN, DRV_BAT_ADC_RESOLUTION, &raw) )
		{
			return RC_ERROR;
		}

		bat.pack[i].present = false;
		bat.pack[i].voltage = (int16_t)(raw / DRV_BAT_DIVIDER_FACTOR);

		if ( 0 > bat.pack[i].voltage )
		{
			bat.pack[i].voltage = 0;
		}

		if ( DRV_BAT_MIN_VOLTAGE <= bat.pack[i].voltage )
		{
			bat.pack[i].present = true; 
		}
	}
		
	return RC_SUCCESS;
}

Thank you so much for your help

Ragards,

David

Kenneth · Accepted Answer

Hi, 
 I can't find anything in specific, but just to rule out any hardware connection/electrical problem connect battery directly to one analog input, and only sample that specific input. 
 I notice you have used: 
 #define DRV_BAT_ADC_PRIMARY_CHANNEL_ID 4 #define DRV_BAT_ADC_PRIMRAY_CHANNEL_INPUT NRF_SAADC_INPUT_AIN4 #define DRV_BAT_ADC_SECONDARY_CHANNEL_ID 5 #define DRV_BAT_ADC_SECONDARY_CHANNEL_INPUT NRF_SAADC_INPUT_AIN5 
 But there is no direct dependency between ID and INPUT, so typically the following is more common to use: 
 #define DRV_BAT_ADC_PRIMARY_CHANNEL_ID 0 #define DRV_BAT_ADC_PRIMRAY_CHANNEL_INPUT NRF_SAADC_INPUT_AIN4 #define DRV_BAT_ADC_SECONDARY_CHANNEL_ID 1 #define DRV_BAT_ADC_SECONDARY_CHANNEL_INPUT NRF_SAADC_INPUT_AIN5 
 I suggest to continue debugging for instance by comparing with the existing examples. What does you prj.conf look like btw. 
 Best regards, Kenneth

Zephyr adc sequence read returns not consistent values in periodic reads

Top Replies