Unable to disable SAADC on NRF52832

[Mike]

Hi,

I'm developing a simple sensor application using s132 on NRF52832_XXAA. The application works fine, except that I have problems disabling the saadc module after conversion. I check battery voltage once per minute and the readings are always correct, but at the first conversion the current consumption jumps from less than 100uA to 7mA and stays there until i reset the chip.

How come the adc draw 7mA? It's a lot. I could live with that since it's for a fraction of a second once per minute. But how come i can't disable it? Is something missing in the code below?

Thanks in advance, Mike

#include "nrf.h"
#include "nrf_drv_saadc.h"
#include "app_error.h"
#include "app_util_platform.h"
#include "nrf_log.h"
#include "adc.h"

static void adc_callback(nrf_drv_saadc_evt_t const *e)
{
}

static void adc_enable()
{
    int32_t r;
	nrf_drv_saadc_config_t cfg;
    nrf_saadc_channel_config_t ch;

	cfg.resolution = NRF_SAADC_RESOLUTION_14BIT;
	cfg.oversample = NRF_SAADC_OVERSAMPLE_DISABLED;
	cfg.interrupt_priority = 7;
	cfg.low_power_mode = 1;	

    r = nrf_drv_saadc_init(&cfg, adc_callback);
    APP_ERROR_CHECK(r);

	ch.resistor_p = NRF_SAADC_RESISTOR_DISABLED;
	ch.resistor_n = NRF_SAADC_RESISTOR_DISABLED;
	ch.gain = NRF_SAADC_GAIN1_6;
	ch.reference = NRF_SAADC_REFERENCE_INTERNAL; // 0.6V internal ref
	ch.acq_time = NRF_SAADC_ACQTIME_10US;
	ch.mode = NRF_SAADC_MODE_SINGLE_ENDED;
	ch.burst = NRF_SAADC_BURST_DISABLED;
	ch.pin_p = NRF_SAADC_INPUT_VDD;
	ch.pin_n = NRF_SAADC_INPUT_DISABLED;

    r = nrf_drv_saadc_channel_init(0, &ch);
    APP_ERROR_CHECK(r);
}

static void adc_disable()
{
    int32_t r;

	r = nrf_drv_saadc_channel_uninit(0);
    APP_ERROR_CHECK(r);
	nrf_drv_saadc_uninit();
}

int32_t adc_sample(int16_t *value)
{
	int32_t r;

	adc_enable();
	r = nrf_drv_saadc_sample_convert(0, value);
	adc_disable();

    APP_ERROR_CHECK(r);
	return r;
}

[Jørgen Holmefjord]

Are you doing some floating point calculations in your code? If you are, please check if you are affected by Errata 87 - CPU: Unexpected wake from System ON Idle when using FPU. The current you are seeing indicates that the CPU is prevented from going to sleep mode.

[Mike]

Thanks Jørgen, indeed it was the issue. I'd never figured that out.

Are there any particular reasons why sd_app_evt_wait() don't have this fix implemented?

[Mike]

The errata you linked to appears to have a typo(?)

_set_FPSCR(_get_FPSCR() & ~(0x0000009F));

should probably be

__set_FPSCR(__get_FPSCR() & ~(0x0000009F));

i.e., missing underscores.

[Jørgen Holmefjord]

Yes, there seems to be a typo. Thanks for reporting this. Most likelyt this is not implemented in sd_app_evt_wait() since this function is supposed to be general for all revisions of the chip. Not all revisions are affected by the same issues, which is why it is important to allways check the errata sheet for your chip revision.

[t_m]

Hello

The link to the Errata page is broken. Can you please send me an updated link. I seem to have similar problem. I do simple floating calculation of adding adc samples in a buffer and calculating the average. The AIN pin stays high (3.3v) during sleep mode.