Dev Academy Lesson 6 Exercise 2 – nrfx ADC is non-blocking but does not wait for conversion to complete

[chris22222]

Working through the dev academy exercise, it appears that the sample code does not wait for the conversion to complete before reading out the sample:

/* STEP 7.1 - Implement timer callback handler function */
void battery_sample_timer_handler(struct k_timer *timer)
{

        /* STEP 7.2 - Trigger the sampling */
        nrfx_err_t err = nrfx_saadc_mode_trigger();
        if (err != NRFX_SUCCESS) {
                printk("nrfx_saadc_mode_trigger error: %08x", err);
                return;
        }

        /* STEP 7.3 - Calculate and print voltage */
        int battery_voltage = ((600*6) * sample) / ((1<<12));

        printk("SAADC sample: %d\n", sample);
        printk("Battery Voltage: %d mV\n", battery_voltage);

}

https://github.com/NordicDeveloperAcademy/ncs-inter/blob/main/v2.6.2-v2.5.2/l6/l6_e2_sol/src/main.c

Would it not be more appropriate to get a call back from the nrfx_saadc_irq_handler when the conversion is complete?

When using the Zephyr ADC driver on the same exact board, it appears the ADC measurements are ~30 mV different.

Can you please confirm if we should be waiting for the conversion to complete here?

[chris22222]

I figured this out.  I found NULL was passed as the event handler to this:

/**
 * @brief Function for setting the SAADC driver in the simple mode.
 *
 * The simple mode allows obtaining a single sample from each requested channel.
 * The conversion can be done in a blocking or non-blocking manner.
 * Sampling is initiated by calling @ref nrfx_saadc_mode_trigger() once.
 *
 * @param[in] channel_mask  Bitmask of channels to be used in the simple mode.
 * @param[in] resolution    Resolution configuration.
 * @param[in] oversampling  Oversampling configuration.
 * @param[in] event_handler Event handler provided by the user. In case of providing NULL,
 *                          the conversion will be performed in the blocking manner.
 *
 * @retval NRFX_SUCCESS             Initialization was successful.
 * @retval NRFX_ERROR_BUSY          There is a conversion or calibration ongoing.
 * @retval NRFX_ERROR_INVALID_PARAM Attempt to activate channel that is not configured.
 */
nrfx_err_t nrfx_saadc_simple_mode_set(uint32_t                   channel_mask,
                                      nrf_saadc_resolution_t     resolution,
                                      nrf_saadc_oversample_t     oversampling,
                                      nrfx_saadc_event_handler_t event_handler);

I wish this was more clearly written out.

Also the offset discrepancy was caused by not calling 

nrfx_saadc_offset_calibrate() prior to starting the conversion in the sample.

[Vidar Berg]

Hello,

Thank you for the feedback. I will relay this to the team responsible for this course. I also think it is worth mentioning about the calibration. The blocking part appears to be covered already in the updated course:

Best regards,

Vidar

[Vidar Berg]

The team has added a note about SAADC calibration in the course now:

 

Thanks again for your feedback.