SPI instance taking up a lot of power when enabled.

[Jay_Shah]

SDK version: 15.3

Soft-device version: 6.1.1

Hi,

I've been working on a low power application where every 10's of uA is important. My application requires SPI as Master on nRF52832 but as soon as I enable my SPI my current shoots up to mA range from uA. 

I used "SPI"example present in "..\nRF5_SDK_15.3.0_59ac345\examples\peripheral" to understand the configuration of SPI. Could you please tell my why would this happen?

I read a couple of articles where people faced similar current consumption issue. Or do I have to do some disabling of SPI to save power?

[Stian Røed Hafskjold]

Hi, the SPI example is not power optimized. nrf_delay_ms() is just using a loop of NOP instructions. You should start the SPI transfer from an RTC interrupt or similar, and keep the CPU in a WFE loop.

I modified the SPI example to include an app_timer which uses the RTC to give you an interrupt every 100ms. The power consumption is 2uA between the transfers. Also remember to disable logging in sdk_config.h.

#include "nrf_drv_spi.h"
#include "app_util_platform.h"
#include "nrf_gpio.h"
#include "nrf_delay.h"
#include "boards.h"
#include "app_error.h"
#include <string.h>
#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"
#include "app_timer.h"

#define SPI_INSTANCE  0 /**< SPI instance index. */
static const nrf_drv_spi_t spi = NRF_DRV_SPI_INSTANCE(SPI_INSTANCE);  /**< SPI instance. */

#define TEST_STRING "Nordic"
static uint8_t       m_tx_buf[] = TEST_STRING;           /**< TX buffer. */
static uint8_t       m_rx_buf[sizeof(TEST_STRING) + 1];    /**< RX buffer. */
static const uint8_t m_length = sizeof(m_tx_buf);        /**< Transfer length. */

/**
 * @brief SPI user event handler.
 * @param event
 */
void spi_event_handler(nrf_drv_spi_evt_t const * p_event,
                       void *                    p_context)
{
    NRF_LOG_INFO("Transfer completed.");
    if (m_rx_buf[0] != 0)
    {
        NRF_LOG_INFO(" Received:");
        NRF_LOG_HEXDUMP_INFO(m_rx_buf, strlen((const char *)m_rx_buf));
    }
}

static void spi_transfer_handler(void * p_context){
        memset(m_rx_buf, 0, m_length);
        APP_ERROR_CHECK(nrf_drv_spi_transfer(&spi, m_tx_buf, m_length, m_rx_buf, m_length));
}

int main(void)
{
    bsp_board_init(BSP_INIT_LEDS);

    APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
    NRF_LOG_DEFAULT_BACKENDS_INIT();

    nrf_drv_spi_config_t spi_config = NRF_DRV_SPI_DEFAULT_CONFIG;
    spi_config.ss_pin   = SPI_SS_PIN;
    spi_config.miso_pin = SPI_MISO_PIN;
    spi_config.mosi_pin = SPI_MOSI_PIN;
    spi_config.sck_pin  = SPI_SCK_PIN;
    APP_ERROR_CHECK(nrf_drv_spi_init(&spi, &spi_config, spi_event_handler, NULL));

    NRF_CLOCK->TASKS_LFCLKSTART = 1;
    while(NRF_CLOCK->EVENTS_LFCLKSTARTED == 0){
    }
    NRF_CLOCK->EVENTS_LFCLKSTARTED = 0;
    APP_ERROR_CHECK(app_timer_init());
    APP_TIMER_DEF(spi_timer_id);
    APP_ERROR_CHECK(app_timer_create(&spi_timer_id, APP_TIMER_MODE_REPEATED, spi_transfer_handler));
    APP_ERROR_CHECK(app_timer_start(spi_timer_id, APP_TIMER_TICKS(100), NULL));

    NRF_LOG_INFO("SPI example started.");

    while (1)
    {
        NRF_LOG_FLUSH();
        __WFE();
    }
}