Help needed for excessive standby current for SPI slave

[Jim Hartmann]

Hello, I am using the nRF51822 with SDK 10.0, S110 softdevice, and a SPI slave interface. The SPI slave interface works, but the standby power consumption of the nRF is high, ~300 uA. I suspect that the high speed crystal is running in the background.

I use an interrupt on a pin to wake the nRF and perform the SPI transfer. Upon waking I signal the master that I am ready with a separate output pin. When the transfer is done I go back into power management via sd_app_evt_wait().

Here is the wake-up pin initialization:

err_code = nrf_drv_gpiote_init();
APP_ERROR_CHECK(err_code);

// Configure event handler for EXTERNAL_WAKEUP_PIN, low accuracy for low power
nrf_drv_gpiote_in_config_t wakePinConfig = GPIOTE_CONFIG_IN_SENSE_LOTOHI(false);

err_code = nrf_drv_gpiote_in_init(EXTERNAL_WAKEUP_PIN, &wakePinConfig, wakeupHandler);

APP_ERROR_CHECK(err_code);

nrf_drv_gpiote_in_event_enable(EXTERNAL_WAKEUP_PIN, true);

SPI Slave initialization:

nrf_drv_spis_config_t spis_config = NRF_DRV_SPIS_DEFAULT_CONFIG(SPIS_INSTANCE_NUMBER);

spis_config.miso_pin = SPIS_MISO_PIN;
spis_config.mosi_pin = SPIS_MOSI_PIN;
spis_config.sck_pin = SPIS_SCK_PIN;
spis_config.csn_pin = SPIS_CSN_PIN;
spis_config.mode = NRF_DRV_SPIS_MODE_1;
spis_config.bit_order = NRF_DRV_SPIS_BIT_ORDER_MSB_FIRST;
spis_config.def = DEF_CHARACTER;
spis_config.orc = ORC_CHARACTER;

// Open the SPI peripheral
err_code = nrf_drv_spis_init(&m_spis, &spis_config, spi_slave_event_handler);
APP_ERROR_CHECK(err_code);

I have tried enabling the SPI interface on-the-fly in my wakeup handler but this did not help. It worked, but the power consumption is the same, e.g.:

Wakeup event:

nrf_drv_spis_init(...)
raise_handshake_output()
wait for transfer event...
lower_handshake_output()
nrf_drv_spis_uninit(...)
sd_app_evt_wait() // wait for next Wakeup Event.

I know that my CSN input pin is high at all times except when the handshake output has been raised to signal the master.

Other details:

// Initialize the SoftDevice handler module.
SOFTDEVICE_HANDLER_INIT(NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_4000MS_CALIBRATION, NULL);

The standby current is measured with a high speed current to voltage device in series with VCC and an oscilloscope where the advertising interval can also be observed.

[John]

It looks like you are using the GPIOTE IN event to wake your device. This is known to burn a lot of power on the nRF51822. You should use the PORT event instead if you need a lower power solution. Search this forum and you'll find a lot of information on how to do this.

[Jim Hartmann]

This was also my understanding of how it is supposed to work. We initially had "true" (high accuracy) and the current was higher. Disabling high accuracy lowered the current significantly but not to the level we need.