Problem using digital IO as MISO.

[Luccas]

Hi, I'm trying to set P0.20 pin as MISO of my spi master to communicate with ADXL372 accelerometer sensor, but when the miso is disconnected the signal received matches with expected and when connected to the DK pin the signal is modified and results are different. 

The expected signal received when disconnected from DK: 

The received signal after connected to P0.20 pin:

I used the default configuration of SPI:

nrf_drv_spi_config_t spi_config = NRF_DRV_SPI_DEFAULT_CONFIG;

spi_config.frequency = NRF_DRV_SPI_FREQ_4M;
spi_config.ss_pin = SPI_SS_PIN; //P0.08;
spi_config.miso_pin = 20;
spi_config.mosi_pin = SPI_MOSI_PIN; //P0.16;
spi_config.sck_pin = SPI_SCK_PIN; //P0.16;
spi_config.mode = NRF_DRV_SPI_MODE_0;


uint32_t ret =nrf_drv_spi_init(&spi, &spi_config, NULL , NULL);

I've already tried to change the MISO pull config on sdk_config.h with no success.

NRFX_SPIM_MISO_PULL_CFG 1

I'm using SDK 15.2.0_9412b96 on PCA10040 V1.1.1 2016.35, with softdevice  s132 6.1.0.

Do you know how can I configure this pin to work properly? 

[awneil]

Have you checked the PCA10040 schematics to see if there's anything else connected to that pin ... ?

[Luccas]

Yes, It has the LED4 connected, I've already cut the SB8 connection to disable the LED. 

[haakonsh]

The signal seems to struggle to pull the miso line high. Have you tried using a pull-up?

ie. "NRFX_SPIM_MISO_PULL_CFG 3" // NRF_GPIO_PIN_PULLUP

[haakonsh]

Have you tried using another pin? That pin's trace might have to much capacitance to be used for SPI. 

[Luccas]

Yes, I've tried another digital pin with same results. Using AIN pin worked, but I dont know why.