The nrfx_spim driver toggles random pin when chip select pin of the driver is not used.

[Gunnar Sigurðsson]

The nrfx_spim driver has an issue when chip select pin of the driver is not used. 

When nrfx_spim_init is called with a nrfx_spim_config_t whose ss_pin is set to NRFX_SPIM_PIN_NOT_USED the value is not assigned to the ss_pin of the spim_control_block_t in the driver.

This results in a toggle of the ss_pin, which has not been initialized, when nrfx_spim_xfer is called.

This problem is present in nrf-sdk version 15.2 and 15.0. 

[MartinBL]

Hi,

Can you upload some project files that I can use to recreate this? Or point me to exactly where in the driver the ss pin is being toggled?

When I test this with the basic SPI example in SDK 15.2.0 the p_cb->ss_pin is set to 0xFF (NRFX_SPIM_PIN_NOT_USED) inside nrfx_spim_init(). Then there is an if-statement inside nrfx_spim_xfer() that checks this and skips the toggling of the ss_pin if it is not used:

if (p_cb->ss_pin != NRFX_SPIM_PIN_NOT_USED)
{
#if NRFX_CHECK(NRFX_SPIM_EXTENDED_ENABLED)
    if (!p_cb->use_hw_ss)
#endif
    {
        if (p_cb->ss_active_high)
        {
            nrf_gpio_pin_set(p_cb->ss_pin);
        }
        else
        {
            nrf_gpio_pin_clear(p_cb->ss_pin);
        }

[Gunnar Sigurðsson]

In the nrfx_spim_init() function the ss_pin of the driver instance in the m_cb array is set if the ss_pin in the config is not NRFX_SPIM_PIN_NOT_USED. If however the p_config->ss_pin is NRFX_SPIM_PIN_NOT_USED the ss_pin of the driver (m_cb[p_instance->drv_inst_idx].ss_pin) instance is not set. The relevant if clause is included below, it starts at line 281 in the driver code.

    if (p_config->ss_pin != NRFX_SPIM_PIN_NOT_USED)
    {
        if (p_config->ss_active_high)
        {
            nrf_gpio_pin_clear(p_config->ss_pin);
        }
        else
        {
            nrf_gpio_pin_set(p_config->ss_pin);
        }
        //nrf_gpio_cfg_output(p_config->ss_pin);
			 nrf_gpio_cfg(p_config->ss_pin,
				  NRF_GPIO_PIN_DIR_OUTPUT,
				  NRF_GPIO_PIN_INPUT_DISCONNECT,
				  NRF_GPIO_PIN_NOPULL,
				  NRF_GPIO_PIN_H0H1,
				  NRF_GPIO_PIN_NOSENSE);
#if NRFX_CHECK(NRFX_SPIM_EXTENDED_ENABLED)
        if (p_config->use_hw_ss)
        {
            m_cb[p_instance->drv_inst_idx].use_hw_ss = p_config->use_hw_ss;
            nrf_spim_csn_configure(p_spim,
                                   p_config->ss_pin,
                                   (p_config->ss_active_high == true ?
                                        NRF_SPIM_CSN_POL_HIGH : NRF_SPIM_CSN_POL_LOW),
                                   p_config->ss_duration);
        }
#endif
        m_cb[p_instance->drv_inst_idx].ss_pin = p_config->ss_pin;
        m_cb[p_instance->drv_inst_idx].ss_active_high = p_config->ss_active_high;
    } 

When nrfx_spim_xfer() is called, after initializing the spim driver with ss_pin ==NRFX_SPIM_PIN_NOT_USED in the config, the ss_pin of the driver has not been initialized. In the code below p_cb->ss_pin is therefore not NRFX_SPIM_PIN_NOT_USED and the code in the if cluase is executed. I am not using the extended mode so the use_hw_ss flag is not checked.

    if (p_cb->ss_pin != NRFX_SPIM_PIN_NOT_USED)
    {
#if NRFX_CHECK(NRFX_SPIM_EXTENDED_ENABLED)
        if (!p_cb->use_hw_ss)
#endif
        {
            if (p_cb->ss_active_high)
            {
                nrf_gpio_pin_set(p_cb->ss_pin);
            }
            else
            {
                nrf_gpio_pin_clear(p_cb->ss_pin);
            }
        }
    }

[MartinBL]

This seems to be the same issue as the one reported here: https://devzone.nordicsemi.com/f/nordic-q-a/33246/sdk-15-spi-driver-sttting-with-ss_pin-nrf_drv_spi_pin_not_used

However, I believe they have fixed it in SDK 15.2.0 where they have added these lines in nrfx_spim_init():

// 'p_cb->ss_pin' variable is used during transfers to check if SS pin should be toggled,
// so this field needs to be initialized even if the pin is not used.
p_cb->ss_pin = p_config->ss_pin;

p_cb is a pointer to m_cb so the ss_pin is set no matter what.

[Gunnar Sigurðsson]

Ok, I've checked the driver in sdk 15.2 and it works as you've noted.

I tested it on sdk 15.0 and missed this assignment when reviewing sdk 15.2.

Thanks!