Latency of 2us when set up the SPIM transmit and receive buffer

[Bernard Chan Lye Hock]

Hi Nordic,

Currently I am using the nRF5_SDK_15.2.0_9412b96 to develop SPI interface with a sensor using SPIM for nRF52832.

Within the spim_xfer(NRF_SPIM_Type * p_spim, spim_control_block_t * p_cb, nrfx_spim_xfer_desc_t const * p_xfer_desc, uint32_t flags) function, I can see that the code is setting up the TX and RX buffer of the SPIM as below:

    nrf_spim_tx_buffer_set(p_spim, p_xfer_desc->p_tx_buffer, p_xfer_desc->tx_length);
    nrf_spim_rx_buffer_set(p_spim, p_xfer_desc->p_rx_buffer, p_xfer_desc->rx_length);

The issue that I am seeing is that it takes a long period of 2us (measured on oscilloscope) to execute the set up the TX and RX buffer.

Why does it takes so long time and what can I do to reduce it?

From the time my SS pin is pulled low to first TX SCK comes out, this delay of 2us is always there. I would like to reduce this to a minimum for my application to work in optimised state.

Please help!

Thanks

[Kenneth]

Hi,

So I assume you have measured that spim_xfer() takes 2us to execute here, and that there is no delay due to wakeup from sleep that take is part of the 2us. 

To speed up transfer you can call spim_xfer() with NRFX_SPIM_FLAG_HOLD_XFER flag to setup the SPI transfer before SS pin. You can get the task to start the transfer by calling nrfx_spim_start_task_get():

https://www.nordicsemi.com/DocLib/Content/SDK_Doc/nRF5_SDK/v15-2-0/group__nrfx__spim#gadb5b212e351bd65958c6be78a957ac65 

Then you only need to call the start task to trigger the SPIM transfer (for instance through PPI or from application).

Best regards,
Kenneth