SPIM data transfert depends on Rx buffer

[Chaabane]

Hello,

I have configured the SPI0 module in master mode with EasyDMA. I use the below code to send data 

    uint8_t rx_buffer[24]="";
    uint8_t tx_buffer[] = "SPIM TEST";

    nrfx_spim_xfer_desc_t  spim_xfer_desc =
    {
        .p_rx_buffer = rx_buffer,
        .rx_length   = sizeof(rx_buffer),
    };

    spim_xfer_desc.p_tx_buffer = ( uint8_t*)tx_buffer;
    spim_xfer_desc.tx_length   = strlen(tx_buffer);        

    result = nrfx_spim_xfer(&_t_spi_inst, &spim_xfer_desc, 0);

In the output I get  the string "SPIM TEST" and with extra 0xFF bytes. I noticed that the total number of bytes corresponds to the rx_length ?

I reduced the rx_length to 4 bytes and I was able to receive the right amount of bytes on the output !

I did the same thing with the spi example and got the same results. Is this the normal behavior of the module ?

Thank you

[RK]

Yes  that's  how SPI works, it's full duplex, every cycle bytes are clocked in AND out, so if you clock in 24 bytes you clock out 24 bytes too. The data clocked out depends on the setting of the 'idle bit' but by default that's 1 so all the extra bytes would be 0xFF. All SPI devices are designed to understand this protocol and ignore bytes they weren't expecting. 

en.wikipedia.org/.../Serial_Peripheral_Interface

[Chaabane]

Thank you for the information