Using 32bit transfers on SPI

Peter,

What device are you using? There are different SPI controllers and their behavior is slightly different.

The basic issue here is that you need the master to issue two 16-bit transfers but make it look to the slave like one 32-bit transfer. The chip select signal plays an important role. If the chip select can be asserted for the two 16-bit transfers, then the slave will see it as one 32-bit transfer. 

For example, on Tiva devices, using the Freescale SPI Frame Format with SPO=0 and SPH=1, the chip select line is asserted for successive data words. So, this mode might work for you. However, using SPO=0 and SPH=0 would not work because the chip select line is pulsed between each data word.

If your particular device does not have such a mode as above, then your next option is to manage the chip select line yourself using a GPIO pin. In this case, do not connect the SPI chip select line but use a GPIO pin instead. In your code, you would assert (usually active low) the GPIO pin just before the SPI_transfer call and de-assert the GPIO pin after the SPI_transfer call returns. Configure your transfer as two 16-bit words.

~Ramsey