I'm trying to configure my STM32L4 as an SPI slave and am having some difficulty. I've integrated the SPI_FullDuplex_ComDMA sample into my application and configured my device as a slave. I can also use my Aardvark to transmit x# of bytes to the STM32 and it will reply with the data in it's Tx buffer. However, the issue is that the STM32 will only reply if I explicitly call the HAL_SPI_TransmitReceive_DMA function. If I don't call this function first I don't see any interrupt on the STM32 when the master device writes to it.
Do I need to explicitly do a read on the Slave STM32 in order for it the master to be able to send it data? If so, how am I supposed to handle the fact that the amount of data being sent by the master is unknown?