35.6.3.9.1 Pointer Operation in Host Mode

In Host mode, the transmit / receive sequence is started by the CPU writing one or more transmit words into the TX FIFO. The CPUWRPTR is incremented by 1 every time the CPU writes a word to the memory array. As soon as the CPUWRPTR becomes not-equal to SPIRDPTR (FIFOSPACE.TXSPACE != 0), the SPI transmits the data pointed to by the SPIRDPTR through MOSI. When a complete word is shifted in, the SPIRDPTR is compared to CPUWRPTR. If they are not equal, SPIRDPTR is incremented, another byte / word is shifted in/out, and so on. When the CPU completes a read from the RX FIFO location (FIFOSPACE.RXSPACE != 0), the CPURDPTR pointer is incremented. When both RX shifter and RX FIFO are full, the Buffer Overflow status bit is set (STATUS.BUFOVF) and optional ERROR interrupt is generated. The module will not respond to SCK transitions while BUFOVF is ‘1’, effectively disabling the module until software reads DATA register.

All pointers increment to their maximum value, dictated by CTRLC.DATA32B bit, and then rolls over to ‘0’. CPURDPTR will not increment past SPIWRPTR. In other words, if CPURDPTR = SPIWRPTR, and the CPU attempts another read, the pointer will stay at the value of SPIWRPTR.