35.6.3.8 32-bit Extension
For better system bus utilization, 32-bit data receive and transmit can be enabled by writing to the Data 32-bit bit field in the Control C register (CTRLC.DATA32B=1). When enabled, write and read transaction to/from the DATA register are 32 bit in size.
If frames are not multiples of 4 Bytes, the Length Counter (LENGTH.LEN) and Length Enable (LENGTH.LENEN) must be configured before data transfer begins. LENGTH.LEN must be enabled only when CTRLC.DATA32B is enabled.
The figure below shows the order of transmit and receive when using 32-bit mode. Bytes are transmitted or received and stored in order from 0 to 3.
Only 8-bit character size is supported.
32-bit Extension Client Operation
The figure below shows a transaction with 32-bit Extension enabled (CTRLC.DATA32B=1). When address recognition is enabled (CTRLA.FORM=0x2) and there is an address match, the address is loaded into the FIFO as Byte zero and data begins with Byte 1. INTFLAGS.RXC will then be raised for every 4 Bytes transferred. For transmit, there is a 32-bit holding buffer in the core domain. Once DATA has been registered in the core domain, INTFLAG.DRE will be raised, so that the next 32 bits can be written to the DATA register.
When utilizing the length counter, the LENGTH register must be written before the frame begins. If the frame length while SS is low is not a multiple of LENGTH.LEN Bytes, the Length Error Status bit (STATUS.LENERR) is raised. If LENGTH.LEN is not a multiple of 4 Bytes, the final INTFLAG.RXC interrupt will be raised when the last Byte is received.
The length count is based on the received Bytes, or the number of clocks if the receiver is not enabled. If pre-loading is disabled and DATA is written to for transmit before SCK starts, transmitted data will be delayed by one Byte, but the length counter will still increment for the first (empty) Byte transmission. When the counter reaches LENGTH.LEN, the internal length counter, Rx Byte counter, and Tx Byte counter are reset. If multiple lengths are to be transmitted, INTFLAG.TXC must go high before writing DATA for subsequent lengths.
If there is a Length Error (STATUS.LENERR), the remaining Bytes in the length will be transmitted at the beginning of the next frame. If this is not desired, the SERCOM must be disabled and re-enabled in order to flush the Tx and Rx pipelines.
Writing the LENGTH register while a frame is in progress will produce unpredictable results. If LENGTH.LENEN is not configured and a frame is not a multiple of 4 Bytes (while SS is low), the remainder will be transmitted in the next frame.
32-bit Extension Host Operation
When using the SPI configured as Host, the Length and the Length Enable bit fields (LENGTH.LEN and LENGTH.LENEN) must be written before the frame begins. When LENGTH.LENEN is written to '1', the value of LENGTH.LEN determines the number of data bytes in the transaction from 1 to 255.
For receive data, INTFLAG.RXC is raised every 4 Bytes received. If LENGTH.LEN is not a multiple of 4 Bytes, the final INTFLAG.RXC is set when the final byte is received.
For transmit, there is a holding buffer for the 32-bit data in the core domain. Once DATA has been registered in the core domain, INTFLAG.DRE will be raised so that the next 32 bits can be written to the DATA register.
If multiple lengths are to be transmitted, INTFLAG.TXC must go high before writing DATA for subsequent lengths.