Mailbox interface

CTRL-AP implements a mailbox interface which enables the CPU to communicate with a debugger over the SWD interface.

The mailbox interface consists of a transmit register MAILBOX.TXDATA with its corresponding status register MAILBOX.TXSTATUS, and a receive register MAILBOX.RXDATA with its corresponding status register MAILBOX.RXSTATUS. Status bits in registers TXSTATUS/RXSTATUS will be set and cleared automatically when registers TXDATA/RXDATA are written to and read from, independently of the direction.

Mailbox transfer sequence

1. Sender writes TXDATA
2. CTRL-AP sets sender's TXSTATUS to DataPending
3. CTRL-AP sets receiver's RXSTATUS to DataPending
4. Receiver reads RXDATA
5. CTRL-AP sets receiver's RXSTATUS to NoDataPending
6. CTRL-AP sets sender's TXSTATUS to NoDataPending

Events

EVENTS_RXREADY is generated when MAILBOX.RXSTATUS changes to DataPending. This indicates that a debugger has written new data to MAILBOX.RXDATA.

EVENTS_TXDONE is generated when the MAILBOX.TXSTATUS changes to NoDataPending. This indicates that a debugger has read the data from MAILBOX.TXDATA.