45.6.3.1 Transmitter Operations

The transmitter performs the same in both Synchronous and Asynchronous operating modes (SYNC = 0 or SYNC = 1). One start bit, up to 9 data bits, one optional parity bit and up to two stop bits are successively shifted out on the TXD pin at each falling edge of the programmed serial clock.

The number of data bits is configured in the US_MR.CHRL and the US_MR.MODE9. Nine bits are selected by writing a ‘1’ to US_MR.MODE9 regardless of the CHRL field. The parity is selected by US_MR.PAR. Even, odd, space, marked or none parity bit can be configured. US_MR.MSBF configures which data bit is sent first. If written to ‘1’, the most significant bit is sent first. If written to ‘0’, the less significant bit is sent first. The number of stop bits is selected by US_MR.NBSTOP. The 1.5 stop bit is supported in Asynchronous mode only.

The characters are sent by writing in US_THR. The transmitter reports two status bits in the Channel Status register (US_CSR): TXRDY (Transmitter Ready), which indicates that US_THR is empty, and TXEMPTY, which indicates that all the characters written in US_THR have been processed. When the current character processing is completed, the last character written in US_THR is transferred into the Shift register of the transmitter and US_THR becomes empty, thus TXRDY rises.

Both TXRDY and TXEMPTY are low when the transmitter is disabled. Writing a character in US_THR while TXRDY is low has no effect and the written character is lost.