57.7.3.1 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, 1 optional parity bit and up to 2 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 selected by the CHRL field and the MODE9 bit in FLEX_US_MR. Nine bits are selected by setting the MODE9 bit regardless of the CHRL field. The parity bit is set according to the PAR field in FLEX_US_MR. The even, odd, space, marked or none parity bit can be configured. The MSBF bit in FLEX_US_MR 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 the NBSTOP field in FLEX_US_MR. The 1.5 stop bit is supported in Asynchronous mode only.

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

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