Motorola ColdFire MCF5281 - Manual (Page 439)

UART Modules
Freescale Semiconductor 23-19
optional parity bit, and the programmed number of stop bits. The lsb is sent first. Data is shifted from the
transmitter output on the falling edge of the clock source.
After the stop bits are sent, if no new character is in the transmitter holding register, the UTXDn output
remains high (mark condition) and the transmitter empty bit (USRn[TXEMP]) is set. Transmission
resumes and TXEMP is cleared when the CPU loads a new character into the UART transmit buffer
(UTBn). If the transmitter receives a disable command, it continues until any character in the transmitter
shift register is completely sent.
If the transmitter is reset through a software command, operation stops immediately (see Section 23.3.5,
“UART Command Registers (UCRn) ”). The transmitter is reenabled through the UCRn to resume
operation after a disable or software reset.
If the clear-to-send operation is enabled, UCTSn must be asserted for the character to be transmitted. If
UCTSn is negated in the middle of a transmission, the character in the shift register is sent and UTXDn
remains in mark state until UCTSn is reasserted. If transmitter is forced to send a continuous low condition
by issuing a
SEND BREAK command, transmitter ignores the state of UCTSn.
If the transmitter is programmed to automatically negate URTSn when a message transmission completes,
URTSn must be asserted manually before a message is sent. In applications in which the transmitter is
disabled after transmission is complete and URTSn is appropriately programmed, URTSn is negated one
bit time after the character in the shift register is completely transmitted. The transmitter must be manually
reenabled by reasserting URTSn before the next message is sent.
Figure 23-19 shows the functional timing information for the transmitter.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3