Datasheet

14-26 MCF5407 User’s Manual
Operation
14.5.2.1 Transmitting in UART Mode
The transmitter is enabled through the UART command register (UCRn). When it is ready
to accept a character, the UART sets USRn[TxRDY]. The transmitter converts parallel data
from the CPU to a serial bit stream on TxD. It automatically sends a start bit followed by
the programmed number of data bits, an 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 TxD
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 transmitter 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 14.3.10, “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, CTS
must be asserted for the character to be
transmitted. If CTS
is negated in the middle of a transmission, the character in the shift
register is sent and TxD remains in mark state until CTS
is reasserted. If the transmitter is
forced to send a continuous low condition by issuing a
SEND BREAK command, the
transmitter ignores the state of CTS
.
If the transmitter is programmed to automatically negate R
TS when a message transmission
completes, R
TS must be asserted manually before a message is sent. In applications in
which the transmitter is disabled after transmission is complete and R
TS is appropriately
programmed, R
TS is negated one bit time after the character in the shift register is
completely transmitted. The transmitter must be manually reenabled by reasserting R
TS
before the next message is to be sent.
Figure 14-30 shows the functional timing information for the transmitter.