15-6
MCF5249UM
MOTOROLA
Operation
USR is set. Transmission resumes and the TxEMP bit is cleared when the CPU loads a new character into
the UART transmitter buffer (UTB). If the transmitter receives a disable command, it continues operating
until the character (if one is present) in the transmit-shift register is completely shifted out of transmitter
TxD. If the transmitter is reset through a software command, operation ceases immediately (refer to
operation after a disable or software reset.
Figure 15-5 Transmitter Timing Diagram
If 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 transmitted and following the
completion of STOP bits TxD, enters in the mark state until CTS is asserted again. If the transmitter is
forced to send a continuous low condition by issuing a Send-Break command, the transmitter ignores the
state of CTS.
Users can program the transmitter to automatically negate the request-to-send (RTS) output on completion
of a message transmission. If the transmitter is programmed to operate in this mode, RTS must be
manually asserted before a message is transmitted. In applications where the transmitter is disabled after
transmission is complete and RTS is appropriately programmed, RTS is negated one bit time after the
character in the shift register is completely transmitted. Users must manually enable the transmitter by
setting the enable-transmitter bit in the UART Command Register (UCR).
C1
C2
C3
C6
C4
Transmitter 4
Enabled
W
Internal
Module
Select
TxRDY
TxD
CTS1
C1
C2
C3
C4
C5
C6
Start5
Break
Stop
Break
Not
Transmitted
Negated since transmit
buffer and shift register
are empty (last character
has been shifted out)
Notes:
1. Timing shown for UMR2[4]=1
2. Timing shown for UMR2[5]=1
3. CN=Transmit 8-bit character
4. Transmitter enable by configuring TCx bits in UCR (see
Table 15-14)5. Start break/Stop break programmed by MISCx bits in UCR (see
TableDisable 7
Trans.
Manually
Asserted
Manually Asserted
by Bit-Set Command
RTS2
(W=Write)
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.