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UART Modules
MCF5213 ColdFire® Integrated Microcontroller Reference Manual, Rev. 3
Freescale Semiconductor
21-19
If the transmitter is programmed to automatically negate U
n
RTS when a message transmission completes,
U
n
RTS must be asserted manually before a message is sent. In applications in which the transmitter is
disabled after transmission is complete and U
n
RTS is appropriately programmed, U
n
RTS is negated one
bit time after the character in the shift register is completely transmitted. The transmitter must be manually
reenabled by reasserting U
n
RTS before the next message is sent.
shows the functional timing information for the transmitter.
Figure 21-19. Transmitter Timing Diagram
21.4.2.2
Receiver
The receiver is enabled through its UCR
n
Section 21.3.5, “UART Command Registers
When the receiver detects a high-to-low (mark-to-space) transition of the start bit on U
n
RXD, the state of
U
n
RXD is sampled eight times on the edge of the bit time clock starting one-half clock after the transition
(asynchronous operation) or at the next rising edge of the bit time clock (synchronous operation). If
U
n
RXD is sampled high, start bit is invalid and the search for the valid start bit begins again.
If U
n
RXD remains low, a valid start bit is assumed. The receiver continues sampling the input at one-bit
time intervals at the theoretical center of the bit until the proper number of data bits and parity, if any, is
assembled and one stop bit is detected. Data on the U
n
RXD input is sampled on the rising edge of the
C1
1
C2
C3
Break
C4
C6
U
n
TXD
Transmitter
Enabled
USR
n
[TXRDY]
W
2
W
W
W
W
W
W
W
U
n
CTS
3
U
n
RTS
4
Manually asserted
by
BIT
-
SET
command
Manually
asserted
Start
break
C5
not
transmitted
C6
C4
Stop
break
C3
C2
C1
1
C1 in transmission
3
UMR2
n
[TXCTS] = 1
1
C
n
= transmit characters
2
W = write
4
UMR2
n
[TXRTS] = 1
internal
module
select
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