MSC8144E Reference Manual, Rev. 3
20-12
Freescale
Semiconductor
UART
20.2.2 Data Sampling
The receiver samples
URXD
at the RT clock rate. The RT clock is an internal signal with a
frequency 16 times the baud rate. To adjust for baud rate mismatch between the baud rate
generated by RT clock and the target baud rate, the RT clock (see Figure 20-9) is
re-synchronized:
After every start bit.
After the receiver detects a data bit change from logic 1 to logic 0 (after the majority of
data bit samples at RT8, RT9, and RT10 returns a valid logic 1 and the majority of the
next RT8, RT9, and RT10 samples returns a valid logic 0).
To locate the start bit, data sampling logic searches for a logic 0 preceded by three logic 1s. When
the falling edge of a possible start bit occurs, the RT clock logic begins to count to 16.
Figure 20-8. UART Receiver Block Diagram
Al
l O
n
es
M
WAKE
ILT
PE
PT
RE
H 8 7 6 5 4 3 2 1 0 L
11-Bit Receive Shift Register
S
top b
it
S
tar
t bi
t
Data
Wake-up
Parity
Checking
MS
B
SCI Data Register R[7–0]
R8
RIE
ILIE
RWU
RDRF
OR
NF
FE
PF
URXD
System
Idle Interrupt Source
RDRF Interrupt Source
SBR[12–0]
Baud-Rate
From UTXD
OR Transmitter
Clock
IDLE
RAF
Sampling
Logic
OR Interrupt Source
Generator
sa
mpl
e
sa
mpl
e
Loop
LOOPS
RSRC
Control
MBus
Содержание MSC8144E
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