NXP Semiconductors MCF5253, Reference Manual, Page: 254 / 648

UART Modules
MCF5253 Reference Manual, Rev. 1
15-8 Freescale Semiconductor
15.3.2.2 Receiver
The receiver is enabled through the UCR located within the UART module. Functional timing information
for the receiver is shown in
Figure 15-6 . The receiver looks for a high-to-low (mark-to-space) transition
of the start bit on RxD. When a transition is detected the start bit is validated 0.5 baud clock after the
transition. If RxD is sampled high, the start bit is not valid and the search for the valid start bit repeats. If
RxD is still low, a valid start bit is assumed and the receiver continues to sample the input at one-bit time
intervals at the theoretical center of the bit.
Figure 15-6. Receiver Timing Diagram
This process continues until the proper number of data bits and parity (if any) is assembled and one stop
bit is detected. Data on the RxD input is sampled on the rising edge of the programmed clock source. The
least significant bit is received first. The data is then transferred to a receiver holding register and the
RxRDY bit in the USR is set. If the character length is less than eight bits, the most significant unused bits
in the receiver holding register are cleared. The Rx RDY bit in the USR is set at the one-half point of the
stop bit.
After the stop bit is detected, the receiver immediately looks for the next start bit. However, if a nonzero
character is received without a stop bit (framing error) and RxD remains low for one-half of the bit period
after the stop bit is sampled, the receiver operates as if a new start bit is detected. The parity error (PE),
C1 C2 C3 C4 C5 C6 C7 C8
RxD
RECEIVER
ENABLED
C6, C7, C8 ARE LOST DUE TO
RECEIVER DISABLED
RxRDY2(S
RO)
FFULL2.5
(SR1)
R R R R R R
|STATUS DATA| |STATUS DATA| |STATUS
DATA| | | | C2
C3 C4
INTERNAL
MODULE
SELECT CS
R R
|STATUS DATA|
|
OVERRUN
(SR4)
RTS1
(OP0)
RESET BY COMMAND
C5
LOST
NOTES:
1. TIMING SHOWN FOR UMR1[7]=1
2. TIMING SHOWN FOR UMR1[6]=0
3. CN = RECEIVED 5-8 BIT CHARACTER
UOP1[0]=1
R = Read