Datasheet
14-30 MCF5407 User’s Manual
Operation
Figure 14-34. Receiver Timing
When the receiver detects a high-to-low (mark-to-space) transition of the start bit on RxD,
the state of RxD is sampled each 16× clock for eight clocks, starting one-half clock after
the transition (asynchronous operation) or at the next rising edge of the bit time clock
(synchronous operation). If RxD is sampled high, the start bit is invalid and the search for
the valid start bit begins again.
If RxD is still low, a valid start bit is assumed and 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 RxD input is
sampled on the rising edge of the programmed clock source. The lsb is received first. The
data is then transferred to a receiver holding register and USRn[RxRDY] is set. If the
character is less than eight bits, the most significant unused bits in the receiver holding
register are cleared.
After the stop bit is detected, the receiver immediately looks for the next start bit. However,
if a non-zero 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 were detected. Parity error, framing error, overrun error, and received break
conditions set the respective PE, FE, OE, RB error and break flags in the USRn at the
received character boundary and are valid only if USRn[RxRDY] is set.
If a break condition is detected (RxD is low for the entire character including the stop bit),
a character of all zeros is loaded into the receiver holding register (RHR) and
USRn[RB,RxRDY] are set. RxD must return to a high condition for at least one-half bit
time before a search for the next start bit begins.
C1 C2 C4 C6 C7
C8
C3 C5
C6, C7, and C8 will be los
t
(C2)
Status
Data
(C3)
Status
Data
(C4)
Status
Data
C5 will
be lost
Reset by
command
TxD
Receiver
Enabled
USRn
[RxRDY]
Overrun
R
TS
4
internal
module
select
USRn
[FFULL]
(C1)
Status
Data
USRn
[OE]
Automatically asserted
when ready to receive
Manually asserted first time,
automatically negated if overrun occurs
UOP0[RTS] = 1