mcf5272 Freescale Semiconductor, Inc, mcf5272 Datasheet - Page 385

• Manufacturer Part Number: mcf5272
• Description: Mcf5272 Coldfire Integrated Microprocessor User
• Manufacturer: Freescale Semiconductor, Inc

MOTOROLA
USRn[RxRDY]
16.5.2.2 Receiver
The receiver is enabled through its UCRn, as described in Section 16.3.5, “UART
Command Registers (UCRn).” Figure 16-26 shows receiver functional 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.
USRn[FFULL]
USRn[OE]
Receiver
Enabled
Overrun
internal
module
select
RTS
RxD
4
UOP0[RTS] = 1
Manually asserted first time,
automatically negated if overrun occurs
C1
Status
Data
(C1)
C2
Figure 16-26. Receiver Timing
Chapter 16. UART Modules
C3
C25
is lost
C26
C26
Status
Data
(C2)
C27
C26, C27, and C82 are lost
Automatically asserted
when ready to receive
Status
Data
(C3)
Status
Data
(C4)
C28
Operation
command
Reset by
16-25
C29