mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 422

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

Serial Communication Interface (SCI)Serial Communications Interface (SCIV4)
status flag is set. If RDRF was already set indicating the receive data register (buffer) was already full, the
overrun (OR) status flag is set and the new data is lost. Because the SCI receiver is double-buffered, the
program has one full character time after RDRF is set before the data in the receive data buffer must be
read to avoid a receiver overrun.
When a program detects that the receive data register is full (RDRF = 1), it gets the data from the receive
data register by reading SCIxD. The RDRF flag is cleared automatically by a two-step sequence normally
satisfied in the course of the user’s program that manages receive data. Refer to
and Status Flags,”
18.3.3.1
The SCI receiver uses a 16× baud rate clock for sampling. The receiver starts by taking logic level samples
at 16 times the baud rate to search for a falling edge on the RxD serial data input pin. A falling edge is
defined as a logic 0 sample after three consecutive logic 1 samples. The 16× baud rate clock divides the
bit time into 16 segments labeled RT1 through RT16. When a falling edge is located, three more samples
are taken at RT3, RT5, and RT7 to make sure this was a real start bit and not merely noise. If at least two
of these three samples are 0, the receiver assumes it is synchronized to a receive character.
The receiver then samples each bit time, including the start and stop bits, at RT8, RT9, and RT10 to
determine the logic level for that bit. The logic level is interpreted to be that of the majority of the samples
taken during the bit time. In the case of the start bit, the bit is assumed to be 0 if at least two of the samples
at RT3, RT5, and RT7 are 0 even if one or all of the samples taken at RT8, RT9, and RT10 are 1s. If any
sample in any bit time (including the start and stop bits) in a character frame fails to agree with the logic
level for that bit, the noise flag (NF) is set when the received character is transferred to the receive data
buffer.
The falling edge detection logic continuously looks for falling edges. If an edge is detected, the sample
clock is resynchronized to bit times. This improves the reliability of the receiver in the presence of noise
or mismatched baud rates. It does not improve worst case analysis because some characters do not have
any extra falling edges anywhere in the character frame.
In the case of a framing error, provided the received character was not a break character, the sampling logic
that searches for a falling edge is filled with three logic 1 samples so that a new start bit can be detected
almost immediately.
In the case of a framing error, the receiver is inhibited from receiving any new characters until the framing
error flag is cleared. The receive shift register continues to function, but a complete character cannot
transfer to the receive data buffer if FE remains set.
18.3.3.2
Receiver wakeup is a hardware mechanism that allows an SCI receiver to ignore the characters in a
message intended for a different SCI receiver. In such a system, all receivers evaluate the first character(s)
of each message, and as soon as they determine the message is intended for a different receiver, they write
logic 1 to the receiver wake up (RWU) control bit in SCIxC2. When RWU bit is set, the status flags
associated with the receiver (with the exception of the idle bit, IDLE, when RWUID bit is set) are inhibited
from setting, thus eliminating the software overhead for handling the unimportant message characters. At
18-14
Data Sampling Technique
Receiver Wakeup Operation
for more details about flag clearing.
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
Section 18.3.4, “Interrupts
Freescale Semiconductor