MC9S12KG128 Datasheet, PDF (343/600 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 10 Serial Communications Interface (SCIV1)
10.4.4.2 Character Reception
During an SCI reception, the receive shift register shifts a frame in from the Rx input signal. The SCI data
register is the read-only buffer between the internal data bus and the receive shift register.
After a complete frame shifts into the receive shift register, the data portion of the frame transfers to the
SCI data register. The receive data register full flag, RDRF, in SCI status register 1 (SCISR1) becomes set,
indicating that the received byte can be read. If the receive interrupt enable bit, RIE, in SCI control
register 2 (SCICR2) is also set, the RDRF flag generates an RDRF interrupt request.
10.4.4.3 Data Sampling
The receiver samples the Rx input signal 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, the RT clock (see Figure 10-13) 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 recovery logic does an asynchronous search for a logic 0 preceded by three
logic 1s.When the falling edge of a possible start bit occurs, the RT clock begins to count to 16.
START BIT
LSB
Rx Input Signal
SAMPLES 1 1 1 1 1 1 1 1 0
0
0
0000
START BIT
QUALIFICATION
START BIT
VERIFICATION
DATA
SAMPLING
RT CLOCK
RT CLOCK COUNT
RESET RT CLOCK
Figure 10-13. Receiver Data Sampling
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 10-11 summarizes the results of the start bit verification samples.
Table 10-11. Start Bit Verification
RT3, RT5, and RT7 Samples
000
001
010
011
Start Bit Verification
Yes
Yes
Yes
No
Noise Flag
0
1
1
0
MC9S12KG128 Data Sheet, Rev. 1.15
Freescale Semiconductor
343