
Chapter 13 Serial Communications Interface (S12SCIV2) Block Description
Freescale Semiconductor
MC9S12Q128
397
Rev 1.10
13.4.4.2
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 ag, 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 ag generates an RDRF interrupt request.
13.4.4.3
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 13-13) 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.
Figure 13-13. Receiver Data Sampling
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 13-11 summarizes the results of the start bit verication samples.
Table 13-11. Start Bit Verication
RT3, RT5, and RT7 Samples
Start Bit Verication
Noise Flag
000
Yes
0
001
Yes
1
010
Yes
1
011
No
0
RESET RT CLOCK
RT1
RT2
RT3
RT4
RT5
RT8
RT7
RT6
RT11
RT10
RT9
RT15
RT14
RT13
RT12
RT16
RT1
RT2
RT3
RT4
SAMPLES
RT CLOCK
RT CLOCK COUNT
START BIT
Rx Input Signal
START BIT
QUALIFICATION
START BIT
DATA
SAMPLING
11
1
11
0
00
LSB
VERIFICATION