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MC9S12Q128_10 Datasheet, PDF (397/644 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers | |||
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Chapter 13 Serial Communications Interface (S12SCIV2) Block Description
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 ï¬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 ï¬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.
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 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 veriï¬cation samples.
Table 13-11. Start Bit Veriï¬cation
RT3, RT5, and RT7 Samples
000
001
010
011
Start Bit Veriï¬cation
Yes
Yes
Yes
No
Noise Flag
0
1
1
0
Freescale Semiconductor
MC9S12Q128
397
Rev 1.10
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