Freescale Semiconductor MC68HC908MR16, MC68HC908MR32 - page 165

Functional Description
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor 165
13.3.3.2 Character Reception
During an SCI reception, the receive shift register shifts characters in from the PTF4/RxD pin. The SCI
data register (SCDR) is the read-only buffer between the internal data bus and the receive shift register.
After a complete character shifts into the receive shift register, the data portion of the character transfers
to the SCDR. The SCI receiver full bit, SCRF, in SCI status register 1 (SCS1) becomes set, indicating that
the received byte can be read. If the SCI receive interrupt enable bit, SCRIE, in SCC2 is also set, the
SCRF bit generates a receiver CPU interrupt request.
13.3.3.3 Data Sampling
The receiver samples the PTF4/RxD pin 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 is resynchronized at
these times (see Figure 13-7):
• After every start bit
• After the receiver detects a data bit change from 1 to 0 (after the majority of data bit samples at
RT8, RT9, and RT10 return a valid 1 and the majority of the next RT8, RT9, and RT10 samples
return a valid 0)
Figure 13-7. Receiver Data Sampling
To locate the start bit, data recovery logic does an asynchronous search for a 0 preceded by three 1s.
When the falling edge of a possible start bit occurs, the RT clock begins to count to 16.
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 13-1 summarizes the results of the start bit verification samples.
RT CLOCK
RESET
RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT2 RT3 RT4 RT5 RT8 RT7 RT6
RT11 RT10
RT9
RT15 RT14 RT13 RT12 RT16
RT1 RT2 RT3 RT4
START BIT
QUALIFICATION
START BIT
VERIFICATION
DATA
SAMPLING
SAMPLES
RT
CLOCK
RT CLOCK
STATE
START BIT LSB
PTF4/RxD