mc68hc908qz16 Freescale Semiconductor, Inc, mc68hc908qz16 Datasheet - Page 190

• Manufacturer Part Number: mc68hc908qz16
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Enhanced Serial Communications Interface (ESCI) Module
15.4.3.1 Character Length
The receiver can accommodate either 8-bit or 9-bit data. The state of the M bit in ESCI control register 1
(SCC1) determines character length. When receiving 9-bit data, bit R8 in ESCI control register 3 (SCC3)
is the ninth bit (bit 8). When receiving 8-bit data, bit R8 is a copy of the eighth bit (bit 7).
15.4.3.2 Character Reception
During an ESCI reception, the receive shift register shifts characters in from the RxD pin. The ESCI 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 ESCI receiver full bit, SCRF, in ESCI status register 1 (SCS1) becomes set, indicating
that the received byte can be read. If the ESCI receive interrupt enable bit, SCRIE, in SCC2 is also set,
the SCRF bit generates a receiver CPU interrupt request.
15.4.3.3 Data Sampling
The receiver samples the 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
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.
To verify the start bit and to detect noise, data recovery logic takes samples at RT3, RT5, and RT7.
Table 15-2
190
•
•
Figure
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)
summarizes the results of the start bit verification samples.
15-7):
RT CLOCK
RT CLOCK
SAMPLES
CLOCK
RESET
STATE
RxD
RT
RT3, RT5, and RT7 Samples
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
Figure 15-7. Receiver Data Sampling
000
001
010
QUALIFICATION
Table 15-2. Start Bit Verification
START BIT
VERIFICATION
START BIT
Start Bit Verification
START BIT
Yes
Yes
Yes
SAMPLING
DATA
Noise Flag
0
1
1
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