MC68HC08AS32 Datasheet, PDF (159/280 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

12.3.1 Data Format
The SCI uses the standard non-return-to-zero mark/space data format illustrated
in Figure 12-4.
8-BIT DATA FORMAT
(BIT M IN SCC1 CLEAR)
POSSIBLE
PARITY
BIT
NEXT
START
START
BIT BIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 ^STOP BIT
BIT
START
9-BIT DATA FORMAT
(BIT M IN SCC1 SET)
POSSIBLE
PARITY
BIT
NEXT
START
BIT BIT 0 BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT 8 STOP BIT
BIT
Figure 12-4. SCI Data Formats
12.3.2 Transmitter
Figure 12-5 shows the structure of the SCI transmitter.
12.3.3 Character Length
The transmitter can accommodate either 8-bit or 9-bit data. The state of the M bit
in SCI control register 1 (SCC1) determines character length. When transmitting
9-bit data, bit T8 in SCI control register 3 (SCC3) is the ninth bit (bit 8).
12.3.4 Character Transmission
During an SCI transmission, the transmit shift register shifts a character out to the
PTE0/TxD pin. The SCI data register (SCDR) is the write-only buffer between the
internal data bus and the transmit shift register. To initiate an SCI transmission:
1. Initialize the Tx and Rx rate in the SCI baud register (SCBR) ($0019) see
12.8.7 SCI Baud Rate Register.
2. Enable the SCI by writing a logic 1 to ENSCI in SCI control register 1 (SCC1)
($0013).
3. Enable the transmitter by writing a logic 1 to the transmitter enable bit (TE)
in SCI control register 2 (SCC2) ($0014).
4. Clear the SCI transmitter empty bit (SCTE) by first reading SCI status
register (SCS1) ($0016) and then writing to the SCDR ($0018).
5. Repeat step 3 for each subsequent transmission.
At the start of a transmission, transmitter control logic automatically loads the
transmit shift register with a preamble of 10 or 11 logic 1s. After the preamble shifts
out, control logic transfers the SCDR data into the transmit shift register. A logic 0
start bit automatically goes into the least significant bit position of the transmit shift
register. A logic 1 stop bit goes into the most significant bit position.
MC68HC08AS32 — Rev. 4.1
Freescale Semiconductor
Data Sheet
159