MC68HC08AS32 Datasheet, PDF (241/394 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Serial Communications Interface (SCI)
Functional Description
17.4.4 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 17.9.7 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.
The SCI transmitter empty bit, SCTE in the SCI status control register
(SCS1), becomes set when the SCDR transfers a byte to the transmit
shift register. The SCTE bit indicates that the SCDR can accept new
data from the internal data bus. If the SCI transmit interrupt enable bit,
SCTIE (SCC2), is also set, the SCTE bit generates a transmitter CPU
interrupt request.
When the transmit shift register is not transmitting a character, the
PTE0/TxD pin goes to the idle condition, logic 1. If at any time software
clears the ENSCI bit in SCI control register 1 (SCC1), the transmitter and
receiver relinquish control of the port E pins.
MC68HC08AS32 — Rev. 3.0
MOTOROLA
Serial Communications Interface (SCI)
Advance Information
241