Datasheet

Pin Name Conventions
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 Data Sheet, Rev. 2.1
Freescale Semiconductor 113
12.3 Pin Name Conventions
The generic names of the SCI input/output (I/O) pins are:
• RxD, receive data
• TxD, transmit data
SCI I/O lines are implemented by sharing parallel I/O port pins. The full name of an SCI input or output
reflects the name of the shared port pin. Table 12-1 shows the full names and the generic names of the
SCI I/O pins.The generic pin names appear in the text of this section.
12.4 Functional Description
Figure 12-3 shows the structure of the SCI module. The SCI allows full-duplex, asynchronous, NRZ serial
communication between the MCU and remote devices, including other MCUs. The transmitter and
receiver of the SCI operate independently, although they use the same baud rate generator.
The source of the baud rate clock is determined by the configuration register 2 bit, SCIBDSRC. If
SCIBDSRC is set then the source of the SCI is the internal data bus clock. If SCIBDSRC is cleared, the
source of the SCI is oscillator output CGMXCLK.
During normal operation, the CPU monitors the status of the SCI, writes the data to be transmitted, and
processes received data.
12.4.1 Data Format
The SCI uses the standard non-return-to-zero mark/space data format illustrated in Figure 12-2.
Figure 12-2. SCI Data Formats
Table 12-1. Pin Name Conventions
Generic Pin
Names
RxD TxD
Full Pin Names PTB4/RxD PTB5/TxD
BIT 5
START
BIT
BIT 0 BIT 1
NEXT
STOP
BIT
START
BIT
8-BIT DATA FORMAT
BIT M IN SCC1 CLEAR
START
BIT
BIT 0
NEXT
STOP
BIT
START
BIT
9-BIT DATA FORMAT
BIT M IN SCC1 SET
BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT 8
BIT 2 BIT 3 BIT 4 BIT 6 BIT 7
PARITY
OR DATA
BIT
PARITY
OR DATA
BIT