SERIAL COMMUNICATION INTERFACE (SCI)
6 - 26
PORT C
MOTOROLA
6.3.2.3.2
SCCR Clock Out Divider (COD) Bit 12
Figure 6-12 and Figure 6-35 show the clock divider circuit. The output divider is controlled
by COD and the SCI mode. If the SCI mode is synchronous, the output divider is fixed at
divide by 2; if the SCI mode is asynchronous, and
1. If COD equals zero and SCLK is an output (i.e., TCM and RCM=0), the SCI
clock is divided by 16 before being output to the SCLK pin; thus, the SCLK out-
put is a 1
×
clock
2. If COD equals one and SCLK is an output, the SCI clock is fed directly out to
the SCLK pin; thus, the SCLK output is a 16
×
baud clock
The COD bit is cleared by hardware and software reset.
6.3.2.3.3
SCCR SCI Clock Prescaler (SCP) Bit 13
The SCI SCP bit selects a divide by 1 (SCP=0) or divide by 8 (SCP=1) prescaler for the clock
divider. The output of the prescaler is further divided by 2 to form the SCI clock. Hardware and
software reset clear SCP. Figure 6-12 and Figure 6-35 show the clock divider diagram.
6.3.2.3.4
SCCR Receive Clock Mode Source Bit (RCM) Bit 14
RCM selects internal or external clock for the receiver (see Figure 6-35). RCM equals zero
selects the internal clock; RCM equals one selects the external clock from the SCLK pin.
Hardware and software reset clear RCM.
6.3.2.3.5
SCCR Transmit Clock Source Bit (TCM) Bit 15
The TCM bit selects internal or external clock for the transmitter (see Figure 6-35). TCM
equals zero selects the internal clock; TCM equals one selects the external clock from the
SCLK pin. Hardware and software reset clear TCM.
6.3.2.4
SCI Data Registers
The SCI data registers are divided into two groups: receive and transmit. There are two
receive registers – a receive data register (SRX) and a serial-to-parallel receive shift reg-
ister. There are also two transmit registers – a transmit data register (called either STX or
STXA) and a parallel-to-serial transmit shift register.
6.3.2.4.1
SCI Receive Registers
Data words received on the RXD pin are shifted into the SCI receive shift register. When
the complete word has been received, the data portion of the word is transferred to the
byte-wide SRX. This process converts the serial data to parallel data and provides double
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Freescale Semiconductor, Inc.
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