
Technical Data
MC68HC705C8A 
—
 Rev. 3
126
Serial Communications Interface (SCI)
MOTOROLA
Serial Communications Interface (SCI)
Break Characters 
—
 Writing a logic 1 to the SBK bit in SCCR2 
loads the shift register with a break character. A break character 
contains all logic 0s and has no start and stop bits. Break 
character length depends on the M bit in SCCR1. As long as SBK 
is at logic 1, transmitter logic continuously loads break characters 
into the shift register. After software clears the SBK bit, the shift 
register finishes transmitting the last break character and then 
transmits at least one logic 1. The automatic logic 1 at the end of 
a break character is to guarantee the recognition of the start bit of 
the next character.
Idle Characters 
—
 An idle character contains all logic 1s and has 
no start or stop bits. Idle character length depends on the M bit in 
SCCR1. The preamble is a synchronizing idle character that 
begins every transmission.
Clearing the TE bit during a transmission relinquishes the 
PD1/TDO pin after the last character to be transmitted is shifted 
out. The last character may already be in the shift register, or 
waiting in the SCDR, or it may be a break character generated by 
writing to the SBK bit. Toggling TE from logic 0 to logic 1 while the 
last character is in transmission generates an idle character (a 
preamble) that allows the receiver to maintain control of the 
PD1/TDO pin.
Transmitter Interrupts 
—
 These sources can generate SCI 
transmitter interrupt requests:
–
Transmit Data Register Empty (TDRE) 
—
 The TDRE bit in the 
SCSR indicates that the SCDR has transferred a character to 
the transmit shift register. TDRE is a source of SCI interrupt 
requests. The transmission complete interrupt enable bit 
(TCIE) in SCCR2 is the local mask for TDRE interrupts.
–
Transmission Complete (TC) 
—
 The TC bit in the SCSR 
indicates that both the transmit shift register and the SCDR are 
empty and that no break or idle character has been generated. 
TC is a source of SCI interrupt requests. The transmission 
complete interrupt enable bit (TCIE) in SCCR2 is the local 
mask for TC interrupts.