Freescale Semiconductor, Inc.
Serial Communications Interface (SCI)
• Character Length — The receiver can accommodate either 8-bit
or 9-bit data. The state of the M bit in SCI control register 1
(SCCR1) determines character length. When receiving 9-bit data,
bit R8 in SCCR1 is the ninth bit (bit 8).
• Character Reception — During reception, the receive shift register
shifts characters in from the PD0/RDI pin. The SCI data register
(SCDR) is the read-only buffer between the internal data bus and
the receive shift register.
After a complete character shifts into the receive shift register, the
data portion of the character is transferred to the SCDR, setting
the receive data register full (RDRF) flag. The RDRF flag can be
used to generate an interrupt.
• Receiver Wakeup — So that the MCU can ignore transmissions
intended only for other receivers in multiple-receiver systems, the
MCU can be put into a standby state. Setting the receiver wakeup
enable (RWU) bit in SCI control register 2 (SCCR2) puts the MCU
into a standby state during which receiver interrupts are disabled.
Either of two conditions on the PD0/RDI pin can bring the MCU out
of the standby state:
– Idle input line condition — If the PD0/RDI pin is at logic 1 long
enough for 10 or 11 logic 1s to shift into the receive shift
register, receiver interrupts are again enabled.
– Address mark — If a logic 1 occurs in the most significant bit
position of a received character, receiver interrupts are again
enabled.
The state of the WAKE bit in SCCR1 determines which of the two
conditions wakes up the MCU.
• Receiver Noise Immunity — The data recovery logic samples
each bit 16 times to identify and verify the start bit and to detect
noise. Any conflict between noise detection samples sets the
noise flag (NF) in the SCSR. The NF bit is set at the same time
that the RDRF bit is set.
Technical Data
128
MC68HC705C8A — Rev. 3
Serial Communications Interface (SCI)
For More Information On This Product,
Go to: www.freescale.com
FREESCALE [ Freescale ]
