
Signal Description
2-12
MC68307 USER’S MANUAL
MOTOROLA
2.7.2 Serial Data (SDA/PB1)
This bidirectional open-drain signal is the data input/output for the M-bus interface. When
programmed as general-purpose input/output, this signal functions as bit 1 of port B.
2.8 UART I/O SIGNALS
The following signals are used by the UART serial I/O module for data and clock signals.
Refer to Section 8 Serial Module for more information on these signals.
2.8.1 Transmit Data (TxD/PB2)
This bidirectional signal can be programmed as the transmitter serial data output for the
UART module. The output is held high ('mark' condition) when the transmitter is disabled,
idle, or operating in the local loopback mode. Data is shifted out on this signal at the falling
edge of the serial clock source, with the least significant bit transmitted first. When pro-
grammed as a general-purpose input/output, this signal functions as bit 2 of the 16-bit sec-
ondary port, port B.
2.8.2 Receive Data (RxD/PB3)
This bidirectional signal can be programmed as the receiver serial data input for the UART
module. Data received on this signal is sampled on the rising edge of the serial clock source,
with the least significant bit received first. When the UART clock has been stopped for
power-down mode, any transition on this pin can optionally restart it. When programmed as
a general-purpose input/output, this signal functions as bit 3 of port B.
2.8.3 Request-To-Send (RTS/PB4)
This bidirectional signal can be programmed as an active-low request-to-send output from
the UART module. When programmed as a general-purpose input/output, this signal func-
tions as bit 4 of port B.
2.8.4 Clear-To-Send (CTS/PB5)
This bidirectional input signal can be programmed as the active-low clear-to-send input for
the UART module. When programmed as general-purpose input/output, this signal func-
tions as bit 5 of port B.
2.9 TIMER I/O SIGNALS
The following external signals are used by the timer module. Refer to Section 6 Dual Timer
Module for additional information on these signals.
2.9.1 Timer 1 Input (TIN1/PB6)
This bidirectional signal can be programmed as a clock input that causes events to occur in
timer/counter channel 1, either causing a clock to the event counter or providing a trigger to
the timer value capture logic. When programmed as a general-purpose input/output, this
signal functions as bit 6 of port B.