
MOTOROLA
2-20
PINS AND CONNECTIONS
M68HC11
REFERENCE MANUAL
ceiver is enabled, the PD0/RxD pin becomes an input dedicated to the RxD function.
When the SCI transmitter is enabled, the PD1/TxD pin becomes an output dedicated
to the TxD function. When the serial peripheral interface (SPI) system is enabled, the
PD2/MISO, PD3/MOSI, PD4/SCK, and PD5/SS pins become dedicated to SPI func-
tions. Even while the SPI system is enabled, the PD5/SS pin can be used as a general-
purpose output by setting the corresponding DDRD5 bit, provided the SPI system is
configured for master mode of operation.
The six port D pins can be configured (port D wired-OR mode (DWOM) control bit in
SPI control register (SPCR)) for wired-OR operation. This option disables the P-chan-
nel device in the output drivers so port D outputs can actively drive low but not high,
allowing two or more such outputs to be connected without contention. Since the P-
channel device is physically present (just turned off), there is an inherent diode from
the output pin to V
DD
so the pin cannot be pulled to a level higher than V
DD
(unlike a
transistor-transistor logic (TTL) open-collector output). An external pull-up resistor is
required on all port D outputs when the wired-OR option is used. The firmware boot-
loader program configures port D for wired-OR operation when the MCU is reset in
bootstrap mode. If the application is using bootstrap mode, either turn off the wired-
OR option after downloading or supply external pull-up resistors on port D output pins.
2.2.10 Ports B and C, STRA, and STRB Pins
These 18 pins are used for general-purpose I/O while the MCU is operating in single-
chip mode. When an expanded mode is used, these 18 pins become a multiplexed ad-
dress/data bus with an address strobe (AS) and a read/write (R/W) control line.
Table
2-2
summarizes the functions of these pins related to the MCU operating mode.
In single-chip modes, no external address/data bus is needed; thus, these 18 pins are
available for general-purpose I/O. Port B is an 8-bit output-only port; port C is an 8-bit
bidirectional I/O port. Any combination of bits in port C can be configured as outputs;
the remaining bits are used as inputs. Several automated handshake I/O functions are
associated with ports B and C. These strobe and handshake functions use the STRA
and STRB pins as strobes and handshake controls. The STRA pin is an edge-detect-
ing input that causes port C data to be latched into a special internal latch register. The
active edge for STRA is software selectable, and any port C pin can be used for gen-
eral-purpose static I/O while other pins are being used for latched inputs. If strobe and
handshake functions are not being used, STRA can still be used as an edge-detecting
interrupt input but cannot be used as a general-purpose static input. The STRB pin is
an output strobe associated with the handshake I/O functions of ports B and C. If hand-
shake functions are not being used, STRB can still be used as a general-purpose out-
put, though it is more difficult to control than a normal port output pin. For a detailed
discussion of the handshake I/O functions of ports B and C, refer to
SECTION 7 PAR-
ALLEL INPUT/OUTPUT
.
When the MCU is operating in expanded modes, these 18 pins are used for an ad-
dress/data bus to allow the central processing unit (CPU) to access a 64-Kbyte mem-
ory space. To save pins, the low-order address and 8-bit data are time multiplexed on
eight pins. During the first half of each bus cycle, address output signals, ADDR[7:0],
are present on these eight pins; during the second half of each bus cycle, these eight