
MOTOROLA
2-28
PINS AND CONNECTIONS
M68HC11
REFERENCE MANUAL
a battery-based system). For additional detailed information concerning the A/D input
pins, see
12.3 A/D Pin Connection Considerations
.
2.4.5 Internal Circuitry — Digital I/O Pin
Figure 2-17
shows the MOS circuitry for an MCU pin capable of operating as an input
or an output. Even when the pin is configured to disable the output driver circuitry, the
MOS transistors still affect the way the pin reacts to illegal levels. The P-channel de-
vice of the output driver [3] forms an inherent diode to V
DD
, and the N-channel device
forms an inherent diode to V
SS
, which is in parallel with the inherent diode of the thick-
field protection device.
Figure 2-17 Internal Circuitry — Digital I/O Pin
When the pin is configured as a high-impedance input, input signals are clamped to
within a diode drop of the V
SS
and V
DD
power-supply rails. When the pin is configured
as an output, the P- or N-channel device provides a low-impedance path to V
DD
or
V
SS
, respectively. The current into or out of the pin should be limited to prevent dam-
age. The specified current limit is 25 mA although these pins can typically withstand
transients of more than 100 mA at nominal room temperature.
The port C and port D I/O pins of the M68HC11 can be configured as open-drain-type
outputs. This configuration disables the gate signal to the P-channel device of the out-
put buffer so the pin cannot be driven to an active-high logic level, but the P-channel
device is still physically present and forms an inherent diode to V
DD
. In a few applica-
tions, the situation will arise where two or more MCU I/O pins are tied to the same
point. Software would be arranged so that no more than one of these I/O pins is con-
figured as an output at any time to avoid output driver contention. In these applications,
the I/O pins should be configured for the open-drain mode so the output drivers are
prevented from high-current contention.
P
N
T
P
PIN
N
[1]
[2]
[3]
INPUT
BUFFER
VDD
N
P
[4]
[5]
OUTPUT
BUFFER
VDD