M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-19
Figure 2-13 Low-Pass Filter for A/D Reference Pins
The port E input pins are used for general-purpose inputs and/or A/D analog inputs.
These inputs are designed so that the digital input buffers are disabled at all times ex-
cept for part of a cycle during an actual read of port E; thus, analog levels near the
switch point of the digital input buffer do not result in high power-supply current drains
as in a normal CMOS input buffer. The buffers are enabled by an extra N-channel de-
vice in series with the N-channel device in the input inverter. During a digital read of
port E, these extra N-channel devices are turned on for part of the read cycle. Because
of this special circuitry, it is not necessary to terminate unused port E pins.
The analog and digital functions of port E do not normally interfere with each other;
thus, any combination of pins can be used as digital inputs while the remaining port E
pins are used for analog inputs. Turning on the digital buffer during an analog sample
may cause small disturbances on the input line, which may cause small errors in the
sampled analog level. The disturbances would be caused by small gate-to-drain and
gate-to-source capacitances and would have to occur very close to the trailing edge
of a sample period to have any noticeable effect. The disturbances are so small (if they
exist) that they probably would not cause any measurable inaccuracy. Since it is so
easy to arrange software to avoid this condition, it is probably easier to avoid potential
disturbances.
2.2.8 Timer Port A Pins
Port A includes three input-only pins, four output-only pins, and one pin that can be
configured to operate as an input or as an output. The input-only pins (PA0/IC3, PA1/
IC2, and PA2/lC1) also serve as edge-sensitive timer input-capture pins. The four out-
put-only pins (PA3/OC5/OC1, PA4/OC4/OC1, PA5/OC3/OC1, and PA6/OC2/OC1)
also serve as main timer output-compare pins. Whenever an output-compare function
is enabled, that pin cannot be used for general-purpose output. These four pins can
be controlled by output compare 1 (OC1) and/or another output compare. The PA7/
PAI/OC1 pin can be used as a general-purpose I/O pin, as a pulse-accumulator input,
or as an OC1 output pin.
2.2.9 Serial Port D Pins
Port D includes six general-purpose, bidirectional I/O pins that can be individually con-
figured as inputs or as outputs. When the serial communications interface (SCI) re-
1K
1
μ
F
VDD
TO VREFH
OF M68HC11
TO VREFL
OF M68HC11