
MOTOROLA
2-22
PINS AND CONNECTIONS
M68HC11
REFERENCE MANUAL
Essentially all current is due to gates that are in the linear region during transitions and
for charging and discharging internal capacitances. Because the input is very high im-
pedance, if it is not connected, the input can oscillate or float to a midsupply level. Ei-
ther of these conditions can result in added power-supply current. The oscillation case
can result in coupling of noise to the power supply. In older CMOS designs, the large
currents caused by an input that floated to midsupply could even induce CMOS latch-
up, which could destroy the integrated circuit. Current design techniques on the
MC68HC11A8 have made latchup due to a floating input unlikely, but it is still impor-
tant to terminate unused inputs to avoid oscillation, noise, and added supply current.
Some inputs on the MCU (RESET, EXTAL, MODA, and MODB) cannot be left unter-
minated in any system.
Figure 2-14 CMOS Inverter
The port E input pins have an extra N-channel device between V
SS
and the bottom of
the N-channel device of the input inverter. Since this extra device is only enabled for
half a cycle during a digital read of port E, it is less important to terminate unused port
E pins than other unused inputs. In cases of very slow bus frequencies, even half a
cycle might be a significant length of time, and unused port E pins could be terminated.
In some battery-powered systems where port E is read often, it would be desirable to
eliminate the potential added supply current.
Since the V
REFL
and V
REFH
pins do not connect to the inputs of any CMOS gates with-
in the MC68HC11A8, these pins do not need terminating if they are not used. Although
termination is not required, it may reduce the risk of damage due to high-voltage static
electricity.
Other than A/D pins, there are two basic types of input pins on the MC68HC11A8 —
an input-only pin and an input/output pin. The best method to terminate unused inputs
is with a pull-up or pulldown resistor for each unused pin. Input-only pins can be con-
nected to each other and then to a common termination point. Although this method is
less expensive and takes less space than individual pull-ups, it is much harder to sep-
arate and use one of these pins if it is needed later. Although input-only pins can be
connected directly to V
DD
or V
SS
, it is better not to because this connection makes it
difficult to change the level at that input. If a pull-up or pulldown resistor is used in-
stead, a signal can easily be connected to the input later. The preferred method of ter-
OUT
VDD
P
N
IN