
M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-25
interfere with normal operation of an MCU signal. For example, a low-pass filter on an
address or data line of an expanded MCU system would introduce significant delays
to these signals, dramatically limiting the maximum operating speed of the system.
2.4.3 Internal Circuitry — Digital Input-Only Pin
Figure 2-15
shows the MOS circuitry for a digital input-only pin. The gates of input
buffer [3] are very high impedance for all voltages that would ever be applied to the
pin. The thick-field protection device [2] normally prevents the pin voltage from reach-
ing levels that could damage the gates of the input buffer. The exact circuitry of the
input buffer may be different for different digital inputs (e.g., to provide hysteresis,
etc.), but only device gates will be connected directly to the pin. Allowing a pin to float
(or be driven) to a midsupply level can result in both the N- and P-channel devices in
the input buffer simultaneously being partially on, which causes excess current and
noise on the V
DD
/V
SS
power supply. Port E inputs are exceptions because they are
specifically designed to be driven by analog levels.
Figure 2-15 Internal Circuitry — Digital Input-Only Pin
If a digital input pin (see
Figure 2-15
) is driven with voltages below V
SS
, the thick-field
protection device [2] forms an inherent diode junction to V
SS
, which conducts when the
pin voltage gets more than a diode drop below V
SS
. As the pin voltage is driven more
negative with respect to V
SS
, current increases. These currents have a tendency to
influence the die substrate in the area around the protection device, thus affecting the
electrical characteristics of devices in the vicinity. When the pin current is increased to
very high levels (typically more than 100 mA, specified limit is 25 mA), physical dam-
age can result.
As voltage at [1] is driven above V
DD
, the protection device will begin to conduct and
tend to clamp the input voltage to protect input buffer [3]. The voltage at which this con-
dition will occur varies significantly from lot to lot and over the operating temperature
range. At room temperature, the pin typically does not draw any current until approxi-
mately 20 V; at 125
°
, the pin may start conducting at a slightly lower level. Up to this
point, the pin appears to function normally and will return a logic one if read. As the pin
P
N
VDD
T
P
PIN
N
[1]
[2]
[3]
INPUT
BUFFER