Zilog Z86193 - Manual (Page 84)

Z8
®
CPU
User Manual
UM001604-0108 Input/Output Ports
77
Z8
®
CMOS Autolatches
I/O port bits that are configurable as inputs are protected against open circuit conditions
using autolatches. An autolatch is a circuit which, in the event of an open circuit condition,
latches the input at a valid CMOS level. This inhibits the tendency of the input transistors
to self-bias in the forward active region, thus drawing excessive supply current. A simpli-
fied schematic of the CMOS Z8 I/O circuit is displayed in
Figure 65 .
The operation of the autolatch circuit is straight-forward. Assume the input pad is latched
at +5V (logic 1). The inverter G1 inverts the bit, turning the P-channel FET ON and the N-
channel FET OFF. The output of the circuit is effectively shorted to V
DD
, returning +5V to
the input. If the pad is then disconnected from the +5V source, the autolatch hold the input
at the previous state. If the device is powered up with the input floating, the state of the
autolatch is at either supply, but which state is unpredictable.
There are four operating conditions which activate the autolatches. The first, which occurs
when the input pin is physically disconnected from any source, is the most obvious. The
Figure 65. Simplified CMOS Z8 I/O Circuit
Open-Drain
PIN
V
DD
OE
Data Out
Data In
Autolatch
G1
N
P
V
DD
N
P