Zilog Z86193 - Manual (Page 86)

Z8
®
CPU
User Manual
UM001604-0108 Input/Output Ports
79
Vil (max), pullup or pull-down resistances must be calculated using Ref = R/Rp. For best
case STOP mode operation, the inputs should be within 200 mV of the supply rails.
In output mode, if a port bit is forced into a tri-state condition, the autolatches force the
pad to V
DD
. If there is an external pulldown resistor on the pin, the voltage at the pin may
not switch to GND due to the autolatch. As displayed in Figure 67 , the equivalent resis-
tance of the autolatch and the external pulldown forms a voltage divider, and if the exter-
nal resistor is large, the voltage developed across it will exceed V
IL
(max). For worst case.
V
IL
(max > V
DD
[R
EXT
÷ (R
EXT
+ R
P
)]
R
EXT
(max) = [(V
IL
(max) ÷ V
DD
) R
P
] ÷ [1—(V
IL
(max) ÷ V
DD
)]
For V
DD
= 5.0 V and I
AO
= 5 µA, V
I H
(max) = 0.8 V:
R
EXT
(max) = (0.16 ÷ 1M) ÷ (1—0.16) = 190 K¾.
Rp increases rapidly with V
DD
, so increased V
DD
will relax the requirement on Rext.
In summary, the CMOS Z8 autolatch inhibits excessive current drain in Z8 devices by
latching an open input to either V
DD
or GND. The effect of the autolatch on the I/O char-
acteristics of the device may be modeled by a current I
AO
and a resistor Rp, whose value is
V
DD
/I
AO
.
Figure 67. Effect of Pulldown Resistors on Autolatches
V
LO
V
IH
(min.)
R
P
R
EXT