The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 11
The MT8808 is shown powered at +5V, but VDD may be anywhere from 4.5V to 13.2V. VEE
is an optional negative supply that enables you to switch negative signals.
The switches do have some resistance, which varies with the supply voltage. At 5V, typical
switch resistance is 120 ohms; at 12V, it drops to 45 ohms. This should cause no problems
in switching standard LSTTL or CMOS signals. If you are routing an analog voltage to a
low-impedance input, the switch resistance may attenuate the signal. Maximum switching
frequency of the chip is 20 Megahertz.
In Figure 11-2, the switch array is controlled by Port A and bit 7 of Port C (PC.7) on an 82C55
PPI. Bringing PC.7 high opens all of the switches. If you don’t need this ability, you can tie
RESET low and use PC.7 for something else.
You can use any output port bits to control the MT8808. Logic high inputs at the MT8808
must be at least 3.5V, though, so if you use NMOS or TTL outputs, add a 10K pull-up resistor
from each output to +5V.
For a simple test of the switches, you can connect a series of equal resistors as shown to the
X inputs. Each X input will then be at a different voltage. To verify a switch closure, measure
the voltages at the selected X and Y inputs; they should match.
Figure 11-2. With the MT8808 switch array, an 8-bit output port can connect
eight X inputs to eight Y inputs, in any combination.
188 The Microcontroller Idea Book