Using an External Pull-Up
In general, an external pull-up is required only when you want to set
the output “high” value greater than
TTL levels. For example, to use a
+12V external power supply, the value of the external pull-up resistor is
calculated as follows:
V
cc
= 12 Vdc
I
max
= I
out low
x safety factor = 1 mA x 0.5 = 0.5 mA
R =
Vcc
I
max
=
12
0.0005
= 24 kΩ
The value of the logic “high” level with the external 24 kΩ pull-up
resistor is calculated as follows:
V
high
= V
cc
x
R
external
R
external
+ R
internal
= 12 x
24 k Ω
24 k Ω + 10 k Ω
= 8.47 Vdc
Driving External Switches
You can use two digital output channels to control an external switch.
For example, you can drive the HP 865X series of microwave switches
using an external power supply and two digital output channels.
The HP 865A switch provides an internal protection diode. The state
of the 2-to-1 multiplexer is changed by setting the appropriate output
bit low (0).
HP 865X Microwave Switch
Digital Output Channel
24 Volt
Power
Supply
–
+
Chapter 8 Tutorial
Multifunction Module
394
To Next Page
Loading...
