ZIO, Motherboard User Manual
Rev. 2.0
Zilogic Systems
Page 10
Detecting External Voltage.
Any external voltage input can be connected to the ZIO
Input
signal through a MOSFET or a BJT. An example circuit using a MOSFET is shown below.
If the input voltage (V
s
) is greater than the threshold voltage of the MOSFET, the
Input
signal will
be low, or else it will be high.
An example circuit using a BJT is shown below.
If the input current (I
s
) is greater than (I
t
= 0.5mA / h
FE
), the
Input
signal will be low, or else it
will be high. For all practical purposes, a (I
t
= 1mA) input current is sufficient to make the
Input
signal go low. The base resistance (R
b
) has to be chosen to make the
Input
signal low, when the
required input voltage is driven.
R
b
= (V
s
- V
be
) / I
t
Connecting an Analog Comparator.
An analog comparator can be used to identify if the input
voltage is larger than a specified reference voltage. Any operational amplifier can be used as a
comparator, but a dedicated comparators like LM339 which provide open collector CMOS/TTL
outputs are suitable for interfacing with logic circuits. An example circuit is shown in the following
diagram.
Isolating inputs using opto-coupler.
As in the case of outputs, inputs can also be electrically
isolated using opto-couplers.
