Analog Devices EV-21569-SOM Manual page 17

Software-Controlled Switches (SoftConfig)
After this generic discussion, there is a detailed explanation of the SoftConfig interface specific to the EV-21569-
SOM carrier board.
The Example of Individual FET Switches figure shows two individual FET switches (Pericom PI3A125CEX) with
reference designators UA and UB. Net names ENABLE_A and ENABLE_B control UA and UB. The default FET
switch enable settings in this example are controlled by resistors RA and RB, which pull the enable pin 1 of UA and
UB to ground (low), respectively. In a real example, these enable signals are controlled by the Microchip I/O expand-
er. The default pull-down resistors connect the signals EXAMPLE_SIGNAL_A and EXAMPLE_SIGNAL_B and
also connect signals EXAMPLE_SIGNAL_C and EXAMPLE_SIGNAL_D. To disconnect EXAMPLE_SIGNAL_A
from EXAMPLE_SIGNAL_B, the Microchip I/O expander is used to change ENABLE_A to a logic 1 through soft-
ware that interfaces with the Microchip I/O expander. The same procedure for ENABLE_B disconnects
EXAMPLE_SIGNAL_C from EXAMPLE_SIGNAL_D.
Figure 3-2: Example of Individual FET Switches
The following figure shows the equivalent circuit to the Example of Individual FET Switches figure but utilizes me-
chanical switches that are in the same package. The default is shown by black boxes located closer to the ON label of
the switches. In order to disconnect these switches, physically move the switch to the OFF position.
Figure 3-3: Example of a Mechanical Switch (Equivalent to Example of Individual FET Switches Figure)
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EV-21569-SOM Manual