I have a device with an active high interlock that I need to control through an external connection. The layout looks something like:

interlock pin  o----------------------------o MCU GPIO
                                    |
                                    |
                                    |
                                     -----/\/\/------o isolated driver

The isolated driver is a ISO7421Fx (http://www.ti.com/lit/ds/symlink/iso7421fe.pdf) digital isolator which connects to an external switch. When the external switch connected to Vcc, the output of the driver will pull the line high...when it is open or grounded, the line is pulled low. The MCU connection is there to read the status of the line, but I also want it to be able to use the MCU to override the external interlock control and manually drive the line high or low.

In the past I've just used 4.7k resistors and called it a day, but I'm not sure in this case. The device says it expects an input impedance of 1 MOhm (on the interlock pin)....so do I take that to mean I should target around 5 uA of current (1MOhm at 5V)?

 

With CMOS devices where the input impedance is high and the current low you have a lot of leeway. Anything between 10kΩ and 100kΩ would be fine.

4.7kΩ works too.

 

okay...looks like I'm just overthinking things.

thanks for your help