Changing feedback mode with a microcontroller

I am attempting to change the feedback mode of an SMC04 using a microcontroller. I connected a pin from the micro to the center pin of the three associated with feedback. If I set the pin to 5v and toggle the reset (using another pin on the micro) they SMC04 does go into analog feedback mode. However, if I set the pin to high impedence and toggle the reset the SMC04 does not go into the no-feedback mode but instead (I think) into the tach. mode. How does the SMC04 determine which mode the system is in? Also, I checked and with the pin on the micro at high impedence it does have about a 100k resistance to ground. Could this be a strong enough pull to change the feedback mode? Thanks!

Hello,

The pin has a 0.01 uF cap to ground; the microcontroller charges it different ways and sees what happens to the line. A 100k resistor gives a time constant of 1 ms, which is how long the microcontroller waits, so your 100k pull-down is enough to discharge the cap past the I/O threshold. If you have an oscilloscope handy, you should be able to look at the pin and see it go high and low and how your microcontroller affects it.

Your best bet is probably to add some more capacitance to the pin. Something like a 0.047 uF cap from the pin to ground should do it.

- Jan

Ah, very clever (I wondered how you were reading three states using a single pin). Thanks for the help!