# Pull down from 0.8V to 0V

Discussion in 'Electronic Basics' started by Dummy, Aug 3, 2004.

1. ### DummyGuest

IC1 is controlled by a microcontroller. Let's say when micocontroller
issues a command to IC1, pin 28 supposed to be pulled down to 0V. If
mesaured voltage at pin 28 of IC1 was 0.8V, what kind of additional
circuit that can be placed at pin 28 to make sure that the pin is
always 0V at certain condition?

2. ### AndrewGuest

You say that 'pin 28 is supposed to be pulled down to 0V'. That will never
be the case with any digital device. It will always output a small positive
voltage, usually ~0.5-0.8V when outputting a logic 0. Look at you
microcontroller's data sheet for VOL (Voltage Output Low)

For example, for the 16F877, VOL = 0.6V with IOL=8.5ma, VDD=4.5V.

What are you driving? Any logic device will expect such a voltage for a
logic 0.

3. ### Norm DresnerGuest

Well, a resistor to ground would pull it down to 0V. An NPN transistor as
an emitter-follower could provide high output. It all depends on the
impedences [current requirements] of the remaining circuits.

Norm

4. ### Rok SitarGuest

Diode and resistor should do the trick.

5. ### Tam/WB2TTGuest

You are probably overloading the pin. Put a noninverting CMOS buffer between
the pin and the load. Make sure the buffer can supply the desired load
current. If you have a pullup resistor on it, try a whole lot higher
value.Also, make sure the VCC and ground pins on IC1 are what you think they
are.

Tam

Tam