Pull down on IO pin

**romeshkumar** · 25th June 2009 05:41 PM

I am very interested in electronics and I have build few basic circuits. I have attached a circuit that I am trying to understand and I need some help. The inputs to the NOR gate PEL1.PWM and PEL1.DIR comes from the micrcontroller AT90CAN128 and they have a pull down resistor to GND. Just curious to know what will happen if I output a 1 or 0 on the GPIO pins of the micro and why do we need the pull down resistor at that place.
Thanks a lot

**BeeBop** · 25th June 2009 06:19 PM

Originally Posted by romeshkumar

I am very interested in electronics and I have build few basic circuits. I have attached a circuit that I am trying to understand and I need some help. The inputs to the NOR gate PEL1.PWM and PEL1.DIR comes from the micrcontroller AT90CAN128 and they have a pull down resistor to GND. Just curious to know what will happen if I output a 1 or 0 on the GPIO pins of the micro and why do we need the pull down resistor at that place.
Thanks a lot

Hi romeshkumar,
Welcome to the best electronic forums on the net!

The pull down insures the pin, an input, stays at a defined level of logic 0. If you leave the pin floating, it is possible the voltage at the input will drift, causing undefined results. The pulldown resistor is of a large enough value that the microcontroller will have no problem overcoming it and setting the input to a logic 1.

Hope this answers your question.

**romeshkumar** · 25th June 2009 06:31 PM

Thanks for your reply. Lets say if I output a 1 to both those pin from micro won't that resistor still pull that pin to GND.

**BeeBop** · 25th June 2009 06:44 PM

Originally Posted by romeshkumar

Thanks for your reply. Lets say if I output a 1 to both those pin from micro won't that resistor still pull that pin to GND.

No, the output from the micro should have an impedance which is much smaller than 100k ohms, and forms a voltage divider with your 100k resistor, so you should get close to Vdd. on those input pins.

**Nigel Goodwin** · 25th June 2009 06:46 PM

Originally Posted by romeshkumar

Thanks for your reply. Lets say if I output a 1 to both those pin from micro won't that resistor still pull that pin to GND.

All the resistors are doing is setting the input impedance of the gates - in this case to 100K - without the resistors they would be VERY, VERY high impedance.

The 100K resistors are far too high to have any effect on the output from the processor, but low enough to stop the inputs drifting high on their own.

**romeshkumar** · 25th June 2009 06:48 PM

Just curious what will happen if the micro has a open collector output. Will this still work.

**Nigel Goodwin** · 25th June 2009 06:59 PM

Originally Posted by romeshkumar

Just curious what will happen if the micro has a open collector output. Will this still work.

No, it wouldn't work at all, that requires a pull-UP resistor.

**BeeBop** · 25th June 2009 07:00 PM

Originally Posted by romeshkumar

Just curious what will happen if the micro has a open collector output. Will this still work.

If your output pin is open collector, then it isn't capable of outputting a logic high, and you must provide a pullup in order to do that. Use a 'normal' IO pin for these and save yourself the headache.

Just a minute faster than me, Nigel!

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