why 100K resistor on pin 2

J

jmalt31

New Member
Hello All,

I am sorry if this has been asked before - I searched but was unable to find it. My question is this why do some people use a 100K resistor and some use a 10K resistor on pin2 when the programing circuit is removed or not there? I know the purpose of it is to keep the pin from floating and thinking that it is being programed again but is there a reason to use one value of resistor over the other or it doesn't make a difference?
 
premelec

premelec

Senior Member
It's there to prevent extraneous signals from confusing the chip into thinking it's got a download coming or any such other information... there are electrical signals all over our environments and any input pin should not be be left floating - q.v. lots of previous posts and manuals comment... The resistor will save you hours of trying to figure out why your unit doesn't work reliably - just do it! :) Have fun...
 
D

Dippy

Moderator
The lower the resistor the more definite a pull-down (or up where appropriate).

The input impedance of PICaxes is very high so you can usually get away with much higher values.
That's it really. So for a simple pull-down stick to 10K, you can't go wrong.

On more sophisticated circuits you may have to review that value.

One general side note, as you go much higher in vlaue then the device will be more susceptible to interference. e.g. an annoying little pulse won't be dragged to ground to easily.
I've certainly had issues when taking pull -downs/ups into the MegOhm range.
Remember with digital devices like PICAxes there will be tiddly transients coming out of it. It's one reason why you decouple.
 
W

westaust55

Moderator
Untested in reality, however one thought would be that if you have a higher value resistance such as 100 kOhm resistor to hold the SERIN pin low, then if you later add some form of separate plug-in programming circuit such as or akin to the AXE029 then the higher resistance value would not need to be removed and should have little influenece on the operation of a programming circuit.
 
J

John West

Senior Member
This is a good example of a circuit where almost any reasonably high resistor value will work, so you use what you have on hand. Sometimes precision is the name of the game. Sometimes not. This time it's not.
 
W

william47316

New Member
sometimes you'll want to tie the program in pin directly to ground with a jumper, especially when you're controlling an inductive 240V AC fan with relays, the back EMF from the fan can cause all sorts of mischief when the relay opens
 
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