Forum - MCS Electronics

 

FAQFAQ SearchSearch RegisterRegister Log inLog in

Internal pull up resistor value?

 
Post new topic   Reply to topic    www.mcselec.com Forum Index -> Various
View previous topic :: View next topic  
Author Message
TSEYFARTH

Bascom Member



Joined: 01 Jul 2006
Posts: 1054

usa.gif
PostPosted: Wed Jun 25, 2008 7:18 am    Post subject: Internal pull up resistor value? Reply with quote

Hello all!

I am not an EE..... however, I searched the data sheet on the atmega16 for the values of the internal pull up resistors. The reason being, I want to verify that the series resistor value I am using at the base of a 2n2222 is appropriate.

Can anyone tell me what the value of the pull up resistor is and/or what you use when 5V presents itself at the base of a 2n2222 resistor? I am thinking in the area of 5-8K as OK.....

Tim
Back to top
View user's profile
AdrianJ

Bascom Expert



Joined: 16 Jan 2006
Posts: 2481
Location: Queensland

australia.gif
PostPosted: Thu Jul 24, 2008 12:57 am    Post subject: Reply with quote

The pullup resistor value is defined on page 291 of the datasheet DC characteristics as Rpu, min 20, max 50 k.

The series resistor you use depends a lot on the current you want at the collector. Very roughly it should limit the current into the base at about the collector current divided by the transistor current gain, or max base current, whichever is less. Note that the transistor gain varies a LOT depending on how hard you want to turn it on. Typically I would assume a gain of around 20 for a hard-on switch.

Since the base voltage is around 0.7 volts with the transistor hard on ( assuming a normal grounded emitter configuration ), then you want the resistor to drop about 4.3 volts at the design base current. Often you need a lower pullup resistor than the internal one to provide this current. I find its usually better to set the pin as output and limit the base current properly, than rely on the internal pullup. They are much better to use just as input pullups on things like switches.

_________________
Adrian Jansen
Computer language is a framework for creativity
Back to top
View user's profile Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    www.mcselec.com Forum Index -> Various All times are GMT + 1 Hour
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum