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difference b/w normal output and open drain output of the pic

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puneetnepsam

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hi,

in the data sheet of the pic24f , it is written that every pin can behave as a normal digital output or a open drain output.
open drain output can be selected with the ODCx register.

can any one please tell me what is the difference between the two?

thanks
Puneet Gupta
 

dknguyen

Well-Known Member
Most Helpful Member
A normal digital output can be driven HI and LOW by the PIC- it's what you would normally think of a digital pin.

Normal digital pins have a pull-up transistor and a pull-down transistor. One transistor pulls the pin HI (by connecting the pin to +V), and the other pulls the pin LO (by connecting the pin to GND). An open-drain output has no pull up transistor and only has the pull-down transistor. This means that instead of being able to make the pin go HI or LO like a normal digital pin, the pin can only go LO or high impedance/floating. This means you need a pull-up resistor to make the pin HI.

The "drain" on the LO transistor is "open" (not connected to a pull-up transistor). Open collector means the exact opposite (the drain on a MOSFET is the equivelant of the collector on a BJT)- it can go HI, but goes high impedance instead of going LO and a pull-down resistor is needed.

http://en.wikipedia.org/wiki/Open_collector
 
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Pommie

Well-Known Member
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The "drain" on the LO transistor is "open" (not connected to a pull-up transistor). Open collector means the exact opposite (the drain on a MOSFET is the equivelant of the collector on a BJT)- it can go HI, but goes high impedance instead of going LO and a pull-down resistor is needed.
Open collector - Wikipedia, the free encyclopedia
Open collector is either low or high impedance and a pullup is required. See your link,


Mike.
 
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puneetnepsam

New Member
thanks for the information....

so in the normal pin case :
in pull up transistor if Vcc is equal 3.3V and if i make the pin high...pin will have 3.3 V...

in the open drain case , i want pin should have 5.5 V making it high,
so i have to connect pull up resistor to the 5.5V??
 
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