difference b/w normal output and open drain output of the pic - Electronic Circuits Projects Diagrams Free

puneetnepsam · 28th May 2009, 06:47 AM

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 · 28th May 2009, 07:04 AM

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

Pommie · 28th May 2009, 07:34 AM

Quote:

Originally Posted by dknguyen

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.

puneetnepsam · 28th May 2009, 07:50 AM

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??

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar
Title	Starter	Forum	Replies	Latest
open drain output.	alphacat	General Electronics Chat	15	23rd May 2009 07:12 PM
Is a 12f output pin an open drain?	smanches	Micro Controllers	3	28th March 2009 08:38 AM
Pic16f628a - Ra4 Open Drain Output	fenderman	Micro Controllers	19	10th February 2009 02:15 AM
PWM on normal output pins?	MrMikey83	Micro Controllers	4	23rd February 2005 03:52 AM
The difference in open-drain and push-pull	elarson	General Electronics Chat	4	3rd January 2004 10:38 AM

28th May 2009, 06:47 AM	#1
puneetnepsam	difference b/w normal output and open drain output of the pic 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

28th May 2009, 07:04 AM	#2
dknguyen	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 __________________ Tanaka Sensei (avatar) says: Please spell it "ridiculous" correctly! Not "rediculous". ^^ Last edited by dknguyen; 28th May 2009 at 07:12 AM.

28th May 2009, 07:50 AM	#4
puneetnepsam	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??