why one port is made open -drain. & y to use pullup resi

[myths]

hey guyz... i m a newbie in embedded field...

i read bout 8051 microcontrollers... and m havin a doubt in that open drain port...
actually m not able to get the reason why it is made open drain n we 've to use pull up resistor..i mean there wud b some reason for this na...

1 reason that i think is to make it use as a bidirectional port....

anybody can plz tell me if i m correct or is there any other reason behind this???

plzzzz tell me[/b]

 

[banjo]

There are two main reasons for open drain outputs. The first is easy bidirectional signalling as you mentioned. The second is to allow a bus arch. with multiple chips hanging on the same wire. Chips on the bus that are inactive will leave their outputs open and thus not load down the line for others trying to communicate.

 

[myths]

Re: why one port is made open -drain. & y to use pullup

thanks a lott....


but m not able to understand it completely....can u plzzzz elaborate it a little bit ..
thanks again

 

[VanThor]

Re: why one port is made open -drain. & y to use pullup

There are two main reasons for open drain outputs. The first is easy bidirectional signalling as you mentioned.

Presence or absence of open drain outputs doesn't matter for bidirectional signalling.
Look at ports P1, P2 & P3 - it's possible to use bidirectional signals on them but these ports don't have open drain outputs (in standard C51).

The second is to allow a bus arch. with multiple chips hanging on the same wire. Chips on the bus that are inactive will leave their outputs open and thus not load down the line for others trying to communicate.

It's possible to do this without open drain port.

 

[banjo]

I think you missed my point. I said open drain main for easy bidirectional signaling. I did not say it was required. It is easy because you do not have to remember to explicitly turn the outputs on and off.

For a multiple chip bus system. Open drain is preferred because you can easily detect bus contention. With open drain, if you release the bus to write a "1" and then sample the pin and find it to be "0", you know another chip is already on the bus. If you try to do this with regular drivers you have some period of time where there are two drivers on the same line. Also with regular drivers, you run the risk of a strong driver overriding a weaker one and missing the collision detection.

Finally, I totally forgot about the other reason for open drain. It allows for easy translation between logic families. Assume that I want to send serial data from a 5V 8051 to a 2.5V chip. If the 2.5V chip is not 5V tolerant on its IO pins, I will damage the part. However, if I use the open drain output and use a pullup resistor to 2.5V instead of 5V, then I can do the interface without a problem.

 

[jhbbunch]

Re: why one port is made open -drain. & y to use pullup

An open drain or an open collector is just a way of saying they have stuck a fet (drain) or a bjt (collector) on the output of a logic chip. This allows for two things. One, you can have a wired-or setup with other such devices, and two, you can drive other outputs with much higher voltage levels. The 7406 ttl device will handle 30V devices for example, not bad considering the ttl logic levels. Of course, it has to be going into a high impedance device like an op-amp or comparator, since it needs a pullup resistor to work(think voltage divider).

 

[myths]

Re: why one port is made open -drain. & y to use pullup

hey thank you all for guiding me.....

anyways in addition for all the reasons u ppl mentioned i got one more reason....actually the open drain condition helps in increasing the fan in & fan out ... value of pull up resistor decide the fan out ,fan in ....

thnx again

 

[mkiani]

Re: why one port is made open -drain. & y to use pullup

this is obvius my friend.
by use of external resistors we can source more current to load and this is a benefith for uc.in new uc we can active internall pull up or use of external pullup resistor and disable internal.