Answer / amir rashid chishti

All other ports have internal pull ups but not post 0
because it is left for user. This port uses multiplexing of
address and data that is why external pull ups are used and
it depends on what purpose we are going to achieve from
this post. When we use external pull ups it becoms general
purpose port.

Answer / shruthi

port 0 is open drain hence it requires pull ups and all other ports r not open drain its not left to user if it has to be used as i/o port its must to connect pull up resistor

Answer / mschaudhari9

When used as outputs, all port pins will drive the state to which
the associated SFR latch bit has been set. Except for port 0,
which will only drive low (not high). When a 0 is written to a
bit in port 0, the pin is pulled low (0). But, when a 1 is
written to a bit in port 0, the pin goes into a high impedance
state - or in other words, "disconnected", no value. To be able
to get a 1 as output, you need an external pullup resistor to
pull up the port (to 1) when the port is in its high impedence
state. Typical values for pullups might be 470 ohm to drive a
LED, and 4.7K or higher to drive logic circuits.

Answer / sat

if your using port0 as i/o pin u must have to connect pull up resistor..
if ur not using pull up resistor it gives logic 0 but will not gives logic 1 instead it flows (high Z)

Answer / shankar

For the better understanding of PORT 0 and the concept of pull up registers, you can check this video out.

http://www.youtube.com/watch?v=t9bN-2nFNEs

Answer / dhamodharan

Becoz, the port 0 was the open collector or open drain from
p 0.0 to p 0.7. So for connecting the pull up resistors we
can drive the output side of the display.

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