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the external pullup is a resistor you put on your board, the internal pullup is a resistor (or usually a transistor) that the semiconductor manufacturer has put inside the chip and connected to a pin.
One major difference, internal pullups have a lot of tolerance so you always need to do worst case calculations, which can be the max. or min. value of the resistor.
thanx a lot for that
one more thing ..pullups r generally associated with the port pin values at the time of reset.
say I want to use a pin wihout internal pullup as an input pin ..do i need to connect a external pull up for that
In AVR uControllers for input pins (for ex PORTA) you write
no internal pullups : PORTA=0x00;
internal pullups PORTA=0xff;
In general you have to use pull ups. For example if you want to read a button that when you press it you have 0V and when you don't 5V you have to use pull ups. Else when you don't press the button the pin will flow and you don't get any good reasult
in 8051 based micro controllers only port 0 does not have the internal pull up
so if u r connecting it directly to any external circuits,u must connect a pullup resistor.to know the function of pullup resistors u can read any book delaing with 8051 family