LPC1768 open drain with pullup characteristics


Just a memo for myself

The LPC1768's internal pullup resistance can be calculated if I can measure voltage on variable resistive load.

This is a memo when I tried to measure it.


On mbed, I set the pin5 as DigitalIn with internal pullup.

#include "mbed.h"

DigitalIn     b0( p5 );

int main()
    b0.mode( PullUp );
    while ( 1 ) {



The result was like this.

There are some comments on this plot.

  • The "V vs R" behavior is not linear. The blue line is saturated at 2.3V when R is very high.
  • The I vs R has gentle slope at R <30k.
  • From this result, it may be true that the internal pullup is made from weak current source ;)

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