LPC1768 open drain with pullup characteristics

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Page last updated 08 Apr 2017, by Tedd OKANO. 0 replies current source, LP1768, pullup

Just a memo for myself

The LPC1768's internal pullup resistance can be calculated if I can measure voltage on variable resistive load.
/media/uploads/okano/mechanism.png

This is a memo when I tried to measure it.

setup

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

#include "mbed.h"

DigitalIn     b0( p5 );

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

/media/uploads/okano/measurement.png

The result was like this.
/media/uploads/okano/internal_pullup3.png

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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