Hiroshi Yamaguchi

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10 years, 11 months ago.

DigitalIn initial value

Hi team,

When pin 8 is open, the following program displays 1 then 0 when compiled with the latest (rev.63) mbed library. But the output should be 0 then 0 according to the DigitalIn handbook.

#include "mbed.h"

DigitalIn digitalIn(p8);

int main() {
    printf("%d\n", digitalIn.read());
    printf("%d\n", digitalIn.read());
}

Any suggestions?

Hiroshi

2 Answers

Erik -
10 years, 11 months ago.

Random idea, but default state of all pins is pull-up enabled. As soon as you make a DigitalIn object it will enable the pull-down, but it takes a very short amount of time to discharge the input capacitance when the pull-down is activated. If you immediatly read the input it might still be high.

Hiroshi Yamaguchi
poster
10 years, 11 months ago.

Erik,

Thanks for the exlanation. If I put a very small delay, less than 1 us, the program displays as I expected.

BTW, this behavior seems to have been introduced since the r44 build, when the refactoringof the core library was made.

Hiroshi