[LPC812] Conflict set/clear event for PwmOut

[toyowata]

Problem

When PwmOut::write() is called with 0 and 0.5 parameter by toggling, output signal level wrongly tied to high level often by write(0).
Original report is here.

Test code

#include "mbed.h"

PwmOut pwm1(D2); // P0_6

int main()
{
    pwm1.period_us(26);
    pwm1.write(0);

    while(1) {
        pwm1.write(0.5); // 50% duty
        wait(1);
        pwm1.write(0);   // should be low level
        wait(1);
    }
}

Signal snapshoot

This is due to the conflict of set/clear event is happened for SCT output.
I have fixed this defect and will make pull-request after some more testing.

[toyowata]

I also confirmed that other SCT enabled MCUs (LPC1549, LPC11U68 and PLC824) do not have this problem.