int LowCount
int HighCount
int RatioPercent

init()
{
    configure digital input interrupt on rising edge
    configure clock to counter
    configure counter input capture on rising edge of digital input
    configure counter to clear on input capture
}

interrupt RisingEdge ISR
{
    LowCount = counter input capture register

    configure digital input interrupt on falling edge
    configure counter input capture on falling edge
}

interrupt FallingEdge ISR
{
    HighCount = counter input capture register
    RatioPercent = (100 * HighCount) / (HighCount + LowCount)

    configure digital input interrupt on rising edge
    configure counter input capture on rising edge
}