1.2 Possible Solutions

The most accurate measurement will consist of the fastest clock source with a timer on the lowest prescale. The lowest timer prescale yields the highest resolution. Higher resolution typically increases the maximum count, which may necessitate compensation for rollover of the timer.

An interrupt can be incorporated into these routines only if non blocking code is used. The accuracy of this implementation may suffer as a result. For example, the Interrupt-on-Change (IOC) pin can cause an interrupt when a rising or falling edge is detected. The pulse measurement can now be completed inside of the ISR without the need to constantly poll the pin. While this may sound ideal, the user must now accommodate for the 3-5 instruction cycle delay that is caused by servicing an asynchronous interrupt.

When absolute accuracy is important, then an external crystal should be used since the internal oscillator block can have a drift up to 5% of its nominal frequency.

All of the measurements in this document including the associated code use an internal 16 MHz system clock with all timers on a 1:1 prescale of the system clock (F_OSC), unless otherwise noted. The low and high waveform length constraints, as well as the accuracy of the measured result are calculated using Table   1 and Table   2. If rollover is accounted for in software-based methods, then the accuracy of the measurement will decrease in proportion to the software routine overhead.

These methods assume a pulse to be active-high and a period to be the time between two rising edges. See Figure   1 and Figure   2 for clarification. A generic duty cycle equation (Figure   3) is also provided based on Figure   1 and Figure   2. There are more method-based equations later in the document.