====== Software PLL ======
===== Purpose =====
External clock syncing is an essential feature for many Eurorack functions. An often-suggested solution for this problem is to employ a PLL ((https://forum.electro-smith.com/t/clock-input-tempo-detection/1869/5)). In this project, we will a

  - write a microcontroller program that blinks an LED in sync with an external clock (or according to an adjustable internal clock if no external clock is applied), taking note of how quickly it responds to changes in clock frequency
  - extend the above to use the internal reference frequency control to set integer division/multiplication of the external clock (when applied)

===== Resources =====
  * https://hackaday.com/2016/03/23/unlock-the-phase-locked-loop/
  * https://arachnoid.com/phase_locked_loop/index.html