In embedded applications, keeping track of time is crucial. Even for the simple task of blinking a led at a certain time interval, we need a reference of time that is constant and precise.

A clock is a piece of hardware that provides us with that reference. Its purpose is to oscillate at a fixed frequency and provide a signal that switches from high to low at a fixed interval.

The most precise type of clock is the crystal oscillator (XOSC). The reason why it is so accurate is because it uses the crystal's natural vibration frequency to create the clock signal. This clock is usually external to the processor itself, but the processor also has an internal clock (ROSC) that is less accurate and that can be used in cases where small variations of clock pulses are negligible. When using the USB protocol, for instance, a more stable clock signal is required, therefore the XOSC is necessary. The crystal oscillator on the Raspberry Pi Pico board has a frequency of 12MHz.

This clock signal is just a reference, and most of the time we need to adjust it to our needs. This is done by either multiplying or dividing the clock, or in other words, elevating or lowering the frequency of the clock. For example, the RP2040 itself runs on a 125MHz clock, so the crystal oscillator frequency of 12MHz is multiplied (this is done using a method called Phase-Locked Loop).

A counter is a piece of hardware logic that counts, as its name suggests. Every clock cycle, it increments the value of a register, until it overflows and starts anew.

The way the counter works here is that it increments/decrements every clock cycle and checks whether or not it has reached its reset value. If is has, then it resets to its initial value and starts all over again.

1. Prepare your setup for Marble Pico interaction with VS Code and pico SDK or use Arduino IDE support according to instructions available here: Ardushop Marble Pico.

1. RP2040 microcontroller datasheet
2. Raspberry Pi Pico datasheet
3. Pico-SDK
4. Pico-SDK Documentation