What is the lowest possible clock frequency at which a microcontroller can still do useful work? Here’s a little project that attempts to explore this weird question. A couple of years ago, while I was studying the then-Atmel ATtiny85 microcontroller, I was fascinated by the fact that I could make it run at a mere 0.5Hz, using an Arduino board with a standard Blink sketch as an external clock source. When I reported this on a local forum, someone wondered what could possibly be done at these speeds. I gave it some thought and answered that if you put a light sensor on your roof to make a clock signal from the day/night cycle, you could probably create a system that will light up an LED every Saturday. Now, that doesn’t sound particularly useful, but it should be remembered that for religious Jews, Shabbat (Saturday) is sacred and important in many ways. Moreover, according to the Hebrew calendar, a new day begins more or less at sunset, not at midnight. So silly as it is, if this system is done right, it will be actually meaningful – if not practical – to some people.”


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