As I wait for some fresh inspiration (oh, and some more JeeNode Micro v3’s) to arrive, let’s go into something that has intrigued me for a long time:

> What’s that “decoupling” stuff all about when designing electronic circuits?

The following information comes from a fantastic discussion with martynj, exploring ways to make this visible on the scope. I really learned a great deal – thank you, sir!

As you probably know, all chip datasheets have these little capacitors connected near the main chips, “for decoupling”. You’ll often see schematics with several of them:

What is decoupling? And decoupling what exactly?

Well, the problem is that power supply lines can get very noisy – lots of fluctuations you don’t really want, and if they get really bad, the chips will start to malfuction, mistaking a “0” for a “1” level or vice-versa. Not to mention the fact that high frequency signals will radiate into free air, turning a circuit on a PCB into an illegal transmitter – whoops!

Let’s not go into the “deeper” stuff: parasitic capacitance & inductance, dielectrics, etc. Instead, let’s just run some experiments with these humble little 0.1 µF “caps”:

As you’ll soon see, they are not capacitors at all – obnoxious little buggers, they are!

But first, the basic idea: due to digital switching, chips tend to generate a lot of very sharp current transitions. On an ATmega running at 16 MHz, these pulses are likely to be most pronounced at around 16 or 32 MHz, i.e. both the up and the down flanks of the main clock causing all semiconductors inside to switch.

Drawing a lot of current very abruptly is like suddenly pulling on a pendulum: it doesn’t immediately follow your request, but lags and has some trouble meeting the requested change. And when released, it tends to overswing, making the problem even worse.

Same with current changes – the power supply can’t quite match it, and so the voltage near the chip drops (and later overshoots) while trying to keep up. It’s the same effect as the lights dimming briefly when turning on a very power-hungry appliance in the house. It’s all about inductance if you really want to know, but let’s just ignore that for now.

Capacitors near the chip are like tiny (but very responsive!) reservoirs of energy, and they can be very effective in compensating for this dip.

So the general advice is: put decoupling caps between VCC and GND, as near the chip as possible, and the cap will “dampen” the demand to give the much longer connections to the power supply time to replenish the energy. Capacitors are great for this – and the closer they are to the consumer, the better they can handle even extremely brisk “spikes”.

Another way of saying this is: capacitors have an infinite resistance at 0 Hz (DC), i.e. they do not leak DC power, but as the frequency increases, they pass more and more current. This “frequency-dependent resistance” is actually called impedance – so as frequency increases, the cap’s impedance decreases. Spikes are – by definition – high-frequency. And the lower the impedance, the more these spikes get… s(h)orted out!

Tomorrow I’ll describe a little test setup to clearly demonstrate the effect of higher and higher frequencies being more and more absorbed by that 0.1 µF capacitor shown above.