Computing stuff tied to the physical world

Dropping below 3.3 V

In AVR, Hardware on Feb 28, 2013 at 00:01

One of the departures from the Arduino world has always been that JeeNodes operate at 3.3 V, whereas the standard Arduino’s all ran at 5 V. Things are changing, with the ARM-based “Arduino Due”, for example, which also runs at 3.3 V. There’s not really a choice: ARM chips all run at 3.3V or less (3.6V max actually, which leaves a nice safety margin).

So what’s this trend towards lower voltages all about, eh?

Well… you may have seen an older post in the Easy Electrons series (whoa, over two years ago.. time flies!). If you think about it for a moment, all the electric energy pumped into a circuits will end up being converted into some other form of energy: a teeny tiny bit of radiated RF energy in the case of a JeeNode, but mostly heat, really.

Odd as it may seem, all that electronic stuff is just a way to turn current into heat. There is a purpose for this, since we’re usually interested in the side effects: some physical sensing and some computation, leading to the information we’re interested in. But the electricity consumption is just an (unavoidable) side-effect.

Whenever we can lower the voltage and/or current consumption, we’ll end up consuming less power as a result (W = I x V). Lowering the voltage on a circuit often leads to a dramatic reduction in power, since part of a circuit is always resistive. Ohm’s law says that voltage = resistance x current, or to put it differently: current = voltage / resistance. So halving the voltage (V) over a resistor also halves the current (I) it draws. The effect: half the voltage leads to a quarter of the power consumption, in purely resistive circuits:

JC's Grid, page 65

Unfortunately, you can’t just use 0.1 V to power a circuit: bipolar semiconductors such as diodes and transistors have a 0.6V threshold voltage, anything below that renders them useless. In the real world, running digital circuits at under 1.0 V is rarely done. Even that 1.8 V level down to which an ATmega and ATtiny can be operated is pushing some limits.

Another reason is that lower voltages make capacitive effects more dominant. The lower you go, the longer it takes to charge or discharge a capacitor. Which explains why the ATmega’s maximum clock speed must drop to 4 MHz with a supply voltage of 1.8 V.

But lowering the clock speed is somewhat self-defeating, as this means that the ATmega and ATtiny have to remain powered-up longer before going back into deep-sleep mode! Lots of trade-offs. Still: a low supply voltage is often a good idea – generally speaking.

Tomorrow, I’ll describe the choices made for the next revision of the JeeNode Micro…