Computing stuff tied to the physical world

What if the supply is under 3.3V?

In AVR, Hardware on May 1, 2013 at 00:01

Welcome to the weekly What-If series, also available via the Café wiki.

To follow up on a great suggestion from Martyn, here’s a post about the different trade-offs and implications of running an ATmega at lower voltages.

The standard Arduino Uno, and all models before it, have always operated the ATmega at 5.0V – which used to be the standard TTL levels used in the 7400 series of chips used in the 1960′s and 1970′s. The key benefit of a single standardised voltage level being that it made it possible to combine different chips from different vendors.

To this day, even though most semiconductor logic has evolved from bipolar junction transistor to CMOS, the voltage level has often been kept at 5V, with slightly adjusted – but compatible – voltage levels for “0″ and “1″, respectively.

Nowadays, chips operate at lower voltages because it leads to lower power consumption and because it is a better fit for batteries and LiPo cells. In fact, lots of new chips operate at 3.3V and will not even tolerate 5.0V.

The ATmega328p is specified to run at a very wide range, from 1.8V all the way up to 5.5V. Which is great for ultra low-power use, supporting different battery options, and even for energy harvesting scenario’ such as a solar panel charging a supercap, for example.

But there are still many trade-offs to be aware of!

The first one is the system clock rate, which is limited (see also this older post):

Screen Shot 2013-04-30 at 21.59.07

If you look closely, you’ll see that 16 MHz is out of spec for 3.3V, the way the JeeNode is running, that is. In practice, this has never caused any known problems, but lowering the voltage further might just be too much.

The good news is that it’s not really the crystal oscillator which is causing problems, but the main circuitry of the ATmega, and that there’s a very easy fix for it: when running at voltages below 3.3V, you should set the ATmega’s pre-scaler to 2, causing the system clock to run at 8 MHz. When running even lower, perhaps under 2.4 V or so, set the pre-scaler to 4, i.e. run the system clock at 4 MHz. This also explains the presence of the divide-by-8 fuse bit: when you need to start up at low voltages, you can force the ATmega to always power up with a clock pre-scaler of 8, and then adjust the pre-scaler under software control after power-up, once the voltage has been verified to be sufficient. Without this setting, an ATmega would not be able to reliably start up at 1.8V, even if it’s meant to run much faster most of the time.

Note that the RF12 driver will still work at 4 MHz, but not less: the interrupt service time will be too slow for proper operation at any slower rate.

Another important issue to be aware of when running a JeeNode at voltages under 3.3V, is that the MCP1702 voltage regulator will no longer be able to regulate the incoming voltage. It can only reduce the input voltage for regulation, so when there is no “headroom” left, the regulator will just pass whatever is left, minus a small “dropout” voltage difference. Hence its name LDO: Low-DropOut.

The problem is that all LDO regulators start consuming (i.e. wasting) more idle current in this situation. See this weblog post for the measured values – which can be substantial!

To avoid this, you should really disconnect the LDO altogether, or at least its ground pin.

A third aspect of running at lower voltages, is that you need to verify that all the parts of the circuit continue to work. This applies to sensors as well as to the RFM12B radio – which should only be operated between 2.2V and 3.8V.

Actually, some experiments a while back showed that the radio could work down to 1.85V, but I suspect that things like transmit power will be greatly reduced at such supply levels.

Lastly, when the supply voltage is lowered, you need to keep some secondary effects in mind: the ADC will operate against a lower reference voltage as well, so its scale will change. One ADC step will be just under 3.3 mV when the supply is 3.3V, but this drops to under 2.0 mV per step with a 2.0V supply.

To summarise: yes, an ATmega/ATtiny can run at voltages below 3.3V, and even an entire JeeNode can, but you need to reduce the system clock by switching the pre-scaler to 2 or 4, and you need to make sure that all parts of your setup can handle these lower voltages.

  1. When you say that everything has switched to CMOS I immediately recall dire warnings about needing to be careful with static electricity. Yet none of guides on this site make a really big point of it, wikipedia doesn’t mention it and I didn’t pay too much attention to it while soldering and everything worked out fine.

    So is it that CMOS isn’t that senstive after all, and that you only need to worry about actual sparking, or is there something about the design of chips that’s changed over the years to make them less sensetive?

    • Hm, that’s a good question. My guess is that the problem has decreased by having ESD protection diodes on all chip pins nowadays. Indeed, I rarely worry about that nowadays, although I do have a static discharge mat on my workbench.

  2. Difficult to generalise, but typically the more complex digital chips have enough die space to integrate in ESD diodes on most, if not all I/O pins. Where it gets tricky is the analogue world – an ESD diode may compromise performance too much (e.g. very low bias current op amp inputs) or with simple discretes (e.g. the gate insulation on a MOSFET is easily compromised).

    A professional prototyping/assembly operation continues to use proper precautions (grounding mat, wrist strap, regular checking of all solder stations for earth leakage etc) since it is easier to retain good practice for the times it is important even if much of the time it isn’t.

    Even here, faking is common. That black foam and those pretty pink parts bags are anti-static right? Hmmm – get your DVM out if you change supplier and check!

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