A sneak preview

July 08, 2013

Jack Ganssle-July 08, 2013

The sleep current war between MCU vendors is bogus.

Regular readers of are aware of the sleep current war raging between MCU vendors. Marketing glitz and web pages are the guns; white papers are the ammo, which is being broadsided with increasing frequency. Everything claimed is probably true, but much of it is misleading and/or irrelevant.

It's truly remarkable just how little current is needed to keep a processor alive today. In the deepest sleep modes even some ARM parts claim just 20 nA (nanoamp) of consumption. That's a mere 0.02 microamp, a number that boggles the mind.

In the desktop world CPUs suck over 100 amps while active, so I salute the MCU community for achieving such astonishingly-low figures.

First, some context. In the trenches of this war lie the lowly CR2032, a typical coin cell that's oft-quoted as the canonical battery for extremely-long-lived systems (Figure 1).

Figure 1. A CR2032 primary cell
A CR2032 has about 220 mAh of capacity (quoted capacities vary a little depending on the vendor), which means it can source one mA for 220 hours. (Note there is some dependency on capacity fast one discharges the battery).

It's nominal voltage is 3.0, perfect for these 1.8 to 2V min MCUs, and the discharge curve has a sharp knee. They are considered "dead" at 2.0 V, which is confirmed by my experiments. Here's data for a number of batteries, all from one batch from the same vendor, discharged at a 0.5 mA rate as shown in Figure 2:

Figure 2. Batteries discharging at a 0.5 mA rate.
Some of the vendors claim their MCUs can run for 20-30 years from a single CR2032. That's may be true in theory, but not in practice. I contacted a number of battery vendors and none guarantee shelf lives over ten years.

One vendor said: "Not recommended for use after shelf life expires as the chemicals in the battery break down and it looses power a lot quicker, and there can be corrosion or leakage."

It's poor engineering practice to use a component beyond its rated specifications.

Though the war is all about battery lifetime of a mostly-sleeping system, that's irrelevant for design engineers. The right question (which no one seems to be asking) is: how much useful work can the system do while awake?

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