button down -> light on

Picture this project – you have a battery operated gizmo with LEDs that has to light up at a given time and light off at another one. Reality is a bit more complex than this, but not that much.

So what could possibly go wrong? How much could cost the development of its firmware?

The difficult may seem on par with “press button – light on”.

As usual devil is in the details.

Let’s take just a component from the whole. If it is battery operated you need to assess quite precisely the charge of the battery, after all – it is expected to do one thing and it is expected to do it reliably. The battery charger chip (fuel gauge) comes with a 100+ pages datasheet. You may dig out of internet a pseudo driver for this chip only to realize that it uses undocumented registers. Eventually you get some support from the producer and get a similar, but different data-sheet, and a initialization procedure leaflet that is close, but doesn’t match exactly neither of the two documents.

Now may think that a battery is a straightforward component and that the voltage you measure to its connectors are proportional to the charge left in the battery. In fact it is not like that, because the voltage changes according to temperature, humidity and the speed with which you drain the energy. So the voltage may fall and then rise even if you don’t recharge the battery.

To cope with this non-linear behavior the fuel-gauge chip keeps track of energy in and energy out and performs some battery profile learning to provide reliable charge status and sound predictions about residual discharge time and charging time. The chip feature an impressive number of battery specific parameters.

Since the chip is powered by the battery itself (or by the charger) and doesn’t feature persistent storage, you have to take care of reading learned parameters, store into some persistent memory, and rewrite them back at the next reboot.

Btw, this is just a part of the system which is described by some 2000 pages programming manuals specific to the components used in this project.

In a past post, I commented an embedded muse article about comparing firmware/software project cost to the alternatives. The original article basically stated that for many projects there is no viable mechanical or electrical alternative.

This specific project could have an electromechanical alternative – possibly designing a clock with some gears, dials and a mechanical dimmer could be feasible. I am not so sure that the cost would be that cheaper. But the point I’d like to make now is that basically you can’t have an electro-mechanical product today – it is not what the customer wants. Even if the main function is the same, the consumer expects the battery charge meter, the PC connection, the interaction. That’s something we take for granted.

Even the cheapest gizmo from China has the battery indicator!

And this is the source of distortion, we (and our internal customer who requested the project) have a hard time in getting that regardless of how inexpensive the consumer device is, that feature may have taken programmer-years to be developed.

This distortion is dangerous in two ways – first it leads you to underestimate the cost and allocate an insufficient budget; second and worse it leads you to think that this is a simple project and can be done by low-cost hobbyists. Eventually you get asked to fix it, but when you realize how bad the shape of the code (and the electronics) is, you are already overbudget with furious customers waving pitchforks, torches and clubs knocking at your door.

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