Yesterday’s post was about how test equipment can differ not only in terms of hardware, but also the software/firmware that comes with it (anyone hacking on the Owons or Rigols yet to make the software more feature-full?).

Here’s another example, where I’m using just about all the bells and whistles of the Hameg HMO series scopes – not for the heck of it, but because it really can help gain more insight about the circuit being examined.

This is my second attempt at understanding what sort of start up currents need to be available for the new JeeNode Micro v3 to properly power up:

I’m applying a 0..2V power up ramp (yellow line) as power supply, using a 1 Hz sawtooth signal. This again simulates an energy harvesting setup where the power supply slowly ramps up (the real thing would actually rise far more slowly, i.e. when using a solar cell + supercap, for example). The current consumed by the JNµ v3 (blue line) is measured by measuring the voltage drop across a 10 Ω resistor – as usual.

The current consumption starts at about 0.85V and rises until the power supply reaches about 1.4V. At that point, the current consumption is about 77 µA. Then the ATtiny84A comes out of reset, enters a very brief high-current mode (much higher than the peak shown, but this is averaged out), and then goes into ultra low-power sleep mode. The sketch running on the JNµ is the latest power_down.ino, here in simplified form:

#include <JeeLib.h>

void setup () {
    cli();
    Sleepy::powerDown();
}

void loop () {}

Note that since this is the new JNµ v3, the RFM12B module never even gets powered up, so there’s no need to initialise the radio and then put it in sleep mode.

The red line uses the Hameg’s advanced math features to perform digital filtering on top of the averaging already performed during acquisition: the averaging keeps the power-up spike visible (albeit distorted), at the cost of leaving some residual noise in the blue trace, while the IIR digital low-pass filter applied to that result then makes it possible to estimate the 77 µA current draw just before the ATtiny84A starts running.

Here’s the zoomed-in view, showing the interesting segment in even more detail:

The IIR filtering seen here is slightly different, with a little spike due to the following power-up spike, so the 86 µA reported here is slightly on the high side.

Note how the Hameg’s storage, high sensitivity, averaging, adjustable units display, variable vertical scale, math functions, on-screen measurements, on-screen cursors, and zooming all come together to produce a pretty informative screen shot of what is going on. Frankly, I wouldn’t know how to obtain this level of info in any other way.

So what’s all this fuss about measuring that 77 µA level?

Well, this is how much current the JNµ draws before it starts running its code. There’s nothing we can do to reduce this power consumption further until it reaches this point. In the case of energy harvesting, the supply – no matter how it’s generated – will have to be able to briefly deliver at least 77 µA to overcome the startup requirements. If it doesn’t, then the supply voltage (presumably a supercap or rechargeable battery) will never rise further, even though a JNµ can easily draw less than a tenth of that current once it has started up and goes into ultra-low power with brief occasional wake-ups to do the real work.

What I’m taking away from this, is that a solar energy setup will need to provide at least 0.1 mA somewhere during the day to get a JNµ started up. Once that happens, and as long as there is enough power to supply the average current needed, the node can then run at lower power levels. But whenever the supercap or battery runs out, another period with enough light is needed to generate that 0.1 mA again.

It all sounds ridiculously low, and in a way it is: 0.1 mA could be supplied for over two years by 3 AA batteries. The reason for going through all this trouble, is that I’d really like to find a way to run off indoor solar energy light levels. Even if it only works near a window, it would completely remove the need for batteries. It would allow me to just sprinkle such nodes where needed and collect data … forever.