Computing stuff tied to the physical world

JeeNode Micro start-up power

In AVR, Hardware on Mar 29, 2013 at 00:01

The JeeNode Micro v3 includes a P-channel MOSFET to control power to the RFM12B radio. This isn’t just a new gimmick – the goal was to “fix” the RFM12B wireless radio’s startup power consumption, which can prevent an ultra-low power source from ever building up a high enough supply voltage for a JeeNode to start up.

Now that the JNµ is in production, it’s time to measure how well such an approach works. Get ready for a bunch of scope screenshots, all based on the same circuit as before:

JC's Grid, page 51

… except that now the entire JeeNode Micro is in there, and I’m using a 10 Ω resistor.

I’ll be applying a 1 Hz ramp signal going from 0.0 to 3.0V using the power booster behind a signal generator, to see exactly what amount of current is being drawn. In all the images below, the yellow trace is the input voltage (i.e. a simulated power supply), and the blue trace is the voltage over the 10 Ω resistor – that means 1 vertical division on the blue trace corresponds to 0.5 mA when the display shows 5 mV/div:


The above image is just a baseline: a simple blink sketch which never enables the radio, and which then toggles some I/O pins every 500 ms. As you can see, the ATtiny84 comes out of power-on reset at about 1.4V and ends up drawing about 3.5 mA at 3.0V.

Fuses are set to low=C2 high=D7 ext=FF, i.e. BOD disabled, startup asap on RC @ 8Mhz.

Now let’s look at the same setup with the JNµ running radioBlip2.ino:


This time, the sketch enables the MOSFET to power up the radio, measures the battery voltage, tries to send out a packet (this will fail at 1.4V), and goes into deep sleep. A short but very quick (and high!) blip before power consumption drops to almost zero.

The third measurement is with a sketch doing nothing but powering down right away:

#include <JeeLib.h>

void setup () {
    cli(); // disable all interrupts

void loop () {}

Which produces this result:


I’ve bumped the scope sensitivity up to its maximum of 1 mV/div (i.e. 100 µA/div) and am now adding a lot of averaging to try and keep the displayed noise levels low. The “blip” is the ATtiny getting out of reset and powering down completely.

As last test, I repeated the above, but now using a sweep of 10 s (0.1 Hz), and filtering the signal through the lowest low-pass setting available, i.e. 5 Hz. This loses the important spike at 1.4V, which is of course still there, but improves the readout of the baseline:


As you can see, the power consumption now never rises above ≈ 60 µA – that’s a ten-fold improvement over what we get with the RFM12B connected to power in the standard way.

The shape of this curve is quite interesting: it’s essentially resistive (since it’s more or less linear), but the current starts at 1.2V, i.e. after overcoming two extra diode drops.

This is the power-up “hump” which any ultra-low power supply based on solar cells or other energy harvesting techniques will need to overcome, so that the ATtiny can switch itself into power down mode and let the supply voltage rise further.

I think that’s an excellent result, and am looking forward to trying a few things out!