Well, I'm seriously lagging on the documentation so unfortunately, nothing much to report. It's not that the documentation is difficult, but it's the mountains of other things I need to wade through before I can get to it. That seems like the case for most things I'm trying to do these days, where the volume of tasks seems to be increasing. I guess its inevitable as the project marches forward that small things start to consume time and energy as well as the major things. 

I'm hoping to get the dev boards done soon, but I've been agonizing over the power supply section recently. I can go with the el-cheapo voltage regulators, but I'm leaning towards using a DC-DC converter on them. TI has a new switching converter out that can boost voltages from 0.3V up to 5V, but it's a bit on th expensive side. The main benefit of using something like this is that the dev boards will be able to accept different types of inputs.

The standard dev board usually has a DC jack and possibly the option to be powered off the USB. Battery power is a different issue where you have to take into consideration the battery types that you want to be compatible with. Lithium batteries (coin cells) usually run at 3V, standard alkaline batteries are 1.5V/cell, and NiMH ones run at 1.2V/cell. For my dev boards, I want to be compatible with both alkaline and NiMH batteries which usually come in the same form factors (AA or AAA) so that you can use disposable (ugh) or rechargeable batteries. That means that you'd have to be able to support the minimum voltage which is 1.2V/cell.

Along with that, I'm planning on some experiments using solar power and super capacitors which presents another pain in the ass. Solar cells run at ~0.6V/cell which represents the diode drop across the PN junction of the cell. To make things more complicated, super caps function as normal capacitors but with a huge amount of capacity. That means that they can run from the full voltage rating all the way down to zero, as opposed to batteries that slowly lose voltage as they discharge and drop off steeply when they're almost empty. 

Of course the drawback of using the switching converter is that the power supply ripple looks pretty ugly from the datasheet which would make sensitive measurements difficult. This kind of goes against the whole concept of wireless sensing if you can't do the sensing part well. The tradeoff I came up with is to use the boost converter for the 5V supply and drop the voltage to 3.3V with a linear regulator. It's a waste of power, I know, but at least you'll have a smooth 3.3V supply which powers most of the main components that I'm planning to have on the board. The 5V supply is just for any peripheral components on the modular connectors that might need it. Not sure if this is the best solution, but at least it's a solution. 

The casual observer would say that I'm overdesigning the power options, which is probably true. However one of the most important factors in wireless sensors is power utilization since that's the lifeblood of a WSN node. Hence, I'm very interested in experimenting with different power options for different situations. Plus, the new DC/DC converter chip I mentioned can accomodate all of the potential inputs (hopefully) since it's designed specifically for energy harvesting types of apps which work at low voltage. So anyways, I've been agonizing over the power supply for the dev boards for quite a while and I think I'm converging on this solution.

Hmmm...what was my point in writing this post again?

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written by Emery Premeaux, June 10, 2009

Well, what is important is the Vref right? Most logic will survive some choppy/noisy power supplies so long as they stay above 3 and under 5.5. The analog side and ref voltages need to be pretty stable... focus on that. Two supplies might at first sound like overkill, but might make things simple/cheap. Or what about a Zener? Im just throwing ideas out.

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written by Akiba, June 10, 2009

Actually, the boards are going to be modular so some future add-on cards or user-generated boards might need the 5V for something like a relay. Most of the main components, like the MCU are running at 3.3V and it also contains the A/D converter. That's why I wanted a stable 3.3V supply which should be okay with a regulator. If worse comes to worse, then I can just use the intrinsic bandgap (~2.5V) from the MCU which should be reliable enough for the sensing. I'm going to be using the boards to run some power experiments, because the real trick is to run at as low a voltage as possible, especially for nodes that can be duty cycled (allowed to sleep). Since power consumption is proportional to voltage, even going down to 3V or 2.8V might add a couple of percentage points to battery life. So it'll be interesting to see how low I can run the voltage without crashing the MCU. It's like a voltage limbo contest...

Anyways, I'll see how things turn out and hopefully iterate until a good platform evolves

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