Hello - I would like to run a nano off of a lipo battery directly. I was planning on going in at the 5v in. I know that this is not within the specs at 16MHz clock but will it work? If not, can I slow the clock down with software or pull the clock? I know I can use a buck booster unit but would rather not. I plan to turn the unit on once a day with a RTC and let it do its thing and then shut down again and just want to use as little power as possible. I also know a pro mini might be an option but have had issues with them
If you have an ISP programmer you can set the Nano to run off the internal oscillator at 8Mhz. The pro mini is probably a better choice for low power operation, depending on exactly what you want to do, particularly if you don't need the USB interface. If you look at the schematic of the Pro Mini board made by SparkFun there is even a jumper you can cut to disconnect the voltage regulator and power LED for low power operation.
You could set the system clock prescaler to divide down the 16 MHz crystal to produce an 8 MHz system clock. That would retain the accuracy of the crystal, but might let the processor better tolerate the low voltage. I know this kind of thing can reduce current draw, but I don't know if it really helps with the out-of-spec voltage situation since the oscillator is still operating at 16 MHz. Keeping the lipo charged to about 3.8V or higher would also help make sure it will work, and would actually be within spec.
I did an IR remote control for my Roku, and used an 8 MHz 3.3V Pro Mini with the power LED and regulator removed, powered directly by an 18650. It sleeps most of the time, at something less than 1µA, and the battery lasts a long time. That gives you crystal accuracy, and the processor runs fine at 8 MHz.
For low power operation get an 8 MHz Pro Mini, remove the power LED and the regulator. The Nano has the USB/TTL chip on board, that always uses power, can't be shut down.
A boost converter has no place in a low power circuit.
Okay - So I wired this up today with a Nano. I took the advice and am running the Nano at rated voltage. The circuit is basically the lipo directly to the Sim800 and the lipo directly to a booster with an output of 9v going to the Nano Vin. The project works as long as I disconnect the power from the Nano and use a USB cable from my laptop. Once I remove the laptop and put the 9v back into the Nano, it will no longer work. The LED is a little dimmer running from the battery and dims a little more when the LED on the Sim800 blinks. I have ordered a 3.3v pro mini to switch it out later but in the meantime, any thoughts?
When you refer to powering the Nano with 9 V, I presume you mean feeding 9 V to "Vin". This is OK as long as you do not connect anything else such as a Sim800 as the on-board regulator cannot supply anywhere near enough current.
