Electronic – Bluetooth vs Bluetooth Smart (Low Energy)

Your question is unanswerable without the costs of the various tradeoffs. You give a relative cost of transmitting and receiving, but have provided no guidance on the cost in your situation of using the RF link. You have also not taken data errors into account. Those mean a message is lost, which may cause retries with their associated costs or perhaps lack of features with their own associated costs.

This whole thing is a cost/benefit analysis. Only you can make judgements on some of the costs, especially when they aren't the same physical quantity. For example, we have no way of knowing how much you value not interfering with other RF communication versus battery life.

This is the kind of thing where Monte Carlo analysis can be useful. You set up a bunch of probabilities, write simulations that model all the various interactions, then measure your various costs as a function of parameters you can tweak.

If you're budgeting 0.3 mA average every µA counts. Not so much of a problem for the microcontroller, but the SD card will consume tens of mAs. You want to have it switched on as little as possible. But the ATmega328P has only 2 kB of RAM, so that means your sample buffer will be full in less than half a minute, and then it's time to write to the SD card. Two times a minute.

I would consider a TI MSP430 instead of the AVR. It's still the lowest power commonly available controller. It will save you the µA you'll need when writing to the SD card. The MSP430F5418A also has 16 kB RAM, so that you have to power on the SD card only once every three and a half minute.

You can run the MSP430 on its low frequency oscillator, and switch to the high frequency DCO (Digitally Controlled Oscillator) for writing to the SD card, so that this takes as little time as possible.

For powering the SD card I would indeed use a high-side switch. The BSS215P is a suitable logic-level P-MOSFET.

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If you don't mind a BGA package a NAND Flash device may be an alternative to the SD card. This one can operate in MMC or SPI mode. It consumes less than an SD card, but still consumes 200 µA in standby, so you'll still want to shut it off with the high-side FET. Be sure to make the I/Os to the chip low before switching off power. That goes for the SD card as well.