ATTENTION: This this apparently didn't interest anyone here, I moved this to the mysenors forum. There is already a new and improved version. While this version does not have known deal breakers, I suggest the new ones.
Hi, I have quite successfully smartified our house with ESP8266, but unfortunately, they are not exactly energy efficient. So I plan to use MySensors for battery powered applications, especially keyfobs. This will control stuff like the garage door opener, so security is a major concern.
Attached is a first design for an 8-button keyfob mit hardware HMAC authentication via an ATSHA204, running on a CR2032 coin cell.
The ATMEGA is supposed to run in power-down mode (400nA with disabled ADC, internal oszillator @ 8MHz), the powered down NRF024L01 (SMD module with ceramic antenna in this case) is supposed to take 900nA in power down mode and the ATSHA <150nA in sleep mode.
So the overall standby consumption should be well below the self-discharge rate of the coin cell.
In standby, either the keypad rows or columns will have interrupts attached via PCINTx_vect and pulled HIGH, while the respective other dimension will be output LOW (or vice versa, have to look up what consumes less power), so that a button press will wake the ATMEGA.
Before I blow the order, perhaps somebody is interested in having a closer look ... this is both my first MySensors project and my first low power project. Also, it is probably the most compact thing I have ever clicked together. (I am so glad about KiCAD's push and shove router ...)
Any comments? Tips? Ideas? Errors?
I would hate to send this to production and end up with 80 unusable PCB 
The two major concerns I could currently think of are:
- Does MySensors implement the sleep mode of the ATSHA? I don't really know much about the chip and it would be a bugger if it sucked my battery dry ...
- Will the coin cell provide enough current for transmission? I added a 22uF tantal, should I include more?
Also, any ideas where I find a cheap ICSP programmer with 3.3V? While the ATMEGA can obviously take 5V, I think the NRF24L01 cannot, and I would try to avoid desoldering it before programming
Not sure if you can open the KiCAD project without my custom libraries.
keyfob.pdf (79.9 KB)
MySensors_Keyfob.zip (43.6 KB)