[OpenTRV-dev] V0.2-Arduino: getting started

[Kevin Wood]

Hi Damon,

> That's definitely in the right sort of territory, at 3.3V rather than 5V for example, though I'd like us to be immediately battery-operable, eg ~2.2V for 2XAA rechargeables.

OK. The AVRs are OK down to 1.8 V, so that should be achievable.

> In effect it is the bootloader I'd like to add for maximum ease of (re)programming, though maybe I can be talked out of it.

Yep, it makes things easier, especially if there is a requirement for an 
"end user" in the field to update it. Having the USB on-board is also an 
advantage here since nothing has a serial port any more, and relying on 
serial-to-USB adaptors is a pain, as they all seem to behave differently.

The Arduino boot loader looks like it just programs the flash and keeps 
out of the way at other times, so can't see it being a problem.

So, the question here, in my mind, is what is the power overhead of 
having a USB interface as "dead weight" in the AVR as opposed to perhaps 
an FTDI chip that's only powered from the USB bus when connected?

> Unfortunately all three radio modules now seem to be end of life.

Hmm. will have to keep an eye out for an alternative.

> The current design uses the DS18B20, but its operating voltage is not low enough for 2xAA.

Oh, yes, that's true. :-(

Something from the Texas TMP102 series, perhaps?

> This is the sort of area I am not at all sure about.  The Leonardo's uC has the USB built-in, and thus may save board area and power consumption ultimately.

Yep, as above, I think, if we are to have easy upgrades in the field the 
choices are:

1) AVR with built-in USB
2) AVR with USB-to-Serial chip
3) AVR only, and use a cable containing a USB-to-Serial chip

Option 1 being preferable, IMHO, especially as it opens up the 
possibility to do more with the USB interface. Maybe a PC application to 
program the schedule on the device, for example?

I've heard the AVRs with built-in USB can be a pain during development, 
though, because the USB interface goes down every time you reset the CPU 
after a download. This is the reason devices like the UNO have a 
separate CPU to provide the USB link, so both of your modules sound 
useful, perhaps at different stages in the development.

Cheers,

Kevin