I'd take the femtoduino over the digistump in this case. The atmega328p is vastly superior to it's low pin cousin, attiny85. I'm sure you could engineer the atmega328p to also talk directly to usb as well as it's merely using the V-USB project.
But you don't even need that. If you glance at the V-USB wiring examples, all you need is a few diodes & crystal depending on the freq you want. For my projects, I do use the attiny85 and use the built-in oscillator. I use 16mhz for usb communication, but drop down to 1mhz after the boot procedure.
Is the FT232R (an FTDI chip) the least expensive way to get USB to Arduino? I'm trying to hookup a wireless nrf24L01+ to USB (which talks to another nrf24l01+ and an atmega328p), and I'm looking at PIC chips which are around $2, whereas the FT232R is $4.
It's not even close to the least expensive way - various Chinese-made USB-to-serial chips are cheaper, as is the Atmega8u2 used on more recent Arduinos. The Atmega8u2 is a fully-programmable microcontroller too, as are the larger chips in the range.
You don't even need that. The attiny85 is sufficient.
See https://github.com/Bluebie/micronucleus-t85 It's a bootloader that detects usb in the boot process. If usb is hooked up, you can upload programs, then it will run whatever code is loaded. This bootloader is the same one used on the digistump.
You need 3 diodes and 2 resistors along with the attiny85 chip to make your own usb to serial converter.
If you don't demand jumping on the arduino ecosystem bandwagon, and want the smallest possible microcontroller, there's always the PIC-10 series. A little 6 pin SMD chip in a six terminal'd SOT-23 outline. Years ago I built a little dev board to make one talk to a pickit programmer and all that and it works fine. Made it flash morse code and ran out of other ideas for what to do with it. I believe the 10F220 is obsolete now but its more advanced cousins are still available in the same family, just the usual "more memory, more everything". I recall SMD "ZIF" sockets are fiddly. Somewhere on my bench I still have what amounts to a "smart" LED throwie using this tech.
There are also SOT-23-6 parts in the TinyAVR series. The ATtiny4,5,9,10. Here's the 10:
http://www.atmel.com/devices/ATTINY10.aspx
They use something different than the conventional ISP protocol though (TPI) -- but I think people have come up with ways to do that even with third party hardware and avrdude.
I believe that it is true that Microchip was there first, though. For a while, the lowest pin-count AVRs were SOIC-8 and DIP-8.
I'm looking forward to seeing some reviews of these when they start arriving. I'm especially curious to see how well (or rather if) they achieve that Arduino IDE/sketch compatibility, it seems like a point that a lot of ARM-based "arduino compatible" kickstarters have been promising but glossing over. I know that PJRC's (ARM-based) Teensy 3 has had an enormous amount of work put into truly making it compatible, hopefully a lot of that is feeding back into the libraries and main Arduino project in ways that will help all ARM-based Arduino-compatibles.
Even without support for Arduino/Wiring, that NRF51822 should be fun to play with! But it looks like NRF51822 breakout boards are already showing up on ebay at about the same price (and with more pins broken out). So I'm not sure if preordering is too interesting at this point if it doesn't deliver on something above and beyond a mere breakout board.
No, and you might have to reset it manually for upload, like some of the early and other barebones Arduino versions (instead of relying on the CTS reset trick).
