When I decided it was time to get away from the larger Arduino boards I started shopping for Pro Mini clones, since they can be had pretty cheaply, and can be fit into just about any application. I discovered multiple different PCB designs and decided to buy one of each of the 3 most common unofficial designs to see if I could identify any difference. In all cases these are 5V 16Mhz boards, as I wanted as close a match to my experience with Arduinos Uno and Nano as possible.
Below is a summary of common Arduino Pro Mini (and clones) PCB designs:
Official designs (I didn't purchase either of these, so please take this with a proverbial grain of salt):
-Old Sparkfun Design: (https://www.sparkfun.com/products/retired/9218) sometimes these are sold a bit cheaper than the rest listed below. If someone has a project that they want to put into large scale production, and the project doesn't happen to use the A6 and A7 pins that are not broken out on this board, I suppose the small savings in cost would add up?
-Current Sparkfun Design: (https://www.sparkfun.com/products/11113) this one breaks out the A6 and A7 pins. Exact copies of this design are popular on Ebay.
Unofficial designs (these are the three designs I purchased):
-Common design: This seems to be the most common unofficial design, so for brevity I refer to it as the 'common' design for pro mini clones. It looks like a modification of the old Sparkfun design, but tweaked to have a smaller reset button, making room for three pins (A6, A7 and one additional ground pin) that weren't broken out in the retired Sparkfun board.
-Two Row design: I will refer to this as the 'two row' design, as it can be recognized by the two five-pin rows (or seven? When I say five I am not counting the pins that are part of the long-edge rows of 12 pins each) along the button side edge for a total of 40 pins broken out. The one I bought has those pins marked as shown. I don't know what MIS, MO9, and SCK pins are but if they look like something you need, buy boards of this design, I guess?
-Big Oscillator design: I refer to this as the 'big oscillator' design as it is recognizable by the large oscillator and smaller microcontroller chip on the board. It has 34 pins. Before the boards arrived I looked at this and thought it was the stupidest looking board with the giant oscillator, but the black board and blue LED actually make me like it best aesthetically, now that I've used it a bit. I did see boards with the Big Oscillator design on blue PCB, if anyone cares.
Besides those listed above I saw a few variations on those listed above: A Big Oscillator design but with the larger microcontroller chip and one extra pin broken out (five pins along the button side edge, rather than four, the extra pin assumed to be another ground) and a Common design with four pins next to the button rather than 3, the extra pin apparently one more VCC, but I didn't bother purchasing either.
The only conclusion I can draw about the different designs is to consider your project needs for the pins that are or are not broken out on the board. All boards loaded up fine once I figured out the timing for resetting and got the drivers installed for the different USB to serial converters I was using (the subject of Part 2).
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