Reusing omitted pins in the Pro Micro board

[alliumnsk]

I want to use a Pro Micro with a keyboard grid of 8x13 and the board has a few less of gpio. I think if I solder some wires directly to MC itself it might be enough... How many would I need to solder 1 or 2? TX0/RX1 can be used as GPIO, right?
p.s.edit: removed "leonardo"

[pert]

What's the micro leonardo? I know about the Arduino Leonardo and the Arduino Micro and there is also the Pro Micro (originally by SparkFun but also lots of clones) but I've not heard of the micro leonardo.

[alliumnsk]

Oh. I guess I'm talking about Pro Micro, updated 1st post.

[pert]

You need 21 pins?

[alliumnsk]

Yes. If I understand correctly atmega32u2 can have 23 max (if borrow XCLK and RST). 21 is ok but they are not used all by the board.
I know I can use multiplexing... (which i'd like to scavenge from something but still didn't do it...)
But I wish to learn more.

[alliumnsk]

oh. just saw than the board uses atmega32u4 not u2 as i thought.
unused pins are 12 26 32 40 41
and two are used by leds? 8 22

[pert]

The pins connected to the RX and TX LEDs are going to be a bit easier to connect to. Those are Arduino pins 17 (PBO) and 30 (PD5). That gets you to 20 pins but that last one will be a bit more challenging since the microcontroller package on the pro micro doesn't have nice big leads sticking out to solder to.

[sterretje]

I'm a coward and hence would not even consider soldering on the microcontroller itself.

If you need more pins, why not use an Arduino Micro?