If you have a spare Arduino board that isn’t being used for anything else then you can use that as a programmer. (That’s effectively what the ATtiny in the FX is - an Arduino board being used as an ICSP programmer.)
I’ve never done it myself but I’m sure there’s plenty of examples and tutorials online, and there might be a few people here who have done it before.
I think this is the closest there is to an ‘official’ tutorial.
I’ve had to program some pro micros ages ago when I was having troubles with them. But the issue is more about the voltages I’m connecting. The link though does suggest powering each one with 3.3V. I have a step down converter than can supply 3V so I might try that when I get the chance.
If you only have a 5V ICSP, you could solder in the 32U4, crystal and a few other support components then burn the bootloader using the ICSP. Once the you’ve verified that the programming was successful, you could then populate the rest of the board.
A problem with this is that if you wanted to burn the bootloader again later for some reason, you would then need a 3.3V ICSP to do it.
I haven’t checked extensively for missing connections or that every button, LED, speaker, etc. is connected to the correct pin but your latest posted schematic looks good to me (at least from a components point of view).
Many say it’s fine to supply 5v, but the display is different to the micro-controller. So perhaps my best option is to program the chip with 5v and desolder the display each time. it won’t be very often that I do reprogram it unless I break it somehow.
Then I’ll go for safety over ease. I’ll be saving myself a lot more if I play it safe. Perhaps I could even make a PCB and design for the programmer to actually supply 3.3v using the original schematic and your modified version.
More things on the list to buy, but in the end it’ll be worth it.
I assumed you’d have to remove the chip because you do that on an Arduino Uno. But if it’ll work that’s good. Though I might have to find a version with a USB port on it so I can connect it to my laptop. But there might also be other alternatives for programming it.
How do you install sketches on your Pro Mini currently? Usually you attach a USB to TTL serial converter, such as the SparkFun Serial Basic Breakout. You can use the same connection when using the Pro Mini as an ICSP.
I have just done a deep check through my schematic using MLXXXp’s suggestions and the original schematic, I believe it is close to it’s final state. I have run the parts through a spreadsheet to calculate the total cost and surprisingly it’s only roughly about:
I’ll be ordering the parts somewhere around next week and then hopefully I can assemble it completely around February-March.