Schematic Review

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.

1 Like

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.

1 Like

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).


The value of R8 going to IREF of the display should be 390K

1 Like

I’ll look into buying a 3.3v board to program it, or I can try to supply the power from my step down converter. I’ll see if I can try it this afternoon.

Do take your time though please, I have about 2-6 months before I can actually order anything, and thank you for checking over it.


Was able to find a small changeable programmer:

I have that one. Be aware that it doesn’t properly do 3.3V. I’ve modified mine to work properly at 3.3V

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.

The flash chip is also at risk. Plus, the display’s flex connector is fairly fragile and could be damaged by multiple solder/desolder operations.

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.

Is there a possibility I could use a 3.3v/8MHz Pro Mini for programming it? I think it’d work if you removed the chip on it. It’d also be a lot cheaper.

Yes, you could use a 3.3V Pro Mini as an ICSP.

What chip do you want to remove? I don’t see why you need to remove any chip.

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.

Would a 3.3v/8MHz pro micro also work?

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 don’t own a pro mini unfortunately at the moment.

They’re quite expensive so it’d be better to get a pro mini.

When determining cost, take into account that you’ll need a USB to serial converter for the Pro Mini but the Pro Micro has built in USB. You could also consider a UNO, which has built in USB.

1 Like

Wouldn’t the fact the Uno operates at 5V make it more awkward to use than a 3.3V Pro Micro?

1 Like

True, I didn’t consider that. However, the UNO provides a 3.3V output. You could use that with level shifters on the ICSP signals.

Overall though, a 3.3V Pro Micro or dedicated 3.3V ICSP would likely be the simplest of what has been suggested so far.


Just wanted to ask if you could by any chance check over my schematic once more please? I’m planning to order the PCBs very soon.

I might check over it again and see if I can make it a bit cheaper.


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:

$43.70 AUD
$31.02 USD
£23.54 GBP
$40.07 CAD

I’ll be ordering the parts somewhere around next week and then hopefully I can assemble it completely around February-March.

And the final (May be updated) schematic:

Here is a slightly updated parts list:

(PPU and Cost is in AUD)

1 Like

2/3 of the Parts have been Ordered!

I just ordered the PCBs and components from Digikey, but I’ll have to wait to order the next parts from eBay.

1 Like