Make the board compatible with ATMEGA328P-AU

[DominikPalo]

I really like your improved design (compared to the original one), but one thing that worries me is the decision to use the ATMEGA4809-AFR rather than the ATMEGA328P-AU MCU (used also in the original v2.4 design), which is currently much much cheaper (like five times...) :

• ATMEGA4809-AFR costs more than 15 € (LCSC)
• ATMEGA328P-AU costs only around 3 € (LCSC)

I know there are still sourcing issues with all ATmega MCUs, but it seems the ATMEGA328P-AU is currently more affordable, although it could change anytime... The nice thing about ATMEGA328P-AU is that it is even used in cheap Arduino clones sold on Ali, so it can be extracted (as Great Scott did in his video) and it is also a bit simpler to hand-solder.

So my idea is to improve your PCB design to support both MCUs: do you think it could be possible to provide pads for both ATMEGA4809-AFR (TQFP-48) and ATMEGA328P-AU (TQFP-32) on the same board, to avoid maintaining two designs, so anyone could decide which MCU would like to populate?

[DerSpatz]

At the time of the design, the availability of the 4809 was much better than the 328P. Now that this has changed, a redesign is warranted. I have already checked, and a change in the design to use the 328P-AU should be pretty easy, and quickly done.

On thing to note is that most 328P boards have an external oscillator, so you can't just solder them directly onto a design without external oscillator (as mine), but you need to change the bootloader (not sure about that) first.

Unfortunately, as both chips are 7x7mm, a design to accommodate both chips would be much harder to do.

I will see when I get around to making the necessary changes in the design.

[bzed]

@DominikPalo thats just lcsc trying to make money. Its less than 3 EUR at mouser.... Yes, soldering by jlcpcb is easier, but for that price difference you could even buy a heat gun when you want a few pads :)

[bwack]

At mouser, the ATMEGA4809-AU is in stock, but the AFR version (125C version) is on order with long lead time. Not sure how hot the microcontroller will get.

[DominikPalo]

@DerSpatz initially I was thinking about putting TQFP-32 inside TQFP-48, but as you have already mentioned, both chips use 7x7mm footprints, so it is not possible... also, putting the secondary pad to the back side of the board wouldn't be a good idea, as it would prevent from using reflow process on such double-side populated boards...

So, it seems the only option is to make a separate PCB design for the 328P-AU - I have some experience with KiCad, so I could definitely help with that next week and provide a PR for your review (we could call it v1.2) :)

On thing to note is that most 328P boards have an external oscillator, so you can't just solder them directly onto a design without an external oscillator (as mine), but you need to change the bootloader (not sure about that) first.

@DerSpatz of course, you are right, but switching between the external/internal oscillator is really easy - it does nothing to do with the bootloader, the oscillator selection is configured by simply setting the value of the LOW fuse (lfuse) inside MCU using the avrdude command/tool. Here is a nice calculator for 238P fuses: https://eleccelerator.com/fusecalc/fusecalc.php?chip=atmega328p

thats just lcsc trying to make money. Its less than 3 EUR at mouser.... Yes, soldering by jlcpcb is easier, but for that price difference you could even buy a heat gun when you want a few pads :)

@bzed I didn't mention LCSC as an example because of their soldering service (I always populate my boards at home ;)), in this case, I prefer LCSC over Moser for two reasons:

1. Shipping fees: in my country, the shipping cost from Mouser is 20 € (they provide free shopping only for orders >50 €), whereas LCSC could deliver an order for less than € 5 (of course, it takes 10-15 days - similar to AliExpress). Therefore, whenever I don't need something urgently, I personally prefer to use LCSC. I ordered components from LCSC a couple of times and I can confirm the delivered parts were always genuine.
2. The ATMEGA328P-AU is currently not available on Mouser anyway...

[bzed]

At mouser, the ATMEGA4809-AU is in stock, but the AFR version (125C version) is on order with long lead time. Not sure how hot the microcontroller will get.

There are 7 -AF left, which should be the same as the -AF, just not on a reel. Actually thought about ordering them :-D

[bwack]

Hehe. Be quick 😅. I have ordered pcbs of heatplate 1.1. I was thinking of swapping out micro in the design first, but then there is the software as well.

[platima]

Hey folks just a note that you don't HAVE to use an external oscillator, and a lot of projects don't. 328P has an internal 8Hz oscillator that is good for MOST use cases, especially short term like this. It's just not massively accurate (or fast of course), so it can drift a bit over time.