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geekhack Projects => Making Stuff Together! => Topic started by: feralfoo on Sun, 01 January 2023, 20:38:03

Title: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: feralfoo on Sun, 01 January 2023, 20:38:03

Happy new year peeps,

I'm taking a stab at an atmega32u2 based custom keyboard PCB called the Exp60 and wanted to ask for feedback on the schematic. You can find all the Kicad 6 files in the following github repo:
https://github.com/christian-schulze/exp60

The schematic:
[attach=5]

[attach=2]

[attach=6]

[attach=3]

[attach=7]

[attach=4]

The project readme covers the design constraints, but to summarise the most important:

• 60% form factor with arrow keys (roughly GH60 dimensions)
• standard 2.25U left shift key
• standard 1U forward slash / question mark key
• hand solder-able so large SMD footprints

Note: I've chosen the atmega32u2 over the atmega32u4 due to space constraints (32 vs 48 pins).

The matrix is based on the following layout:
[attach=1]

Couple things I think might be missing:

• ground planes under USB receptacle, crystal and MCU

I'd love to hear any feedback you have, and please feel free to use any of this in your own designs.

Title: Re: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: Applet on Wed, 04 January 2023, 02:31:55

Looks good. Two things I spotted: Ucap should be 1uF and you'll need a pulldown on HWB for the bootloader to work as expected out of the box. I'd recommend to use PC4 as col8 and just put a pulldown resistor on HWB.

Title: Re: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: feralfoo on Wed, 04 January 2023, 05:31:32

Quote from: Applet on Wed, 04 January 2023, 02:31:55

Looks good. Two things I spotted: Ucap should be 1uF and you'll need a pulldown on HWB for the bootloader to work as expected out of the box. I'd recommend to use PC4 as col8 and just put a pulldown resistor on HWB.

Awesome, thanks for the feedback @Applet, have updated the schematic and PCB.

Title: Re: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: Applet on Wed, 04 January 2023, 06:25:38

Currently, you have different ground symbols for shield and signal ground, but they are not connected. Just mentioning so you are aware that shield is floating (unless I'm mistaken on KiCad behavior).

Title: Re: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: feralfoo on Wed, 04 January 2023, 07:35:09

Quote from: Applet on Wed, 04 January 2023, 06:25:38

Currently, you have different ground symbols for shield and signal ground, but they are not connected. Just mentioning so you are aware that shield is floating (unless I'm mistaken on KiCad behavior).

Ah so the SHIELD connector on the USB receptacle? Looking at the following two projects:

• https://github.com/Unified-Daughterboard/Unified-Daughterboard
• https://github.com/ai03-2725/JP60

It looks like signal ground is connected to shield via a ferrite part:

Unified-Daughterboard
[attach=1]

JP60
[attach=2]

I originally had this in my design (based on the JP60 by ai03) but removed it thinking it was not required. Do you think I should add it back in?

Title: Re: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: Applet on Wed, 04 January 2023, 14:08:17

Yeah, I'd recommend it

Title: Re: Request for feedback on atmega32u2 based schematic (Exp60)
Post by: feralfoo on Wed, 04 January 2023, 15:15:52

Quote from: Applet on Wed, 04 January 2023, 14:08:17

Yeah, I'd recommend it

Cool cool, ferrite part wired in. Appreciate the time you've taken to look through all this!