Hello,
Thanks for your answers!
Nice to know I can use the ATMega @8Mhz. You are both perfectly right about the decoupling capacitors, when I checked the boards I designed for my working chain project, I see immediately that I put 2 caps per Maxim chip (0.1µF and 10µF). And on my current breadboard setup, each eBay module has at least one cap.
I got overconfident: when I designed my clock I just took a glance at the MAX7219 datasheet, which on the first page does not mention the caps (see attachments)... and I forgot them stupidly!
I'll write that down on my idiot checklist 
As for the LEDs without resistors, it's just a shortcut I took with Eagle because I wanted a smaller footprint for my resistors. So whenever you see 2 LEDs in series on the schematic, it's in fact 1 LED and 1 resistor.
Do I need a decoupling & supply capacitor on the power connector since the clock will be powered on the USB port of a computer? Could they prevent unwanted reboots?
Thanks again,
Fred
