I need to build a couple of displays for a drag race setup; basically displaying alternately speed and time with a few extra options to display the winning lane or a fault. My plan is to get handy with a router and pop out the segments from a few boards and then wire in by hand up to four LEDs per segment. You can see by the layout of the board (pic attached) that the pad layout makes it straightforward for figuring out what wire goes. I've set the segments mirrored left-right so I can attach it to the back of the display without the need to stand on my head backwards and get confused with the mess of wires.
This design is based on the Texas Instruments TLC5925
chip (~$1.50); it's a current sink, shift register LED driver that handles up to 45ma per channel at 17V. The advantage here is that there's no need for wiring a whole lot of resistors as I can just put the LEDs in series and power the segments from a lead acid battery -- I've not seen any product anywhere that would be as cheap or more fitting than designing my own. Although I've left open the opportunity to talk to the shift registers directly I'd like to put an ATmega328 on so I can speak to it using serial, and that's where my questions all pile up:
1) I've read that the crystal (resonator) is needed when serial will be used; however it wouldn't be necessary at slow speeds (in the neighborhood of 2400 baud), correct? I've not seen anyone mention what kind of top speed should be expected without an external crystal.
2) If I'm not doing any analog input/output is the cap between GND and AREF required?
3) Did I screw up the wiring of the reset pin or miss anything required for the programming of the ATmega?
Any other comments/criticism is most welcome.