Dragon 65c816 SBC

[GamerFox]

So, from the design thread in the newbie forum, I refined (mostly by removing things that would only increase complexity for a first-time board) the design. The Dragon v1.0 runs at a 4 MHz clock speed and has:

• One 65C22 VIA to provide an SPI bus and GPIO
• DS1305 real-time clock
• FRAM to hold data not needed in the system ROM (preferences et al)
• Serial port provided by a MAX3100 and MAX3223
• MicroSD card slot

The power will be provided by a DC converter board of my own design, mostly so I can use the power supply again and again with later projects.

Future plans include replacing the glue logic ICs with CPLDs and maybe adding a CPLD blitter for display purposes (either a TFT screen or VGA output, haven't decided yet)

[GARTHWILSON]

Let me recommend running the planes between each pair of pads, for much better AC performance, so signals' ground return current doesn't have to veer away from the signal line and go clear around the end of the IC. There's plenty of room to get it between pads.

[GamerFox]

On both the power and ground planes? (power on page 2, ground on page 3)

[GARTHWILSON]

At least the ground plane. Since you have traces on layer 4 that cannot "see" the ground plane, and thus are using the power plane as the ground plane, hopefully you have the power plane bypassed to ground at every power pin. Ideally you'd also have it bypassed also everywhere there's a via from layer 1 to layer 4, to keep the return path continuous next to its signal trace; but I've never seen hobbyist boards go to that extent. 74HC logic definitely won't need it. It's easy to run it between pin pads though. See this post: viewtopic.php?f=12&t=6818&p=87925#p87925

[GamerFox]

Is this different from decoupling caps?

[GARTHWILSON]

same thing, going from power to ground

[floobydust]

Unless I can't find it... I don't see a Reset circuit for the CPU...

[GamerFox]

floobydust wrote:

Unless I can't find it... I don't see a Reset circuit for the CPU...

The reset circuit's on the power board

[GamerFox]

Here's a new version of the PCB.