日本語
So I haven't done a primarily SMT PCB before, but I am forced to with this design because the atmega32u4 only comes in the TQFP package. The design is fairly simple, just connection to two OLED screens via SPI, one at 5v and one at 3.3v, served through 4050 as a level shifter. The SPI bus ideally I would be running at 10MHz but it might be closer to 5Mhz or even 2.5Mhz depending on how things turn out. With the design breadboarded with wires/probes everywhere I'm running it at 2.5MHz. The 3.3v display is connected directly to the PCB via the 2x8 header, and the 5v display via a short (10cm) ribbon cable.
I have a first pass done on my layout, with gnd on the top layer and +5v on the bottom. I'm a bit worried about some areas, to the right of U2 there is a part of the gnd plane that is only connected through a pin of the atmega. This doesn't seem great to me so I'm thinking I should take another pass with the layout and try to open up a few more areas. I'm wondering if I need to be concerned with this? Or if I should switch the gnd plane to the bottom as I've read on the forum that you should run higher speed tracks over a gnd plane.
All of my designs up until now have been lower speed through hole, so I'm a bit lost in this area.
- Comments(1)
A****min
May 15.2019, 16:49:21
The key thing to think about is current loops - for every high speed rising edge on the SPI bus, there will be an equal current pulse flowing through the ground trace between the driver and display. Try visualising where these loops will go (including flowing through the ground trace) - you want to try and make sure the ground is a good clear path.
If you're worried about having poorly connected areas of the plane, GND on both the top and bottom might make more sense, as you can then stitch them together with static vias.
