Electronic Circuits and Projects Forum

Originally Posted by ericgibbs

hi alex,
What are the dimensions of the pcb area that will require a four layer pcb layout.?

You could consider the main pcb as being double layer and a small 4 layer 'piggy back' pcb for the USB2 module.

Leave sufficient area clear on the main pcb for the 4 layer pcb.
The 4 layer pcb would be mounted on pickup pins on the main pcb.

I have used a similar method for mounting smaller pcbs to a main pcb.

If this explaination is not clear enough, please ask.

Hi, thanks for your advice. You've been illustrative and I totally understand what you mean. I just had this 6 layer ARM9 PCB shipped here which I can plug into my test board. It's a lot cheaper than a big ARM9 board with everything on it.

I did think about stacking a mini 4-layer PCB above a motherboard. But the USB interface is the only thing in my system that requires a 4-layer PCB, i.e, this 4-layered area only covers a corner of the TQFP128 chip, and extends less than 2 inches beneath D+, D- tracks, to the USB connector. There's a diagram in the attachment which shows its shape.

Such a small area is worthy of the 4-layer material, but it's no right way building a circuit board. To carry the chip, this mini board has to cover at least the area of the entire TQFP128 area, and then there comes the problem with the rest of the pins. So either I implement a 4-layer PCB with a lot of chips that don't actually deserve it, or I keep everything including the USB2.0 tracks on a 2-layer PCB, and take extreme precautions to maintain signal integrity.

Do you think it has to take a 4-layer PCB for USB2.0 layout? Thank you for your time.

Thanks 3v0. I'm checking this link out. And I'll be careful with those ppl there.

BTW, 18F4550 is USB2.0 compatible, but it doesn't work at high-speed. The CY7C68013A complies with USB2.0 spec and also works at 480Mbps high speed, hence requires something at a higher std?

I'll return after reading the thread. Thank you again.

Originally Posted by 3v0

I doubt that the USB will be the bottleneck.

FWIW I like the designs that use an OP AMP instead of transitors. I think the OP AMP is a bit more rugged and easier to replace if you do blow it out.

Well my guess is, that not following the exact specifications won't make significant difference, not even noticeable. Maybe with everything by the book, an error caused by EMI may occur once in a year's time, and with slight modification, the chance may be higher as once in 8 month's time. But either way it's acceptable for common use.

I've always liked more integrated solutions, as long as they don't raise much cost. It makes the design tight and more reliable, and yes, easier to repair.

Originally Posted by ericgibbs

hi alex

Yet another link!, showing pcb artwork for using just two layers.


http://www.smsc.com/main/anpdf/an119.pdf

Hi Eric, I'm reading it right now. It's quite informative, and I think I borrow a lot of ideas from this document. Thank you, I think with this example, I will be able to figure out a way to lay out a great PCB.

Plus, I just came up with this idea, that I may attach a small PCB to the main board over the USB tracks, with intact single copper layer connected to GND on the main board. This might act as a shield and keep the USB tracks in an interlayer. I don't know if it will work the way I think it would, but I'm sure a single layer PCB or just a piece of metal piggy back is much, much cheaper than a four layer one with larger area.