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| 8MHz trace - best practices |
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| i_am_fubar:
I have a wireless video transmitter that needs an 8MHz clock input. To save board space, my plan is to generate it from a PIC and output it via a CCP pin. The trace is 1.6" long and running on a double sided 1.6 FR4 board. Knowing that this is going into RF territory, what considerations should be made? Cheers |
| fourfathom:
1.6" is pretty short, and the PIC isn't going to have blazingly fast signal edges, so you probably don't need to do anything. You might put a series resistor (say 100 Ohms) at the output of the PIC to reduce potential ringing on the 8M clock. How accurate does this clock need to be? |
| i_am_fubar:
Cheers, Looked at the wavelength of 8MHz, and it's well outside of dimensions of trace :) Good question on accuracy.... I don't know. Transmitter datasheet is a bit poor. Can always tune the PIC if needed. |
| Siwastaja:
8 MHz frequency is irrelevant; it's a square wave, so the edge rate defines the actual frequency content. Adding a tiny series resistor (such as a 0402 package) physically as close as the driving pin as possible indeed slows down the edge (and works as series termination). Remember to keep the ground plane contiguous below the trace and make sure there are ground vias close to the ground pins of the ICs at both ends, so that the current has a return path as close as the trace as possible. Not super important at such edge rates, but good to have some practice. The 2-layer 1.6mm board adds some extra distance to the return current you can't completely deal with, but it's ok. |
| Zero999:
I don't see how using a PIC would save any space. It would be huge, compared to some of the tiny oscillator modules available. https://eu.mouser.com/ProductDetail/ECS/ECS-1612MV-080-CN-TR?qs=sGAEpiMZZMt8zWNA7msRCpVJ%252BfotoivODluv3UN%252B9JI%2FSHESqfVcag%3D%3D https://uk.farnell.com/multicomp/mcsjk-3n-8-00-3-3-25-b/oscillator-8mhz-3-2mm-x-2-5mm/dp/2854266 |
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