Design Requirements, Constraints, and Recommendations for 6.25MHz x16 PLL Crystal

gis667en11

Hi everyone,

I'm thinking of getting a Professional Development Board. When I do, I'd love to experiment with using the 6.25MHz crystal for 100MHz operation. Later, I'd like to design and build PCBs that use the 6.25MHz. However, I cannot find any threads in the forum regarding the actual necessary changes to the design of your circuit to accommodate the new crystal safely. I've read here and there that decoupling the crystal is a good idea, and I've read that some people have experienced overheating of their prop as a result of their hack.

So, if anyone knows of another discussion in the forum on these topics, I'd love to be pointed to it. If such a thread doesn't exist, let's chat about what you think good practices would be with using the 6.25MHz.

Heater.

I have Props here running at 6.5536MHz for a clock frequency of 104.8576MHz.

You will need to be sure you have good PSU regulation, proper routing of power and ground to the Prop (all four connections) decoupling caps close to the power pins. XTAL and it's caps close to the XTAL pins. Probably want to keep other signals away from those.

I have never heard anyone saying that an over clocked Prop overheated. I doubt you will even feel it warm up.

Have a search for Cluso99's TriBlade, RamBlade and other designs to see how to do this properly. Clusso has a lot of experience with overclocked Prop designs.

Publison

You would use all the proper design considerations that are used with a 5 MHz xtal. There is no extra considerations for a 6.25 MHz xtal. I swap them all the time.

gis667en11

Thanks guys, that puts me at ease. However, looking at the 1.2 Prop manual, it says under the pin descriptions that for XI and XO external capacitors aren't necessary. I could have sworn I'd seen someone solder a tiny smd capacitor in parallel to the leads of an upgraded crystal. Would this be necessary?

Publison

If you use the Xtal from Parallax, it has the correct capacitance to match the Propeller, (20 pf). No additional components needed.

https://www.parallax.com/product/251-06250