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I am finding myself in a position where I am needing 3 different crystals in my application. Two for the PSoC (RTC and MHz crystal for CAN) and one for a 915 MHz ZigBee transceiver. What I would like to do is instead have the two on the PSoC and then output a clock signal to the transceiver. My question is, assuming I can route the clock trace on the board to not pick up a lot of noise, how much noise would I be adding to the clock signal by routing it out a GPIO pin from the MHz ECO (16MHz crystal). I wouldn't be routing through the PLL. Just straight from the ECO to the GPIO. Obviously the phase noise, jitter, etc. needs to be pretty small for the RF reference frequency, but I haven't found any specs for the ECO. Anyone have any thoughts?
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PSoC 3
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I am not aware of any specs in datasheet or TRM, maybe someone on forum
knows. You might file a CASE, and ask for specs/characterization data -
To create a technical case at Cypress -
“Support”
“Technical Support”
“Create a Case”
You have to be registered on Cypress web site first.
If you have a high speed DSO you can certainly measure jitter with that,
keep in mind it will be displayed as DSO jitter included, so look at its specs.
http://www.tek.com/application/jitter-measurement-and-timing-analysis
Also raise thedirect route to a GPIO issue. Looking at TRM and ap notes I do not
see a way to do this directly to a GPIO, the clock tree hands off everything
to a mux into DSI. However ap notes sometimes can be incomplete, so raise
the question.
Some useful ap notes -
http://www.cypress.com/?docID=41149 AN54439 Clocking on PSOC
http://www.cypress.com/?rID=40990 AN60631
http://www.cypress.com/?rID=39677 AN57821 Board layout info
http://www.cypress.com/?rID=43337 AN61290
I would not recommend you use the Mhz ECO output pin to xtal for a clock source, as
that adds additional Cload and the tolerance of Cload would challenge specs.
Lastly if you google "high speed clock layout practices" there is a wealth of
info on NET that would cover PCB routing and noise pickup issues.
Regards, Dana.