1 Reply Latest reply on Feb 14, 2014 4:50 PM by DaKn_263916

    External MHz Oscillator for RF Reference Frequency

       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?

        • 1. Re: External MHz Oscillator for RF Reference Frequency

          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 -








          “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.














          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.