1 Reply Latest reply on Feb 14, 2014 4:50 PM by user_14586677

    External MHz Oscillator for RF Reference Frequency

    peter.taylor

       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
          user_14586677

          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 -

             

           

             

          www.cypress.com

             

          “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.