External MHz Oscillator for RF Reference Frequency

Tip / Sign in to post questions, reply, level up, and achieve exciting badges. Know more

cross mob
Anonymous
Not applicable

 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?

0 Likes
1 Reply
ETRO_SSN583
Level 9
Level 9
250 likes received 100 sign-ins 5 likes given

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.

0 Likes