2 Replies Latest reply on Feb 17, 2016 9:38 AM by scott

    Crystal drive level for CY7C65632

    scott

      I reviewed the Guidelines on System Design doc for USB HX2VL families.  We are using the CY7C65632.  Section 3.2.1, Crystal Drive Level, isn't making sense to me.  The required power level is extremely high, such that it's impossible to find small crystals which would work.  Most small crystals have a max. drive level of 100 uW, and the Guide says 600 uW but then provides a formula to calculate the drive power.  It's the formula which isn't making sense.  Shouldn't the voltage Vx be the RMS, not the peak voltage?  And why is the "2" outside the brackets?

         

      Let's look at an example calculation, using typical values for crystals we're considering.

         

      Vx = 3.36 V

         

      Co = 7 pF

         

      CL = 18 pF

         

      R = 60 ohms

         

      f = 12 MHz

         

      Power from the formula is 2 * [pi * 12E6 * (7 + 18)E-12 * 3.36] ^ 2 * 60 = 1.2 mW.  This is an absurdly high drive level!  Is there an error in the formula?

         

      Thanks,
      Scott

        • 1. Re: Crystal drive level for CY7C65632
          gsns

          Hi scott,

             

          The formula is correct. There is small typo in the formula. p=I^2R =( Vx/sqrt(2)Z)^2 R. but z = 1/WC w= omega here.

             

          substituting w = 2pifc we will get the 2 in numerator.

             

          Please check the drive level using the formula for the crystal which is being used in the HX2VL .

             

          This is the Part we use in the DVK:

             

          Part no : ECS-120-20-4 (http://www.mouser.com/ds/2/122/ecs_hc_49us-543840.pdf)

             

          Thanks,

             

          Krishna.

          • 2. Re: Crystal drive level for CY7C65632
            scott

            Hello Krishna,

               

            Thank you for your reply.  I think you are saying that the final formula is correct, but that the intermediate step should have shown the sqrt(2).  Is that what you're saying?

               

            If the formula is correct, is there something wrong with my calculation, which resulted in 1.2 mW?

               

            Second, the crystal you are using in the DVK is a 500 uW max. crystal, which is less than the 600 uW from the Guide and less than the number I calculated.  I am happy to use a 500 uW crystal, but I want to make sure the formula matches.

               

            Thanks,
            Scott