7 Replies Latest reply on Apr 7, 2019 6:00 AM by MaH_4035971

    capacitor values for WCO

    MaH_4035971

      Hello, I am designing a circuit based on the CYBLE-416045-02 module and obviously I paid a close look at the schematics of the PSoC63 BLE prototyping kit and I was a bit surprised to see the values of C37 and C38. The PSoC63 Hardware design consideration (AN218241) reads on page 10 that the capacitor value can be calculated with the formula C = 2*CL-3pF and that the capacitors should be C and 2C.

       

      • First of all, that's different from the conventional oscillator circuit where both capacitors have the same value, could you please explain why ?

       

      • CYBLE-416045-02 uses a 32.768KHz crystal (ECS-.327-12.5-34B-TR), which has a load capacitance of 12.5pF. Using Cypress's own formula the capacitors should be 22pF and 44pF (except that 44pF does not exist but 43pF is close enough). Why did you choose C37=33pF and C36=18pF ? I guess you must have measured the actual stray capacitance of the PCB, which may be different than 3pF but even that does not explain the discrepancy between theory and practice. Did you use this alternative formula Cl=(C1*C2)/(C1+C2) + Cs with Cs=0.85pF ?

       

      • The AN95089 on crystal selection and tuning describes the fine tuning using internal trim cap. Does that mean that one possible strategy would be to slightly undersize the external cap and adjust with the internal ones ?

       

      • There is no ECO on the PSoC63 BLE prototyping kit. Would you recommend adding one for BLE applications ? In that case do I really need the WCO ? I am worried about power consumption.

       

      Thanks !

        • 1. Re: capacitor values for WCO
          EktaN_26

          Hello Matt,

           

          Can you please have a look at the link: https://www.cypress.com/file/457821/download , The PSoC 6 BLE Prototyping Kit has both ECO and WCO.

           

          Best Regards

          Ekta

          • 2. Re: capacitor values for WCO
            MaH_4035971

            Hello Ekta,

             

            Thank you for your reply but the prototyping kit has only WCO. The pads are there but Y1, C34 and C35 are not soldered on the board.

             

            IMG_2914.jpg

             

            Best regards

             

            Matt

            • 3. Re: capacitor values for WCO
              himam_31

              Hello,

               

              The BLE ECO and WCO are already included in the kit (pre-installed in Ez-BLE Creator Module) and user need not solder it. These are part of the BLE module. Please refer to the "A.2.6.5 System Crystals" in the user guide.

               

              Thanks,

              Hima

              • 4. Re: capacitor values for WCO
                MaH_4035971

                Whether ECO is there or not is a secondary issue (BTW if you look at the picture you'll notice that it is not), the main question was about the capacitor values that do not match the formula from the application note and the data sheet. I simply wanted to understand why, that's all.

                 

                Thanks

                • 5. Re: capacitor values for WCO
                  AchimE_41

                  Hi Matt,

                   

                  In the PSoC 6 based module there is only the BLE ECO included, no WCO.

                   

                  For low-power applications, with BLE using the low power modes or an accurate time is needed (RTC) a WCO should be added to the system. In case of BLE the WCO ensures that the timings are met while the device is in DeepSleep (Main clock is off).

                   

                  An external ECO for the CPU is only needed if the IMO accuracy does not meet the project requirements.

                   

                  Yes, it is using the alternative formula. I assume it is done because lower capacities will result in a lower power consumption, but I am not sure.

                  If I have to guess the formula in the Hardware Design Guide is wrong and should be C = 2*CL - CL/2

                  That way it returns the same results for 6pF and 12.5pf as the formula in the Crystal Oscillator Selection Guide (AN95089)

                  And basically it is just a complex way to say that C = 3/2 CL isn't it?

                  Considering: the 2:1 ratio, C1 = C and C2 = 2*C1 = 2*C

                  CL = (C*2*C)/(C+2*C) = (2*C²)/(3*C) = (2*C)/3 --> C = 3/2*CL

                   

                  regards,

                  Achim

                  2 of 2 people found this helpful
                  • 6. Re: capacitor values for WCO
                    MaH_4035971

                    Crystal clear ! Thanks Achim !

                     

                    There is still a small discrepancy: C2, i.e. C37 on the prototyping kit, is not exactly equal to 2*C1, i.e. C36 (33 and 18 pF respectively) but I got the idea. As long as C = 3/2 CL it does not matter if C1=C2 or C2=2*C1 right ?

                     

                    Cheers !

                     

                    Edit:  I found this useful answering my own question : http://www.nkg.com.hk/pdf/NKG-TIT_OscFreqDeviations(E).pdf and this http://geyer-electronic.de/fileadmin/user_upload/box/GEYER-Crystal-or-Oszillator-how-to-apply.pdf

                     

                    "C1 is smaller than C2 in order to enhance the start-up performance of the circuit"

                     

                    • 7. Re: capacitor values for WCO
                      MaH_4035971

                      Well after a bit more digging, here is what I found. Hopefully someone else may find that useful:

                       

                      There are actually 2 ECO ! One is called "ECO for BLE" and is already included in the module as shown on the schematics page 20 of the datasheet (https://www.cypress.com/file/428336/download) and is connected to pins X0 and X1 (M5 and M4 on the 116-BGA package) The other is just call ECO, is optional and is connected to pins P12.6 and P12.7 (G5 and H5).

                       

                      That's very confusing because it is not mentioned anywhere in and even some of Cypress' application specialists ignored it... So to summarise:

                                                                             ECO-BLE     ECO     WCO

                      CYBLE-416045-02                              y                  n            n

                      PSoC63 BLE prototyping kit               y                   n            y