7 Replies Latest reply on Dec 20, 2016 4:18 PM by Intel Corporation

    How to supply RTC backup power

    wpd

      Hello all,

      I found the following discussion: RTC 1307 - Real Time Clock |Intel Communities but the result of the discussion (and the associated documentation) left me without an answer to my fundamental question, and without access to the PMIC data sheet, I don't know where else to turn.

       

      Should I expect to be able to attach a 3V coin cell battery to V_VBAT_BKUP and to use that to provide backup power to the RTC when main power is removed?

       

      With the corollary: What happens if I leave the PMIC in its default state (2.5V, 10uA)?

       

      It seems to me that the design intent is that a super cap would be attached to V_VBAT_BKUP, and that the PMIC would charge it to 2.5V at a rate of 10uA.  But, from the discussions, it also seems that I could attach a 3V coin cell.  Will the PMIC simply disable charging if it detects 3V?  Will it blow up if (as much as it can with a 3V coin cell) if I back feed it with 3V on that pin?

       

      And, as long as I have your attention... how much current does V_VBAT_BKUP draw anyway?  How long should I expect the RTC to be maintained (whether I power it with a super cap or a coin cell)?

       

      Thank you for your help.

       

      --wpd

        • 1. Re: How to supply RTC backup power
          Intel Corporation
          This message was posted on behalf of Intel Corporation

          Hi wpd,

           

          Thanks for your interest in the Intel Edison Platform.

           

          The information in the thread RTC 1307 - Real Time Clock |Intel Communities is correct, I think that what might confused you is that the pin changes if you are using the Compute Module or the Kit for Arduino. Please refer to this hardware guides for more information:

           

          1) Intel Edison kit for Arduino (Intel® Edison Kit for Arduino* Hardware Guide ).
          2) Intel Edison Compute module (Hardware Guide for the Intel® Edison Compute Module ).

           

          I hope you find this information useful, have a nice day!

           

          Best Regards,
          -Jose.

          • 2. Re: How to supply RTC backup power
            wpd

            Hello Jose,

            Thank you for your reply.  I am not confused about the pinout.  I am confused about whether I can safely (i.e. without damaging anything) attach a coin cell battery to the V_VBAT_BKUP pin on the compute module, and whether I need to recompile the kernel to change the PMIC configuration should I do so.

             

            Can you tell me:

            1) Can I attach a 3V coin cell to the V_VBAT_BKUP pin and expect that to provide backup power to the RTC?

            2) If I do so, do I need to change the PMIC configuration?
            3) How much current does the RTC draw when the main power supply for the compute module is shut off?

             

            Thank you for your help.

             

            --wpd

            • 3. Re: How to supply RTC backup power
              Intel Corporation
              This message was posted on behalf of Intel Corporation

              Hi wpd,

               

              In the hardware guide of the Compute Module you will find the answer to all your questions.

               

              Hardware Guide for the Intel® Edison Compute Module . See section 3.5, Table 5 and 4.13.

               

              I hope you find this information useful, have a nice day!

               

              Best Regards,
              -Jose.

              • 4. Re: How to supply RTC backup power
                wpd

                Hello Jose,

                Thank you once again for your help.  Section 4.13 of the hardware guide states that "The PMIC has a dedicated charging subsystem for a backup battery supply that could be either a rechargeable coin cell batteries or super-capacitors."

                 

                I still find this section confusing.  Can you please tell me if I need change the programming of the PMIC if I attach a normal (non rechargeable) coin cell battery?

                 

                Can you also tell me how much current does the RTC draw when the main power supply for the compute module is shut off?

                 

                Thank you.

                 

                --wpd

                • 5. Re: How to supply RTC backup power
                  Intel Corporation
                  This message was posted on behalf of Intel Corporation

                  Hi wpd,

                  I would recommend to use a rechargeable battery, since it has a charging subsystem and it may damage a normal battery. 

                  Now regarding how much current does the RTC draw? All the hardware information available is in the guide I provided in my last reply. May I ask why you need this specific information?

                  I will be waiting for your reply, have a nice day!

                  Best Regards,
                  -Jose.

                  • 6. Re: How to supply RTC backup power
                    wpd

                    Hello Jose,

                    Thank you once again.  And thank you for your patience.  Given your advice, I will use a rechargeable battery.

                     

                    I asked about the current draw because I don't see it listed in the specifications for the compute module.  I do see an example in section 4.13 as an "Example implementation", but that is different than a formal specification. The 8uA value given in the example seems a little high to me as compared to similar specifications for other SoC devices.

                     

                    Given a formal specification for current draw and "Minimum RTC retention voltage", I can do the math to size a supercap or battery and determine how long I expect the RTC to be maintained.  The example given in your document is great, but it is just that: an example.  It is not a formal specification.  Given that you keep pointing me at the same document, and given that the data sheets for the PMIC and the SoC are not publicly available, I shall have to content myself with the example.

                     

                    Thank you for your help.

                    --wpd

                    • 7. Re: How to supply RTC backup power
                      Intel Corporation
                      This message was posted on behalf of Intel Corporation


                      Hi wpd,

                      We are always happy to help!

                      I will communicate your concerns of the lack of documentation regarding the PMIC and the SoC to the Department in charge of this.

                      If you have any more questions, please don't hesitate to ask. 

                      Have a nice day!

                      Best Regards,
                      -Jose.