3 Replies Latest reply on Jan 8, 2015 1:02 PM by userc_7511

    BCM20736 UART Rx Voltage level

      Hi Sir,

       

      I have a electrical question.

      If our host soc's io voltage level is 2V5 and I real measure is around 2V7.

      Is it okay to connect to BCM20736 Rx?

       

      The Soc Tx -> BCM20736 Rx (2V7) and the Soc RX <- BCM20736 Tx (3V3).

       

      I do a simple transmission and it seems that the packets went wrong.

      So, I want to confirm if a level shift IC is required for such combination?

       

      Thanks.

        • 1. Re: BCM20736 UART Rx Voltage level
          MichaelF_56

          Per my understanding of the Electrical Characteristics in the SoC datasheet for the VDDO(PUART)/VDDM domain (HCI UART), the voltages you specified for Tx/Rx may not be sufficient for both HCI/PUART operation, depending on how the BLE part is being powered.

           

          Example:

           

          For VDDO/VDDM of 2.5V, our part supports the following levels:

           

          • 0.4V = Vil
            1.875V = Vih
          • 0.4V = Vol
            2.1V = Voh

           

          Copying userc_6547 and jota_1939431 so they can confirm.

          max-elec.png

          dig-vihvil.png

          • 2. Re: BCM20736 UART Rx Voltage level

            For a BCM powered at 3.3 volts, if your host is putting  2.5 or 2.7 volts on the BCM Rx line it should be sufficient to be recognized as a 'high' level.  But what about the low?  BCM is expecting less than 0.4volts for the low.  What is your Host Soc putting out for a low voltage?  If it's less than 0.4 volts you should be fine.

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            • 3. Re: BCM20736 UART Rx Voltage level

              On the other hand, if your BCM is powered at 3.7 volts, then the 2.7 volts coming from your host is below spec for the Rx Input on the BCM.  What is your BCM powered at?  It wasn't clear from your post.  You mentioned 3.3 volts, but is that at the Tx pin or is that your battery/regulator voltage that powers the BCM?  If 3.3 is coming from your Tx pin, that means your BCM VDDO must be around 3.7 volts.  In that case, maybe you can lower the voltage on the BCM VDDO to get a better level match.

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