4 Replies Latest reply on Nov 11, 2020 8:21 PM by MoTa_728816

    I2C - Fall-time

    ivle_1343546

      Our application ( Fast-mode : 400Khz) the measured fall time is only about 4ns - but I2C spec's the minimum allowed fall time according the spec would be 20 x (Vdd/5.5) = 20 x (3.3V / 5.5V) = 12ns .

       

      1. Could it cause any problems(side effect)

      2. How to increase the fall time to meet spec ?

      I2C timing.png

       

      訊息由此人編輯:ivan lee

        • 1. Re: I2C - Fall-time
          MoTa_728816

          Hi,

           

          > 1. Could it cause any problems(side effect) ?

          Some I2C device connected to the bus may not work correctly.

           

          > 2. How to increase the fall time to meet spec ?

          Off my head, we usually apply 2.2K ~ 10K resistor for pull-up.

          Depending on the value of this resistor the fall/raise time will be changed.

          Lager resistor makes the transition slower and smaller resistor makes the transition faster.

           

          The calculation described in the I2C specification may be a good reference.

          17 Application Information

          17.1 Slope-controlled output stages of Fast-mode I2C-bus device

          (I'm referencing The I2C-BUS specification Version 2.1 Jan-2000 from Philips Semiconductors)

           

          moto

          • 2. Re: I2C - Fall-time
            ivle_1343546

            Hi, Moto,

             

            We try to modify resistor for pull-up before , It can improve rising time, but it is not useful for fall time.

            Please give us some suggestion to increase the fall time.

             

            BR

            Ivan

            • 3. Re: I2C - Fall-time
              MoTa_728816

              Dear Ivan-san,

               

              Then how about adding some resistor serial to the SDL/SCL pin,

              so that current drain time will be increased?

               

              Best Regards,

              12-Nov-2020

              Motoo Tanaka

              • 4. Re: I2C - Fall-time
                MoTa_728816

                Dear Ivan-san,

                 

                Although it must be obvious, please keep the value of series resistor

                Vdd * Rseries / (Rpull-up + Rseries + (internal resistance of I2C pin)) < VL (Low level voltage).

                001-schematic.JPG

                 

                Best Regards,

                12-Nov-2020

                Motoo Tanaka