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LAUNCHXL-F28379D: The some circuit problem for Ecap

Zhenjia Chen
Prodigy 140 points
Part Number: LAUNCHXL-F28379D
Other Parts Discussed in Thread: ISO7741

Hi ~I want to ask some question for LaunchPad28379 Board
First, I want to use Ecap module in LaunchPad28379.So my  signal input through J11
The circuit as picture below.I also use IC ISO7741. And IC output should be 5V pulse wave.
And connect 51ohm and 1nF to J11

I don't know why need 1K ohm and 1nF in board(as second picture shows)??
If consider 1k ohm and 1nF in board.How can I calculate RC time constant for my pulse wave which ISO 7741 output?
And also I want to know how does dsp can determine waveform rising and falling edge for Ecap? 

  • 0
    TI__Mastermind 42195 points
    Zhenjia Chen said:
    I don't know why need 1K ohm and 1nF in board(as second picture shows)??

    Please note that F2837x microcontrollers are not 5V tolerant, you will need to level shift any 5V signals. This is taken care of on several pins in the f28379d LaunchPad design. These pins are intended to be used as an encoder interface, for this application the pull-up and decoupling cap are a good idea.

    Zhenjia Chen said:
    If consider 1k ohm and 1nF in board.How can I calculate RC time constant for my pulse wave which ISO 7741 output?

    RC charging time can be evaluated easily. https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-time-constant  

    Zhenjia Chen said:
    And also I want to know how does dsp can determine waveform rising and falling edge for Ecap?

    The eCAP observes the GPIO buffer and depending on the eCAP's configuration captures the value of the TSCTR when the configured level transitions occurs. 

    Regards,
    Cody 

  • 0 in reply to Cody Watkins
    Prodigy 140 points

    Thanks your apply Cody~

     These pins are intended to be used as an encoder interface, for this application the pull-up and decoupling cap are a good idea.


    I knew this application for QEP interface.so need the level shift to change voltage level.But i want to ask why need pull-up and decoupling cap for application.

    RC charging time can be evaluated easily.

    I want to confirm the answer with you.The eqivalent R=51 C=2nF right?

  • 0 in reply to Zhenjia Chen
    TI__Mastermind 42195 points

    Pull-ups help hold the pins in a known good state. And decoupling caps provide a path for noise to be shunted to ground.

    The RC filter design depends on what frequency noise you are expecting and what frequency you're expecting your feedback signals to toggle. Ideally the Fc of your filter is well below your noise such that it can dampen the noise, at the same time Fc must stay a reasonable amount above your intended signal frequency or your signal will be distorted. 

    Don't worry about it too much, estimate it with a calculator and then adjust the R and C when you are debugging your circuitry. If your Fc is too low you might not get any signal, or your signal may look like a sawtooth or shark-fin waveform.

    Regards,
    Cody 

  • 0 in reply to Cody Watkins
    Prodigy 140 points

    Thanks for share information with me.I got it .thank you~~