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Altera_Forum's avatar
Altera_Forum
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15 years ago

CPLD MaxII, pull up resistor and led

Hello

I have to drive 26 Leds through the CPLD MaxII.

I have seen that this CPLD have internal pull up resistor that can be enable or not and apparently we can choose their value.

My question is: can we use those resistor for current limitation?

5 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    --- Quote Start ---

    apparently we can choose their value

    --- Quote End ---

    No, they are fixed. Output drive strength can be selected low or high. But you'll normally need external current limiting resistors to set the current of connected LEDs.
  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    Ok thanks.

    So have you any idea to avoid the use of 26 resistors?

    I think about implementing a led driver in the CPLD but I'm not sure that it will decrease my number of resistor.
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    If you don't need high average LED currents, multiplexing the LEDs in a matrix can save output pins and resistors. Using common chip resistor arrays, 26 shouldn't be a problem, however.

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    The maximum forward current is 50ma. I will use PWM to drive the LED.

    Do you think it is ok for multiplexing.

    I don't know what is multiplexing and how to do that in the CPLD, I am a begginer. I will check that (but if you want or have time to explain feel free :)).

    The other proof that I am a begginer, is that I didn't know what was a common chip resistor (I've already check).

    Thanks for your help