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Danlar81's avatar
Danlar81
Icon for New Contributor rankNew Contributor
3 years ago

VHDL AND Logic working as OR logic

Hello,

I am fairly new to VHDL programming.

I purchased a Udemy online course to begin learning along with a cheap Chinese development board which is used in the course, see link below. The dev board uses a Cyclone IV EP4CE6E22 FPGA.

I have installed Quartus Prime Lite 20.1 to write the VHDL code and program the device.

However, I am running into problems trying to implement simple AND gate logic. I have mapped two external switch inputs and one LED output, but if I press either one of the external switches the LED illuminates, so therefore the logic is behaving as an OR.

I have tried all the switch inputs and all LED outputs individually with no logic and the correct LED illuminates when the assigned switch is depressed.

So I am getting very confused as to why I would be seeing OR type logic when I have implemented an AND.

I have attached a screenshot of my VHDL code, so it can be confirmed that I have implemented the AND operation correctly.

Is there any further settings that I would need to be aware of in Quartus, that may explain the behavior I am experiencing?

I'm still no ruling out, that this could be related to the development board, however I would prefer some further feedback prior to trying to send it back to receive a refund.

Please advise.

<FPGA Dev Board>

Hdl
Platform

6 Replies

  • FvM's avatar
    FvM
    Icon for Super Contributor rankSuper Contributor
    3 years ago
    Hi,
    and turns into or when the signals are active low.
    • Danlar81's avatar
      Danlar81
      Icon for New Contributor rankNew Contributor
      3 years ago

      Hi FvM,

      Yes that is correct. When the switch is being depressed, it is grounding the the input, or active low.

      Does this mean I would need to introduce some further conditions so the AND logic responds to active low signals?

      • roeekalinsky's avatar
        roeekalinsky
        Icon for Contributor rankContributor
        3 years ago

        Hi @Danlar81 ,

        If the switches are active low but the LED is active high, then to achieve your intended "and" logic you need to just negate the switch values:

        (not sw1) and (not sw2)

        which is equivalent to:

        sw1 nor sw2

        And if the LED is also active low, which is likely (check your hardware!), then you also need another negation on the above result. Which would be:

        not ( (not sw1) and (not sw2) )

        which is equivalent to:

        not (sw1 nor sw2)

        which is equivalent to:

        sw1 or sw2

        You didn't say what the intended scope was of your online course, but it sounds like you're unfamiliar with the basic properties of boolean algebra? If that's the case, you have homework to do.

        https://en.wikipedia.org/wiki/Boolean_algebra

        A common pitfall with these kinds of online courses is the false promise of instant gratification -- introducing you to advanced topics without requiring that you first learn the necessary foundational knowledge, telling you that you can run without first learning to crawl. If it sounds too good to be true... Buyer beware.

  • ShengN_altera's avatar
    ShengN_altera
    Icon for Super Contributor rankSuper Contributor
    3 years ago

    Hi,

    I had tested the code in one of the device DE1-SOC Cyclone V and seems like the code working fine (check screenshot attached):

    11: Led illuminate

    10: Led no illuminate

    01: Led no illuminate

    00: Led no illuminate

    May be led of Cyclone IV EP4CE6E22 FPGA a bit different which is active low.

    Thanks,

    Best Regards,

    Sheng

    • Danlar81's avatar
      Danlar81
      Icon for New Contributor rankNew Contributor
      3 years ago

      Hi,

      Thank you for checking the code.

      I had to add inverters to all the inputs and outputs to get it to work they way I wanted.

      Appreciate you verifying.