Verliog - & and | operations backwards?

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Check to see if the LEDs are active low. Thats usually the problem

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Thank you for the reply. The problem that I am having is that when I use the "&" operator the truth table that I get is for an "or" gate and vice versa when I use the "|" operator. (also my LED is active when it has a high input)

One thing I noticed is that you are using posedge clk but you are using blocking assignment. Did you supply clock to your module?

You can try:

always @ (in_1, in_2)

begin

out_1 = (in_1 & in_2);

end

or if you have a clock supplied:

always @ (posedge clk)

begin

out_1 <= (in_1 & in_2);

end

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Thank you for the reply. The problem that I am having is that when I use the "&" operator the truth table that I get is for an "or" gate and vice versa when I use the "|" operator. (also my LED is active when it has a high input)

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Are you absolutely sure about that? Because from what you describe it seems that both your inputs and outputs are active low. In your example code, if you assume that in_1 and in_2 are normally '1' and they become '0' when you press the key, then your out_1 will be normally '1' and become '0' if one or both of the two keys is pressed. Therefore if the LED is also active low, it will light when one or both of the keys is pressed, implementing what looks like an 'or' function, even if you used the '&' operator.

Remember that an AND gate with all its inputs and outputs inverted is an OR gate (and vice versa)

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What is the dev. board you have?

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Thanks for the reply. The board I am using is this http://www.21eda.net/product_184.html (sorry its in chincese :/)

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One thing I noticed is that you are using posedge clk but you are using blocking assignment. Did you supply clock to your module?

You can try:

always @ (in_1, in_2)

begin

out_1 = (in_1 & in_2);

end

or if you have a clock supplied:

always @ (posedge clk)

begin

out_1 <= (in_1 & in_2);

end

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Thank you for you reply. I am using an on board clock running at 50Mhz.

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Are you absolutely sure about that? Because from what you describe it seems that both your inputs and outputs are active low. In your example code, if you assume that in_1 and in_2 are normally '1' and they become '0' when you press the key, then your out_1 will be normally '1' and become '0' if one or both of the two keys is pressed. Therefore if the LED is also active low, it will light when one or both of the keys is pressed, implementing what looks like an 'or' function, even if you used the '&' operator.

Remember that an AND gate with all its inputs and outputs inverted is an OR gate (and vice versa)

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I think you have found the cause. I looked back at the schematic for my circuit and realised that I had totally overlooked my LED. I thought it was using an NPN transistor to drive it but it was in fact a PNP transistor. This means that my circuit has two naturally high inputs and one naturally high output. Thus making an OR gate operation, I rewrote part of the code to this

out_1 <= ~in_1 & ~in_2;

Now my circuit acts like an NAND gate (better but not quite). Do you know a way in which I can now further improve this to be a AND gate operation? Many thanks for your help!

Found a solution to my own problem! I invert the inputs and then the output which gives a normal gate operation.

out_1 <= ~(~in_1 & ~in_2);

Many thanks for your help!

Which means the inputs are active low (e.g. if it is a push button it means you get a zero when the button is pressed), and the output is inverted (e.g. the LED is driven by a n-channel FET, NPN transistor, or connected with anode to VCC and cathode to the pin).

The boolean reduction of your ~(~a & ~b) is simply (a | b), so if all inputs and the output have inverted meaning then you need to do the OR function.