constants true and false

[ybakos]

One of my students has asked me a hardware question I don't know the answer to. The question is, "how do the hdl constants true and false actually get implemented in typical hardware?"

I understand that we can always ask the ALU for a 0 or a 1, or that we can take physically take any input pin and not wire it to anything (resulting in 0 for input). But is this what is actually done in the "real world" of logic circuit implementations?

For example, how would the following hdl be physically implemented in hardware?

And(a=false,b=true,out=someInternalPin);

Thanks for any hardware nerds who can shed some light on this.

[Warren Toomey]

For true, you wire the pin to the voltage that represents true, usually +5V for TTL. For false, you wire the pin to 0V, which is usually ground or the reference voltage. Usually the power supply has enough "fan out" to ensure that the high voltage is always close enough to true!

Warren

[marz]

what does the term "fan out" mean?

[cadet1620]

"Fan-out" means how many inputs can be connected to an output.  For example, typical TTL devices have a fan-out of 20, so you can theoretically connect up to 20 inputs to a single output and still have sufficient voltage and current for the circuit to work.  Typically, however, if you try to push this limit you run into other physical problems like capacitive loading and noise caused by long PC board traces.

--Mark