Basic doubt about signals in VHDL

[InfiniteLoop]

Hey

I start learning VHDL with a Xilinx Spartan 3E SK and bassically my question is that if I assign a '0' to a std_logic signal it will be connected to ground? If not, how can I connect a signal to ground or simulate a virtual ground?

Thanks everybody, and sorry for my poor english... I understand but not know how to speak/write english very well

 

[shaiko]

If this signal drives logic - then the pin of that logic will be connected to ground.

to simulate:
x <= '0' ;

 

[aruipksni]

Hey

I start learning VHDL with a Xilinx Spartan 3E SK and bassically my question is that if I assign a '0' to a std_logic signal it will be connected to ground? If not, how can I connect a signal to ground or simulate a virtual ground?

Thanks everybody, and sorry for my poor english... I understand but not know how to speak/write english very well

1/ it won't be connected to ground.
2/ virtuall ground mabee but only ... a/ if you connect it to output pin and drive it to '0'
or
b/ put the output it to 3-state , and use internal pull-down - but this will only work for small currents, and can gepeordise your fpga.
3/ why do you want to ground a signal or use it as ground ?
4/ only thing that comming to my mind is that you want to activate some external device that need a GND connection , like a horn, or electric lock.
in this case you will need to connect your output pin to a relay device, that will be connected to GND.

 

[shaiko]

aruipksni,

A signal that's '0' is essentially connected to ground (switched to ground by the logic device's transistors)...

 

[TrickyDicky]

Not '0' is logic '0', not ground. So, for example, it would not be recommended to connect the ground pin of a chip to an FPGA pin held at '0', as the level may not match the level of ground on the ground plane on a board.

 

[K-J]

Hey

I start learning VHDL with a Xilinx Spartan 3E SK and bassically my question is that if I assign a '0' to a std_logic signal it will be connected to ground? If not, how can I connect a signal to ground or simulate a virtual ground?

If you're just trying to simulate, then yes the statement "a <= '0';" will simulate exactly the same as if 'a' was connected to ground.

If you're talking about synthesizing a design that you will download to a device then there is nothing that will connect a signal to ground. Instead what happens is that the logic that gets implemented will be optomized. A simple example:

d <= c or b or a;
a <= '0';

What gets synthesized then will be

d <= c or b;

There will be no signal 'a' implemented because the effects of 'a' have been incorporated into the logic of any signal that 'a' affected.

Kevin Jennings