About the values in VHDL std_logic_vector

[parag_paul]

The std logic package has some values defined as follows,

U unitialized
X forcing unknown
1 forcing 1
0 forcing 0
L weak 0
H Weak 1

Now, does a assign like
a:= 1
mean that it has been forced a value of 1. Is that correct , Does it
mean it will hold , until another, assign happens.

 

[Andy]

The std logic package has some values defined as follows,

U unitialized
X forcing unknown
1 forcing 1
0 forcing 0
L weak 0
H Weak 1

Now, does a assign like
a:= 1
mean that it has been forced a value of 1. Is that correct , Does it
mean it will hold , until another, assign happens.

From your syntax, "a" must be an integer type variable. 1 is a number

(integer). '1' is logic high in std_logic system.

VHDL variables operate just like variables in other languages;
assignments to them are in force until another assignment to the same
variable is executed.

If on the other hand, you meant to say:

a <= '1';

then we have a signal assignment.

If the assignment appears outside of a process or subprogram, then it
is a concurrent assignment, and creates a driver, with a constant
value of '1', continually driving your signal.

If the assignment appears inside a process, then it is a sequential
assignment. Only one driver is created for that process, regardless of
how many assignments are made within the process. Only the last
executed assignment prior to the process suspending is used. Updates
take effect when the process suspends (i.e. waiting for another event,
etc.).

If multiple processes and/or concurrent assignment statements drive
the same signal, the effective value for the signal must be resolved
from all of the drivers. There are no "forcing" values in the
std_logic system: drivers with '1' and '0' on the same signal will
result in an 'X', etc.

Andy

 

[parag_paul]

Is there a forcing value system in VHDL itself. I mean something like
force and release in verilog.

I was confused with the definitions of 1 and 0 and though , H and L
are the appropriate ones.

Other than the std_logic what are the other value systems available in
VHDL

 

[Jonathan Bromley]

Is there a forcing value system in VHDL itself. I mean something like
force and release in verilog.

Not in the language, no. Simulators usually provide the
force functionality.

It would be possible to invent a new resolution function for
std_ulogic (or any other data type) providing a "forcing"
value, but you need to be aware that simulators are highly
optimized for the standard implementation of std_logic and
any change to it would drastically worsen simulator performance.
Synthesis tools generally do not support any kind of non-standard
resolution function.

I was confused with the definitions of 1 and 0 and though , H and L
are the appropriate ones.

For what?

Other than the std_logic what are the other value systems available in
VHDL

Any that you care to write. There are built-in data types for
boolean (FALSE, TRUE) and bit ('0', '1'). Others you would need
to write for yourself.
--
Jonathan Bromley, Consultant

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