Driver
Formal Definition
A container for a projected output
waveform of a signal. The value of the signal is a function of the
current values of its drivers. Each process that assigns to a given
signal implicitly contains a driver for that signal. A signal
assignment statement affects only the associated driver(s).
Description
Each signal assignment statement defines a driver for each scalar
signal that is a target of this assignment. In case of signals of
complex type, each element has its own driver. Inside processes each
signal has only one driver, no matter how many assignment to it are specified.
When an assignment statement is executed, a new value is assigned to
the signal driver. The value of the signal is determined based on all
its drivers using the resolution function.
Examples
signal DataBus :
Std_Logic_Vector (7 downto
0) := "ZZZZZZZZ";
P1: process (A,B)
begin
. . .
DataBus
<= "11111111";
end process P1;
P2: process (A,B)
begin
. . .
DataBus
<= "00000000";
end process P2;
Signal DataBus is assigned values in two processes, therefore it will
have two drivers (one per each process). The assignments will result
in a change of the value of respective drivers, which will result in
assigning the "XXXXXXXX" value to the DataBus.
Important Notes
-
Drivers are not associated with signal declarations but with signal assignments.
-
If a signal has more than one driver in an architecture, it must be
of a resolved type.
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