In many digital sytems, buses are used to connect a number of output drivers to a common signal. For example, if open-collector or open-drain output drivers are used with a pull-up load on a signal, the signal can be pulled low by any driver, and is only pulled high by the load when all drivers are off. This is called a wired-or or wired-and connection. On the other hand, if tri-state drivers are used, at most one driver may be active at a time, and it determines the signal value.
VHDL normally allows only one driver for a signal. (Recall that a driver is defined by the signal assignments in a process.) In order to model signals with multiple drivers, VHDL uses the notion of resolved types for signals. A resolved type includes in its definition a resolution function, which takes the values of all the drivers contributing to a signal, and combines them to determine the final signal value.
A resolved type for a signal is declared using the syntax for a subtype:
subtype_indication ::= [ resolution_function_name ] type_mark [ constraint ]
The resolution function name is the name of a function previously defined. The function must take a parameter which is an unconstrained array of values of the signal subtype, and must return a result of that subtype. To illustrate, consider the declarations:
type logic_level is (L, Z, H);
type logic_array is array (integer range <>)
of logic_level;
function resolve_logic (drivers : in logic_array) return logic_level;
subtype resolved_level is resolve_logic logic_level;
In this example, the type logic_level represents three possible states for a digital signal: low (L), high-impedance (Z) and high (H). The subtype resolved_level can be used to declare a resolved signal of this type. The resolution function might be implemented as shown in Figure 6-1.
This function iterates over the array of drivers, and if any is found to have the value L, the function returns L. Otherwise the function returns H, since all drivers are either Z or H. This models a wired-or signal with a pull-up. Note that in some cases a resolution function may be called with an empty array as the parameter, and should handle that case appropriately. The example above handles it by returning the value H, the pulled-up value.
function resolve_logic (drivers : in logic_array) return logic_level;
begin
for index in drivers'range
loop
if
drivers(index) = L then
return
L;
end
if;
end loop;
return H;
end resolve_logic;
Figure 7-1. Resolution function for three-state logic