11.3.1.1 Example 1

In the following VHDL example, the signal “critical” goes through three logic levels.

Figure 11-20. Logic Levels on Critical Paths - Three Logic Levels

if ((( Crtical='0' and Obi='1' and Sar='1')
or CpuG='0') and CpuR='0') then
Des <= Adr;
elsif (((Crtical='0' and Obi='1' and Sar='1')
or CpuG='0') and CpuR='1') then
Des <= Bdr;
elsif (Sar='0' and ..........

The signal “critical” is a late arriving signal. To reduce the logic level usage on “critical”, imply priority by using an if-then-else statement. As a result, the signal “critical” goes through one logic level, as shown below.

Figure 11-21. Logic Levels on Critical Paths - One Logic Level

if (Critical='0') then
if (((Obi='1' and Sar='1')
or CpuG='0') and CpuR='0') then
Des <= Adr;
elsif (((Obi='1' and Sar='1')
or CpuG='0' and CpuR='1') then
Des <= Bdr;
end if;
end if;