Synthesis 243
In the preceding technology library, the intrinsic delays are given in the
cell description. The loading delay is calculated based on the load applied
to the output pin o1and the resistance values in the cell description. The
value calculated for the wire delay depends on the die size selected by the
user. Selecting a wire model scales the delay values. Finally, the input
slope delay is calculated by the size of the driver, in this example, A1, and
the capacitance of the gate being driven. The capacitance of the gate
being driven is in the technology library description.
Technology libraries can also contain data about how to scale delay
information with respect to process parameters and operating conditions.
Operating conditions are the device operating temperature and power
supply voltage applied to the device.Synthesis
To convert the RTL description to gates, three steps typically occur. First,
the RTL description is translated to an unoptimized boolean description
usually consisting of primitive gates such as ANDand ORgates, flip-flops,
and latches. This is a functionally correct but completely unoptimized
description. Next, boolean optimization algorithms are executed on this
boolean equivalent description to produce an optimized boolean equivalent
description. Finally, this optimized boolean equivalent description is
mapped to actual logic gates by making use of a technology library of the
target process. This is shown in Figure 9-8.Translation
The translation from RTL description to boolean equivalent description
is usually not user controllable. The intermediate form that is generated
is usually a format that is optimized for a particular tool and may not
even be viewable by the user.
All IF,CASE, and LOOPstatements, conditional signal assignments, and
selected signal assignment statements are converted to their boolean
equivalent in this intermediate form. Flip-flops and latches can either be
instantiated or inferred; both cases produce the same flip-flop or latch
entry in the intermediate description.