U. Glaeser

FIGURE 2.50 Diffusion-area sharing for better layout.
FIGURE 2.51 Comparison of pass-transistor circuit and CMOS circuit for 2-input NAND/NOR logic.
CMOS circuit provides better performance in terms of area and delay. However, a PTL circuit still provides
lower power consumption because of its pMOS-free structure, which enables small capacitance.
Pass-transistor circuits are more suitable for implementing logic functions in which some signals are
selected by other signals. In contrast, CMOS circuits are more suitable for implementing NAND/NOR
logic (or AND/OR logic). Thus, PTL and CMOS mixed structures, in which logic corresponding to a
selector is implemented with PTL circuits and other logic is implemented with CMOS circuits, are
attractive [33,34]. In this section, such mixed-logic circuits called pass-transistor and CMOS collaborated
logic (PCCL) are described.
The key to PCCL is finding CMOS-beneficial parts and selector-beneficial parts in the logic functions.
To accomplish this, the BDD-based method shown in Fig. 2.52(a) is used, in which first an entire PTL
circuit is constructed from a multilevel BDD or decomposed BDD, and then some parts are replaced
with CMOS circuits. The key to this procedure is to find CMOS-beneficial functions based on the BDD.
This is accomplished as follows: those selectors with one of two inputs fixed to V dd or Gnd operate as
AND or OR logic (NAND or NOR logic) rather than as a selector, so they are good candidates to be replaced
with CMOS circuits, as shown in Fig. 2.52(b). Using this method, logic functions can be categorized into
pro-selector functions and pro-AND/OR functions.
Figure 2.53 shows a detailed example of the PCCL synthesis flow. For the logic function shown in
Fig. 2.53(a), the multilevel BDD shown in Fig. 2.53(b) is constructed. The PTL shown in Fig. 2.53(c) is
then obtained from the BDD. Then, in the synthesized PTL, selectors in which one of the two data inputs
is fixed to V dd or Gnd are searched for. As described before, however, because of the low-power characteristics
of the pass-transistor circuits, the power consumption will increase if all these pass-transistor
© 2002 by CRC Press LLC
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