12-12-2012, 05:54 PM
[color=#FF0000]Introduction to CMOS VLSI Design[/color
Layout
Introduction to CMOS.ppt (Size: 1.81 MB / Downloads: 245)
Clever layout circumvents usual line of diffusion
Use wide transistors on critical path
Eliminate output inverters
Carry-Skip Adder
Carry-ripple is slow through all N stages
Carry-skip allows carry to skip over groups of n bits
Decision based on n-bit propagate signal
Carry-Select Adder
Trick for critical paths dependent on late input X
Precompute two possible outputs for X = 0, 1
Select proper output when X arrives
Carry-select adder precomputes n-bit sums
For both possible carries into n-bit group
Tree Adder
If lookahead is good, lookahead across lookahead!
Recursive lookahead gives O(log N) delay
Many variations on tree adders