CMOS Adders for the Simplified MIPS Processor

Specifications Needs to be fast: well under 1ns Needs to fit width of bitslice: 80λ Needs to be a reasonable length: <1500 λ Bitslice layout ~2000λ Total core area 3500λ x 3500λ

Exploring the Options Static CMOS? –Easy to design and layout –Not very fast though Dynamic CMOS? –Not easy to design –Fast Dynamic CMOS Manchester Carry Chain

Static CMOS Options Ripple Carry Adder –Simple to design and layout –Small footprint –Slow: Cout must propagate through all bits Carry Lookahead Adder –More complex design and carry logic –Significantly larger footprint –Faster than ripple carry

Static CMOS Options cont. Carry Skip Adder –Faster than ripple carry adder –Slower than carry lookahead adder –Smaller footprint than carry lookahead adder –Larger footprint than ripple carry addder 16-bit carry skip adder with 4-bit carry lookahead groups

Static CMOS Ripple Carry Adders Zhuang Full Adder –Fast: Transmission gates as MUXs –Low transistor count (22) leads to small layout –Already have it laid out, tested and speced

Static CMOS Ripple Carry Adders Full Adder structure –Higher transistor count (28) –Larger transistors (8x for some pMOS!) –Carry out is no longer critical path

Static CMOS Ripple Carry Adders Delay with no parasitics modeled Worst case determined to be Cin S Cout = 1 B set to Cin then set to 1 Time from when Cin at 50% until Cout a 50% –1.035ns(!!!)

Zhuang Full Adder Same test case as before Delay found to be.662ns Significanly faster than other static CMOS implementation

Zhuang Full Adder Simulated with parasitics added Same test pattern applied Delay now found to be.970ns Still faster than other adder without parasitics Use Zhuang full adder

Zhuang Full Adder Layout Had Zhuang layout from before Fit nicely in bitslice in IP library –80λ width –Exports at correct places –Perfect! Zhuang Full Adder layout completed

Zhuang Full Adder Simulation Exported layout to SPICE Same test case as used before Delay is now 1.23ns Completed ALU ready to be placed in bitslice ~550 λ

Adders Wrap Up Total bitslice size ~2500λ Well under 3500λ Distorted (squished horizontally, too long to display) view of completed bitslice.

Adder Wrap Up Not fast enough though –Dynamic CMOS? –Static carry lookahead use more space?