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Carry Skip 4-Bit Blocks in Pass and DCVSL Logic

Carry Skip 4-Bit Blocks in Pass and DCVSL Logic. Michael Morgan Department of Electrical and Computer Engineering Microsystems Prototyping Laboratory Mississippi State University. Carry Skip. Poor Man’s Acceleration Method Pass transistor implementation adds 25 transistors

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Carry Skip 4-Bit Blocks in Pass and DCVSL Logic

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  1. Carry Skip 4-Bit Blocks in Pass and DCVSL Logic Michael Morgan Department of Electrical and Computer Engineering Microsystems Prototyping Laboratory Mississippi State University

  2. Carry Skip • Poor Man’s Acceleration Method • Pass transistor implementation adds 25 transistors • DCVSL implementation adds 40 transistors • Try to speed up worst case – propagate • 0 + 1 happens with 50% probability • The average length of the carry chain in a k-bit addition is log2(1.25k)1

  3. Carry Skip Block Diagram

  4. Pass Transistors Overview • Advantages • Low power • Good for Mux logic • Easily sized • Sizes decrease down a path • Disadvantages • No drive strength • Signals degrade due to channel resistance—must buffer • PMOS cannot pull down to Gnd • NMOS cannot pull up to Vdd • Good for Mux logic only Control Out In

  5. DCVSL Overview • Differential Cascade Voltage Switch Logic 2

  6. DCVSL Overview (continued) • Advantages • 2 PMOS per logic gate • Dual rail • Disadvantages • Dual rail • Sizing – incorrect sizing will cause functional failures • Power dissipated through crowbar current

  7. Pass/DCVSL Raw Data

  8. Results • DCVSL is faster • 200% for Tplh • 25% for Tphl • Why? Pass transistors have no drive • Pass transistors consume 29% less power • Why? No drive • Also, DCVSL has crowbar current

  9. Conclusions • Stick to CMOS! • Pass transistors may seem novel, but must buffer • I used full buffers • DCVSL must be sized correctly, or gates will not even function! • Also crowbar current can be power-hungry and slow

  10. References • [1] Dr. J. C. Harden “Basic Addition,” Slide 14, http://www.ece.msstate.edu/classes/ece8053/presentations/08f02-basadd.ppt. • [2] Dr. B. Reese “DCVSL”, Slide 1, http://www.ece.msstate.edu/~reese/EE8273/lectures/dcvsl/dcvsl_files/frame.htm.

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