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U. Glaeser

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clock-powered nodes accounted for 80% of the total dissipation when energy-recovery was disabled.<br />

Table 21.2 summarizes the characteristics for both AC-1 and MD1.<br />

Comparison Study<br />

To compare the effectiveness of clock-powered logic against conventional CMOS, an equivalent fully<br />

dissipative microprocessor, DC1, was implemented. DC1 shares the same instruction set with AC-1,<br />

i.e., it does not include the MDX instructions. DC1 was implemented following the MD1 design flow<br />

for the same 0.5 µm CMOS process. All the three metal layers were available for routing. DC1 uses the<br />

same pipeline timing with MD1. Circuit-wise, DC1 is based on static CMOS as is MD1. The main<br />

difference compared to MD1 is that clocked buffers were replaced with regular drivers and latches were<br />

replaced with sense-amp, edge-triggered flip-flops [31]. DC1 uses a single-phase clock, which was distributed<br />

automatically by the place and route CAD tool following an H-tree. Clock is gated away from<br />

unused blocks. The DC1 core size is 1.8 mm × 1.9 mm. The core contains 21 k transistors.<br />

The three processor cores were compared through PowerMill simulations since DC1 was not fabricated.<br />

All three SPICE netlists were extracted from physical layout using the same CAD tool and extraction<br />

rules. It was not possible to simulate the clock-powered processors operating in energy-recovery (i.e.,<br />

resonant) mode due to limitations of the simulation software. Instead both of them were simulated<br />

operating in conventional mode. For AC-1, the conventional clock driver was used to generate the two<br />

© 2002 by CRC Press LLC<br />

TABLE 21.2 AC-1 versus MD1 Summary<br />

AC-1 MD1<br />

ISA 16-bit RISC 16-bit RISC plus MDX instr.<br />

Word width 16 bits 16 bits<br />

Pipeline structure 5 Stages 5 Stages<br />

Logic style Dynamic Static<br />

Pipeline style As shown in Fig. 21.13(a) As shown Fig. 21.15(a)<br />

Transistor count 12,700 28,000<br />

Cell design Custom Custom<br />

Layout method Custom Synthesized<br />

Clock-power nodes 10% 5%<br />

Power accounted to clock-powered<br />

nodes at no energy recovery<br />

90% 80%<br />

Resonant clock driver FETs position On-chip Off-chip<br />

Conventional clock driver Yes No<br />

EN<br />

Din EN<br />

Din (a)<br />

(b)<br />

FIGURE 21.25 Clocked buffers with conditionally enabled outputs; when disabled, output is either clamped to<br />

ground (a) or is at high impedance (b) ([29] © 2000 IEEE).<br />

V iso<br />

V iso<br />

ϕ D<br />

ϕ D<br />

D out<br />

D out<br />

C L<br />

C L

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