DVFS Example

Example: core area 64 mm²; average net length 0.1 mm; 400K gates/mm², a=0.25.

Net capacitance = 0.1 mm × 1 fF/mm × 400K × 64 mm² = 2.5 nF.

Supply Voltage	Clock Freq	Static Power	Dynamic Power	Total Power
(V)	(MHz)	(mW)	(mW)	(mW)
0.8	100	40	24	64
1.35	100	67	68	135
1.35	200	67	136	204
1.8	100	90	121	211
1.8	200	90	243	333
1.8	400	90	486	576

Workstation microprocessors dissipate tens of Watts: hence cooling fans.

The table shows example power consumption for a circuit when clocked at different frequencies and voltages. The important thing to ensure is that the supply voltage must be sufficient for the clock frequency in use: too low a voltage means that signals do not arrive at D-type inputs in time to meet set up times.

Power consumption versus frequency is worse than linear: it goes with a power law.

In the past, chips were often core-bound or pad-bound. Pad-bound meant that the chip had too many I/O signals for its core logic area: the number of I/O's puts a lower bound on the perimeter of the chip.

Today's VLSI technology allows I/O pads in the middle of the chip and designs are commonly power-bound.