15.01.2013 Views

U. Glaeser

U. Glaeser

U. Glaeser

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

FIGURE 21.1<br />

1<br />

21.2 Overview of Clock-Powered Logic<br />

The overall organization for a clock-powered microsystem is shown in Fig. 21.1. Adiabatic charging<br />

requires a time-varying voltage signal as a source of ac power. Rather than introduce a new power supply,<br />

this power source can be naturally supplied in a synchronous digital system through the clock rails.<br />

Depending on the implementation, the E-R clock driver may or may not be on the same chip as the<br />

clock-powered logic. The clock phases that are generated by the clock driver synchronize the operation<br />

of the clock-powered logic as well as power the large-capacitance nodes through special latches, called<br />

E-R latches. E-R latches operate in synchrony with the clock phases, so their placement effects the timing<br />

and partitioning of logic functions into logic blocks. Placement of the E-R latches is determined not only<br />

by the location of the large capacitance nodes, but also by system-level factors such as circuit latencies<br />

and overall timing, e.g., pipelining.<br />

Data representation is different between clock-powered and dc-powered signals. Nodes that are<br />

dc-powered are logically valid when their voltage levels are sufficiently close to the voltages supplied by<br />

the power rails, i.e.,<br />

© 2002 by CRC Press LLC<br />

Clock<br />

Driver<br />

Abstract block diagram for a clock-powered microsystem.<br />

V<br />

dd<br />

. . .<br />

E-R<br />

Latch<br />

Logic E-R<br />

Latch<br />

Logic E-R<br />

Latch<br />

CMOS Chip<br />

. . .<br />

Logic<br />

Clock-Powered Nodes<br />

and GND. Clock-powered signals are valid only when the clock phase is valid.<br />

The presence of a pulse that is coincident with a clock phase defines a logic value of one. The absence of a<br />

pulse defines a logic value of zero. When the clock phase is zero, the logical value of the clock-powered<br />

signal is undefined.<br />

The co-existence of clock-powered and dc-powered nodes necessitates signal conversion from pulses<br />

to levels and vice versa. Levels are converted to pulses in the E-R latches, which receive dc-powered<br />

signals as inputs and pass clock pulses to the output. As discussed in detail later in this chapter, depending<br />

on the style of the logic blocks, either pulses are implicitly converted to levels, or, special pulse-to-level<br />

converters must be introduced between the E-R latches and the logic blocks.<br />

The total average energy dissipation per cycle, Etot,<br />

of clock-powered microsystems consists of two<br />

terms and is given by:<br />

E tot<br />

∼<br />

∑<br />

i<br />

a i<br />

RiC i 2<br />

---------C<br />

T<br />

iVϕ s<br />

2<br />

ajC jVdd (21.4)<br />

The first term models the clock-powered nodes that are adiabatically switched for Ts<br />

>> RiCi.<br />

The second<br />

term models the dc-powered nodes that are conventionally switched. In Eq. (21.4), ai<br />

and aj<br />

denote the<br />

switching activity for clock- and dc-powered nodes, respectively; Ci<br />

and Cj<br />

denote the capacitance of<br />

clock- and dc-powered nodes, respectively; Ri<br />

is the effective resistance of the charge-transfer path between<br />

the clock driver and the clock-powered nodes; Ts<br />

is the transition time of the clock signal; Vϕ<br />

is the clock<br />

voltage swing; and Vdd<br />

the dc supply voltage.<br />

The benefit of applying clock-powered logic can be readily evaluated from Eq. (21.4). Capacitance<br />

information can be extracted from layout, assuming the various parasitic and device capacitances have<br />

been accurately characterized. Activity data for the different nodes can be determined for specified input<br />

data sets from switch-level and circuit-level simulation. As shown later, in clock-powered microprocessors,<br />

1<br />

Portions in sections 21.2, 21.4, 21.6 and subsection “Static Logic” reprinted, with permission, from [23] © 1999<br />

IEEE.<br />

+<br />

∑<br />

j

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!