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U. Glaeser

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FIGURE 31.9 Current-control feedback loop.<br />

FIGURE 31.10 Slew-rate control using resistors (a) and controlled pre-driver (b).<br />

the fastest operating condition. The slowest operating condition would cause excessive ISI. Early designs<br />

of drivers use devices that correlate inversely with transistor speed. In the example shown in Fig. 31.10(a),<br />

an output device can be broken into segments and each segment turns on sequentially [41]. The delay<br />

can be introduced using polysilicon resistors, which are not very sensitive to PVT. More recent methods<br />

(Fig. 31.10(b)) control the rate at which the pre-driver turns on the output device. By using a control<br />

voltage that tracks PVT, 11 the pre-driver resistance or current stays constant and consequently the slew-rate.<br />

Transmitter Pre-Emphasis<br />

When the data rate exceeds the channel bandwidth, designers compensate for the filtering by equalization.<br />

Because of the ease of implementation, many high-speed links equalize at the transmitter by pre-distorting<br />

the signal to emphasize higher frequencies [6,14,50]. Early pre-emphasis designs were known as advanced<br />

pull up/down (APU/D) [12], which were applied to driving large capacitances. The technique turns on<br />

11 The control voltage can be the voltage of a VCO whose frequency is locked to an external reference clock via a<br />

PLL [49].<br />

control<br />

register<br />

© 2002 by CRC Press LLC<br />

S 0<br />

S 1<br />

sig<br />

R o<br />

V s<br />

V ref =V sw/2<br />

df<br />

replica<br />

driver<br />

d[N:1]<br />

N binary sized<br />

devices<br />

d0<br />

d1<br />

df d0<br />

F<br />

d1<br />

w 2w<br />

F should give I max I drv at SS corner (S 0 =..=S N =1)<br />

output<br />

voltage<br />

pre-driver<br />

V bias<br />

Setting the control register<br />

time<br />

d o<br />

U/D<br />

cnt<br />

FSM<br />

control<br />

register<br />

LoadEn<br />

ck ref<br />

ph comp<br />

+ filter<br />

ctrl<br />

to real<br />

buffers<br />

pre-driver<br />

(a) (b)<br />

PLL<br />

VCO<br />

R o<br />

V s

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