Product specifications

Design Guide - VT82C694X Apollo Pro133 with VT82C686A
Preliminary Revision 0.5, November 19, 1999 36 Motherboard Design Guidelines
Technologies, Inc.
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2.3.2.6 AGP Clock Signals
Layout recommendations for the AGP clock are shown in Figure 2-28. Typically, 22 ohm series terminations are recommended
for the AGP clock. A typical 22 pF bypass capacitor is also required for the AGP clock (GCLKO) to the AGP slot. Depending on
how the system is designed, the value of the bypass capacitors for the PCI clocks may vary. For clock alignment considerations, 3
more inches than that of the AGP clock trace to the AGP slot are added to the feedback AGP clock trace length. (Note: the 3
inches represents the estimated distance from the GCLK pin of a AGP slot to the GCLK pin of the VGA chip on an AGP video
card.) Some layout guidelines for the AGP clock are listed below:
• The trace length of the two separate GCLKO signals from pin N4 of the VT82C694X chip to the damping resistors
should be equal and less than 1 inch.
• Both C1 and C2, which should be placed very close to the VT82C694X chip and the AGP slot respectively, are used to
control the AGP clock alignment. (See Figure 2-28).
VT82C694X
(North Bridge)
AGP Slot
GCLKO
GCLK
0 ~ 33
ohm
C2
0 ~ 33 pF
L
GOUT
L
GOUT
+ 3"
GCLK
C1
0 ~ 33 pF
N4
N5
(as short as possible)
(near the slot)
(near the chip)
B7
Figure 2-28. AGP Clock Layout Recommendations