User's Manual

Intel
®
820 Chipset Design Guide 2-41
Layout/Routing Guidelines
2.7.9 Compensation
The MCH AGP interface supports resistive buffer compensation (RCOMP). Tie the GRCOMP pin
to a 40 Ω 2% (or 39 Ω 1%) pull-down resistor (to ground) via a 10 mil wide, very short (<0.5”)
trace.
2.7.10 AGP Pull-ups
AGP control signals require pull-up resistors to VDDQ on the motherboard to ensure they contain
stable values when no agent is actively driving the bus. The signals requiring pull-up resistors are:
•
1X Timing Domain Signals
—FRAME#
— TRDY#
—IRDY#
—DEVSEL#
—STOP#
—SERR#
—PERR#
—RBF#
—PIPE#
—REQ#
—WBF#
— GNT#
— ST[2:0]
It is critical that these signals are pulled up to VDDQ (NOT 3.3V).
The trace stub to the pull-up resistor on 1X timing domain signals should be kept to less than
0.5 inch to avoid signal reflections from the stub.
The strobe signals require pull-up/pull-downs on the motherboard to ensure they contain stable
values when no agent is driving the bus.
Note: INTA# and INTB# should be pulled to 3.3V – not VDDQ.
•
2X/4X Timing Domain Signals
— AD_STB[1:0] (pull-up to VDDQ)
— SB_STB (pull-up to VDDQ)
— AD_STB[1:0]# (pull-down to ground)
— SB_STB# (pull-down to ground)
The trace stub to the pull-up/pull-down resistor on 2X/4X timing domain signals should be
kept to less than 0.1 inch to avoid signal reflections from the stub.
The pull-up/pull-down resistor value requirements are shown in the table below:
The recommended AGP pull-up/pull-down resistor value is 8.2 KΩ.
Rmin Rmax
4 KΩ 16 KΩ