Intel® 820E Chipset
R
70 Design Guide
2.8.9. 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 inch) trace.
2.8.10. AGP Pull-Ups
AGP control signals require pull-up resistors to VDDQ on the motherboard, to ensure that they maintain
stable values when no agent is actively driving the bus. The signals requiring pull-up resistors are:
• 1× timing domain signals
FRAME#
TRDY#
IRDY#
DEVSEL#
STOP#
SERR#
PERR#
RBF#
PIPE#
REQ#
WBF#
GNT#
ST[2:0]
It is critical that these signals be pulled up to VDDQ (not 3.3 V).
The trace stub to the pull-up resistor on 1× timing domain signals should be kept at 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 that they maintain
stable values when no agent is driving the bus.
Note: INTA# and INTB# should be pulled to 3.3 V, not VDDQ.
• 2×/4× 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 2×/4× 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 following table.
RMIN R
MAX
4 kΩ 16 kΩ
The recommended AGP pull-up/pull-down resistor value is 8.2 kΩ.