Intel® 820E Chipset
R
Design Guide 69 During a 3.3 V AGP 2.0 operation, VREF must be 0.4 VDDQ. However, during a 1.5 V AGP 2.0 operation, VREF must be 0.5 VDDQ. This requires a flexible voltage divider for VREF. Various methods of accomplishing this exist, such as the example in the following figure. Figure 39. AGP 2.0 VREF Generation and Distribution
AGP
device

1.5-V AGP Card

VDDQ
GND
R9
300
1%
R11
200
1%
C10
0.1 µF
VDDQ
REF
GND

GMCH

C8
500 pF
REF
U6
MOSFET
R7
1 k
+12 V
TYPEDET#
VrefCG
VrefGC
VDDQ
Notes:
1. The resistor dividers should be placed near the GMCH. The AGP REF signal must be 5 mils wide and rout ed 10 mils from adjacent signal s.
2. R7 is the same resistor as R1 in the figure AGP VDDQ Gener ation Example Circuit.
R6
1 k
R5
82
R2
1 k
R4
82
C9
500 pF
(Note 2)
C8
500 pF
AGP
device

3.3-V AGP Card

VDDQ
GND
C10
0.1 uF
VDDQ
REF
GND

GMCH

R6
1 k
R2
1 k
R5
82
R4
82
REF
U6
MOSFET
+12 V
TYPEDET#
VrefCG
VrefGC
VDDQ
C9
500 pF
R9
300
1%
R7
1 k
(Note 2)
R11
200
1%
Notes:
1. The resistor dividers should be placed near the GMCH. The AGP REF signal must be 5 mils wide and rout ed 25 mils from adjacent signal s.
2. R7 is the same resistor as R1 in the figure AGP VDDQ Gener ation Example Circuit.
AGP2_Vref_gen-dist
Place C10 close to MCH.
Place C10 close to MCH.
The flexible VREF divider shown in the preceding figure uses an FET switch to switch between the locally generated VREF (for 3.3 V add-in cards) and the source-generated VREF (for 1.5 V add-in cards). Use of the source-generated VREF at the receiver is optional and is a product implementation issue beyond the scope of this document.