Intel VC820 Design Guide - Page 63
VDDQ Generation and TYPEDET#, Ground Reference
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Figure 2-30. Top Signal Layer Layout/Routing Guidelines 2.7.7 Must add six 0.01 uF ceramic 603 Type Capacitors Ground Reference It is strongly recommended that, at a minimum, the following critical signals be referenced to ground from the MCH to an AGP connector (or to an AGP video controller if implemented as a "down" solution) utilizing a minimum number of vias on each net; AD_STB0, AD_STB0#, AD_STB1, AD_STB1#, SB_STB, SB_STB#, G_GTRY#, G_IRDY#, G_GNT# and ST[2:0]. In addition to the minimum signal set listed above, it is strongly recommended that half of all your AGP signals be reference to ground depending on board layout. An ideal design would have the complete AGP interface signal field referenced to ground. The recommendations above are not specific to any particular PCB stackup, but are applied to all Intel® Chipset designs. VDDQ Generation and TYPEDET# AGP specifies two separate power planes (VCC and VDDQ). VCC is the core power for the graphics controller. VCC is always 3.3V. VDDQ is the interface voltage. In AGP 1.0 implementations VDDQ was also 3.3V. For the designer developing an AGP 1.0 motherboard, there is no distinction between VCC and VDDQ as both are tied to the 3.3V power plane on the motherboard. Intel®820 Chipset Design Guide 2-37