Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification

Rev. 1.0

CLK

(@Device #1)

CLK

(@Device #2)

V_ih

T_skew

V_il

V_ih

Vtest

T_skew

Vtest

Figure 3-13: Clock Skew Diagram

/ NOTE

For systems that employ SSC (spread spectrum clocking), the additional clock skew from the different

tracking speeds of the component’s PLL must be accounted for in skew calculations.

3.5.3.3 External Pull-ups/Pull-downs

AGP3.0 signals do not require external resistor pull-ups or pull-downs on the motherboard, since the

internal terminations serve this purpose during normal operation. The AGP3.0 devices must ensure that

the electrical interface is in its de-asserted state during reset (see section 3.5.7). The asynchronous

signals (see section 3.4.2) may require pull-up resistors as dictated by a particular platform design.

Universal platforms that support AGP2.0 and earlier modes need to provide pull-ups and pull-downs as

required by the AGP2.0 spec. These sustainers must be integrated into the AGP target device to avoid

the stubs and vias associated with external resistor sustainers.

3.5.4 Motherboard Level Specification

3.5.4.1 Topology

The AC timings and electrical loading specified for the AGP3.0 interface are optimized for one host

component on the motherboard and one I/O component on an add-in adapter card. In the material that

follows, the AGP3.0-WS (workstation) topologies listed here consider symmetric stripline interconnect

only. AGP3.0-DT (desktop) topologies listed here consider microstrip interconnect only. The WS and

DT designations indicate the likely usage of these topologies, but are not meant to constrain them to

these system types. Further variations can be conceived based on anticipated cost considerations

associated with different platform implementations.

3.5.4.2 System Timing Budget

Timing budget issues for the entire channel have already been addressed in the previous sections.

Further details on requirements for the motherboard portion of the channel are addressed in what

follows: