Accelerated Graphics Port Interface Specification
AGP3.0 Interface Specification
Rev. 1.0
64
pull-up device against a standard load device of 50 Ω. The method of setting the driver pull-up to the
proper resistance value is implementation specific and is not covered in this specification.
V
DDQ
R
ON_N
= Z
pd
V
OH
= 0.80V
V
OL
= V
SS
= 0.00V
Driving Device
R
TT
= Z
pd
Receiving Device
V
SS
V
SS
R
ON_P
= Z
pd
(V
DD
/V
OH
- 1)
T-line impedance = Z
0
ref
V
Figure 3-9: Determination of Device RON Values
3.4.1.2 Vref Generation
As opposed to AGP2.0, Vref for AGP3.0 is highly recommended to be locally generated. This includes
both the motherboard and graphics card. The Vref signal should be implemented with 1% tolerance
resistance values from V
DDQ
, and must be maintained within tolerance while sinking as much as 50 µA
of leakage current. Vref is 350 mV for a nominal V
DDQ
and varies proportionally to the actual value of
V
DDQ
. The generated Vref needs to be properly decoupled to ground at the receiver to manage switching
currents; decoupling elements must be as close to the receiver’s interface as possible. There must be
less than ±15 mV of coupling to the Vref signals (at the receiver pin) from adjacent signal lines. Specific
decoupling methods are platform dependent, and therefore, not specified. For more information on
AGP3.0 Vref generation see the AGP3.0 Design Guide.
AGP3.0 does not define a new set of connector pins for passing along the AGP3.0 Vref data. Instead,
the Vrefcg connector pin is supplied by the motherboard (or core-logic) to provide AGP2.0 Vref or
AGP3.0 Vref to the graphics card based on the configuration detected. Likewise, the Vrefgc pin is
supplied by the Graphics Card to provide AGP2.0 or AGP3.0 Vref to the motherboard based on the
configuration detected. The connector signals TYPEDET#, GC_DET#, and MB_DET# are used to
establish the channel’s configuration. The rules governing configuration determination are described in
Table 17.
3.4.1.3 Driving the Interface to a High Level
When the AGP3.0 interconnect is being driven to a high value the pull-up of the driver is enabled while its
pull-down is disabled (see Figure 3-10).