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160
Intel
82865G/82865GV GMCH Datasheet
Functional Description
5.3.3.1
Mechanism for Detecting AGP 2.0, AGP 3.0, or
Intel
DVO
Two new signals are provided in the AGP 3.0 specification to allow for detection of an AGP 3.0
capable graphics card by the motherboard and an AGP 3.0 capable motherboard by the graphics
card respectively.
The signals are:
GC_DET#: Pulled low by an AGP 3.0 graphics card; left floating by an AGP 2.0 graphics
card.
MB_DET#: Pulled low by an AGP 3.0 motherboard; left floating by an AGP 2.0 motherboard.
The 3.0 capable motherboard uses GC_DET# to determine whether to generate VREF of 0.75 V
(floating GC_DET# for 2.0 graphics card), or 0.35 V (GC_DET# low) to the graphics card. This is
sent to the graphics card via the VREFCG pin on the AGP connector.
Similarly, the 3.0 capable graphics card uses MB_DET# to determine whether to generate VREF of
0.75 V (floating MB_DET# on 2.0 motherboard), or 0.35 V (MB_DET# low) to the motherboard.
The card could also use this pin as a strap to determine 2.0 or 3.0 mode. Note, however, that
VREFGC is not used by the GMCH. Instead, VREFCG is used to account for the DVO ADD card,
where VREFGC is not connected.
The GMCH detects whether the graphics card connected is AGP 2.0 or AGP 3.0 via the voltage
level driven into the GVREF pin (0.35 V{< 0.55 V} = AGP 3.0; 0.75 V{>0.55 V} = AGP 2.0).
GVREF is driven by VREFCG on the motherboard.
An ADD card pulls the GPAR/ADD_DETECT# pin low and leaves GC_DET# pin unconnected.
Since GC_DET# is unconnected, the VREF generator generates an AGP 2.0 compliant voltage,
0.75 V to the GVREF pin. This is detected by the VREF voltage comparator, which drives a “l(fā)ive”
AGP 3.0 mode detect signal to GPAR (and other AGP I/O buffers), as well as the pull-up/pull-
Table 28. AGP 3.0 Downshift Mode Parameters
Parameter
AGP 2.0 Signaling
(All Data Rates)
AGP 3.0 Signaling
(4X Data Rate)
AGP 3.0 Signaling
(8X Data Rate)
Data rate
1X, 4X
4X
8X
VREF level
0.75 V
0.35 V
0.35 V
Signaling
2.0 (1.5 V)
3.0 signaling
(0.8 V swing)
3.0 signaling
(0.8 V swing)
Polarity of GREQ, GGNT,
GDEVSEL, GFRAME, GIRDT,
GTRDY, GSTOP, RBF, WBF
Active low
Active high
Active high
Polarity of SBA
normal (111 = idle)
inverted (000 = idle)
inverted (000 = idle)
GCBE polarity
GC/BE#
GC#/BE
GC#/BE
Strobe definition
Strobe
Strobe#
StrobeFirst
StrobeSecond
StrobeFirst
StrobeSecond
DBI used
No
Disabled on xmit
Yes
PIPE# allowed
Yes
No
No
Commands supported
AGP 2.0 commands
AGP 3.0 commands
AGP 3.0 commands
Isoch supported
No
(Not supported)
No
Calibration cycles included
No
Yes
Yes