Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
41
Table 17: Selecting Platform Mode of Operation
* The UAGP and UAGP8x cards will ground the TYPEDET pin on the connector, the AGP3.3V card will not. The 3.3V
motherboard will ignore it in any case.
Please note that the combinations that do not physically plug into the AGP slot are not listed above.
The detection of card type and mode of operation is resolved during power-up reset of the system.
Once the mode of operation is determined, the signaling scheme cannot be changed. When the
motherboard and card mutually decide to operate in AGP3.0 Mode, both set the AGP3.0_MODE in the
configuration register AGPSTAT [3]. This bit is ‘0’ for AGP mode of operation.
2.4.2.2 Speed Determination
AGP3.0 specification allows both 8x and 4x speeds of operation while using the AGP3.0 signaling
scheme. Note that the 4x mode using AGP3.0 signaling is not the same as the 4x mode when using
AGP 1.5V signaling. The latter mode obeys the AGP2.0 specification for signaling and protocol while the
former complies with the AGP3.0 specifications. Some key differences include:
• Strobes in 8x signaling are not differential and use only the rising edge as the active edge.
• Assertion levels for various signals have changed due to the termination scheme.
• Protocol changes such as removal of PIPE, High Priority reads, Read Long etc. also impact 4x
speed operation while in 8x signaling mode.
The speed selection is based on the signaling mode of operation. As described in the previous section,
the status of AGPSTAT[3] register is set by hardware to indicate the signaling mode. When this bit is
cleared, AGPSTAT[2:0] are used to specify the speeds supported as described in the AGP2.0
specification. When AGPSTAT[3] is set to pick the 8x signaling mode, AGPSTAT[2:0] takes on a
different meaning, described in Table 18.
Table 18: Supported Speed Description
AGPSTAT[3] AGPSTAT[2:0] Signaling Mode Speed Supported
0 xxx AGP2.0 See AGP2.0 Spec
1 001 AGP3.0 4x only
1 010 AGP3.0 8x only
1 011 AGP3.0 4x and 8x
1 1xx AGP3.0 Reserved
Motherboard Graphics
Card
MB_DET pin
on
motherboard
GC_DET pin
on Graphics
Card
TYPEDET pin
on Graphics
Card
Mode Speed of
Operation
AGP3.3V/UAGP/
UAGP3.0
AGP3.3V Don’t Care Open Open AGP3.3V 1x, 2x
AGP3.3V UAGP/UAGP3.0/
AGP3.3V
Don’t Care Open Don’t Care /
Ground *
AGP3.3V 1x, 2x
AGP1.5V/UAGP AGP1.5V/UAGP Open Open Ground AGP1.5V 1x,2x,4x
AGP1.5V/UAGP AGP3.0 Only Open Ground Ground Illegal Not supported
AGP1.5V/UAGP UAGP3.0 Open Ground Ground AGP1.5V 1x,2x,4x
AGP3.0 Only AGP1.5V/UAGP Ground Open Ground Illegal Not supported
UAGP3.0/U1.5V
AGP3.0
AGP1.5V/UAGP Ground Open Ground AGP1.5V 1x,2x,4x
AGP3.0/UAGP3.0/
U1.5VAGP3.0
AGP3.0 Only Ground Ground Ground AGP3.0 4x, 8x
AGP3.0/UA GP3.0/
U1.5VAGP3.0
UAGP3.0/
U1.5VAGP3.0
Ground Ground Ground AGP3.0 4x, 8x