24888 Rev 3.03 - July 12, 2004 AMD-8151
TM
AGP Tunnel Data Sheet
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In summary, Stop Grant broadcasts with SMAF fields specified by DevA:0xF0[ICGSMAF] enable the clock
gating window and STPCLK deassertion broadcasts disable the window. If LDTSTOP# is asserted while the
clock gating window is enabled, then clock gating occurs.
Also, DevA:0xF0[ECGSMAF] may be used in a similar way to disable A_PCLK and the internal clock grids
associated with the AGP bridge. The same rules for the clock gating window that apply to DevA:0xF0[ICGS-
MAF] also apply to DevA:0xF0[ECGSMAF]. If clock gating is enabled, then A_PCLK is forced low within
two clock periods after LDTSTOP# is asserted. It becomes active again within two clock periods after LDT-
STOP# is deasserted. It is required that there be no AGP-card-initiated upstream or downstream traffic while
A_PCLK is gated. In addition, it is required that there be no host accesses to the bridge or internal registers in
progress from the time that LDTSTOP# is asserted for clock gating until the link reconnects after LDTSTOP#
is deasserted.
4.4 Tunnel Links
HyperTransport link A supports CLK receive and transmit frequencies of 200, 400, 600, and 800 MHz. Link B
supports frequencies of 200 and 400 MHz. The side A and side B frequencies are independent of each other.
4.4.1 Link PHY
The PHY includes automatic compensation circuitry and a software override mechanism, as specified by
DevA:0x[E8, E4, E0]. The IC only implements synchronous mode clock forwarding FIFOs. So only the link
receive and transmit frequencies specified in DevA:0x[D0, CC][FREQB, FREQA] are allowed.
4.5 AGP
The AGP bridge supports AGP 3.0 signaling at 8x and 4x data rates and 1.5-volt AGP 2.0 signaling at 4x, 2x,
and 1x data rates. 64-bit upstream and 32-bit downstream addressing is supported. AGP 3.0 dynamic bus inver-
sion is supported on output signals in 8X mode only, not in 4X mode; dynamic bus inversion on input signals is
supported in both 4X and 8X modes.
4.5.1 Tags, UnitIDs, And Ordering
The IC requires three HyperTransport
TM
technology-defined UnitIDs. They are allocated as follows:
• First UnitID is not used. This is to avoid a potential conflict with the host (because it may be zero; see
DevA:0xC0[BUID]).
• Second UnitID is used for PCI-mode upstream requests and responses to host requests.
• Third UnitID is used for AGP (high priority and low priority) upstream requests.
The SrcTag value that is assigned to upstream non-posted AGP requests increments with each request from 0
to 27 and then rolls over to 0 again; the first SrcTag assigned after reset is 0. Up to 28 non-posted link requests
may be outstanding at a time. The SrcTag value that is assigned to non-posted PCI requests is always 28.
