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Motherboard Layout and Routing Guidelines
2-20
Intel
®
440GX AGPset Design Guide
It is always best to reduce the line length mismatch wherever possible to insure added margin. It is
also best to separate the traces by as much as possible to reduce the amount of trace to trace
coupling.
The clock lines on the motherboard can couple with other traces. It is recommended that the clock
spacing (air gap) be at least two times the trace width to any other traces. It is also strongly
recommended that the clock spacing be at least four times the trace width to any strobes.
The clock lines on the motherboard need to be simulated to determine the their proper line length.
The motherboard needs to be designed to the type of clock driver that is being used and
motherboard trace topology. These clocks need to meet the loading of the receiving device as well
as the add-in trace length.
Additionally, control signals less than 8.5 inches can be routed 1:1, while control signals greater
than 8.5 inches should be routed 1:2.
Some of the control signals require pull-up resistors to be installed on the motherboard. AGP
signals must be pulled up to VCC
3.3
using 8.2K to 10K pull-up resistors (refer to Section 3.5.1,
“82443GX Interface” on page 3-10). Pull-up resistors should be discrete resistors, as resistor packs
will need longer stub lengths and may break timing. The stub to these pull-up resistors needs to be
controlled. The maximum stub length on a strobe trace is < 0.1 inch. The maximum stub trace
length on all other traces is < 0.5 inch.
2.8.2 On-board AGP Compliant Device (“Down” Option) Layout
Guidelines
Routing guidelines for the device ‘down’ option are very similar to those when the device is ‘up’.
Some modifications need to be made when placing the graphics device on the motherboard, due to
the various trace spacing.
Table 2-13. Source Synchronous Motherboard Recommendations
Width:Space Trace Line Length Line Length Matching
1:1 (Data) / 1:2 (Strobe) Data / Strobe 1.0 in < line length < 4.5 in -0.5 in, strobe longest trace
1:2 Data / Strobe 1.0 in < line length < 9.5 in -0.5 in, strobe longest trace
Table 2-14. Control Signal Line Length Recommendations
Width:Space Board Trace Line Length Pull-up Stub Length
1:1 Motherboard Control signals 1.0 in < line length < 8.5 in < 0.5 in (Strobes < 0.1in)
1:2 Motherboard Control signals 1.0 in < line length < 10.0 in < 0.5 in (Strobes < 0.1in)
1:2 (1:4 to Strobe) Motherboard Clock