Intel 440GX manual On-board AGP Compliant Device “Down” Option Layout Guidelines

Motherboard Layout and Routing Guidelines

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.

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

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.

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

Some of the control signals require pull-up resistors to be installed on the motherboard. AGP signals must be pulled up to VCC3.3 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.

2.8.2On-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.

2-20 Intel®440GX AGPset Design Guide