Mobile Intel Pentium 4 Processor - M and Intel 845MP/MZ Chipset Platform Design Guide

Mobile Intel
®
Pentium
®
4 Processor-M and Intel
®
845MP/845MZ Chipset Platform
148 Design Guide
R
Table 49. End of Line Termination Topology BCLK [1:0]# Routing Guidelines
Layout Guideline Value Illustration Notes
BCLK Skew between agents 400 ps totalBudget:150 ps
for Clock driver250 ps for
interconnect
1, 2, 3, 4
Differential pair spacing S max. Figure 80 5, 6
Spacing to other traces 4 S- 5 S mils Figure 80 --
Line width 4.0 mils Figure 80 7
Systemboard Impedance –
Differential
100Ω± 15% -- 8
Systemboard Impedance – odd
mode
55Ω ±15% -- 9
Processor routing length – L1,
L1': Clock driver to Rs
0.5” max Figure 79 13
Processor routing length – L2,
L2'
0 – 0.2" Figure 79 13
Processor routing length – L3,
L3’: RS-RT node to Rt
0 - 0.5" Figure 79 13
Processor routing length – L4,
L4': RS-RT Node to Load
2 – 8" Figure 79
MCH-M routing length – L1, L1’:
Clock Driver to RS
0.5” max Figure 79 13
MCH-M routing length – L2, L2' 2– 8" Figure 79 13
MCH-M routing length – L3, L3’:
RS-RT node to Rt
0 – 0.2" Figure 79 13
Clock driver to Processor and
clock driver to Chipset length
matching (L1+L2)
+260 mils
-190 mils
Figure 79 10
BCLK0 – BCLK1 length
matching
±10 mils --
Rs Series termination value 33 Ω ± 5% 11
Rt Shunt termination value 55 Ω ± 1% (for 55 Ω MB
impedance)
12
10.3.1.2. Source Shunt Termination Topology
The recommended termination for the differential bus clock is a “Source Shunt Termination.” Refer to
Figure 79 for an illustration of this terminology scheme. Parallel Rt 49.9 resistors perform a dual
function, converting the current output of the clock driver to a voltage and matching the driver output
impedance to the transmission line. The series resistors Rs provide isolation from the clock driver’s
output parasitic, which would otherwise appear in parallel with the termination resistor Rt.
The value of Rt should be selected to match the characteristic impedance of the system board and Rs
should be between 20 and 33 Ohms. Simulations have shown that Rs values above 33 ohms provide no
benefit to signal integrity but only degrade the edge rate.