FSB Design Guidelines
R
54
Intel
®
855PM Chipset Platform Design Guide
4.1.4.1.3.
Topology 1C: Open Drain (OD) Signals Driven by the Processor – PROCHOT#
The Topology 1C OD signal PROCHOT#, should adhere to the following routing and layout
recommendations. Table 10 lists the recommended routing requirements for the PROCHOT# signal of
the processor. The routing guidelines allows the signal to be routed as either a micro-strip or strip-line
using 55
± 15% characteristic trace impedance. Figure 18 shows the recommended implementation
for providing voltage translation between the processor’s PROCHOT# signal and a system receiver that
utilizes a 3.3-V interface voltage (shown as V_IO_RCVR).
Series resistor Rs is a component of the voltage translation logic and serves as a driver isolation resistor.
Rs is shown separated by distance L3 from the first bipolar junction transistor (BJT), Q1, to emphasize
the placement of Rs with respect to Q1. The placement of Rs a distance L3 before the Q1 BJT is a
specific implementation of the generalized voltage translator circuit shown in Figure 24. Rs should be
placed at the beginning of the T-split from the PROCHOT# signal. The pull-up voltage for termination
resistor Rtt is V
CCP
(1.05 V).
Intel recommends that PROCHOT# be routed using the voltage translation logic shown in Figure 18.
The receiver at the output of the voltage translation circuit can be any system receiver that can function
properly with the PROCHOT# signal given the nature and usage model of this pin. PROCHOT# is
capable of toggling hundreds of times per second to signal a hot temperature condition.
Figure 18. Routing Illustration for Topology 1C
Intel
Pentium M
Processor
L1
L3
L2
Rs
3.3V
3904
3904
L4
3.3V
Q1
Q2
R1
R2
V_IO_RCVR
System Receiver
VCCP
Rtt
Table 10. Layout Recommendations for Topology 1C
L1
L2
L3
L4 Rs R1 R2-
Rtt
Transmission
Line Type
0.5” – 12.0” 0” – 3.0” 0” – 3.0” 0.5” – 12.0” 330
± 5% 1.3 k
± 5% 330
± 5% 56
± 5%
Micro-strip
0.5” – 12.0” 0” – 3.0” 0” – 3.0” 0.5” – 12.0” 330
± 5% 1.3 k
± 5% 330
± 5% 56
± 5%
Strip-line
