Intel 815 - APIC Clock Voltage Switching Requirements; Figure 15. Voltage Switch for APIC Clock from Clock Synthesizer to Processor

Universal Socket 370 Design
R
Intel
®
815 Chipset Platform Design Guide 39
4.2.9 APIC Clock Voltage Switching Requirements
The processor’s APIC clock is also specified at different voltage levels depending on whether it is
for the Pentium III processor (CPUID=068xh) / Celeron processor (CPUID=068xh) or whether it
is for a future 0.13 micron socket 370 processor. There is no overlap in the range of accepted
voltage levels for the two processor groups, so a voltage switch is required to ensure proper
operation. Figure 15 shows an example implementation.
Figure 15. Voltage Switch For APIC Clock from Clock Synthesizer to Processor
30 Ω
130 Ω
IOAPIC
APICCLK_CPU
TUAL5
MOSFET N
API_CLK_SW
NOTE: The 30 Ω resistor represents the series resistor typically used in connecting the APIC clock to the
processor.