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R
5.6
CLKREF Circuit Implementation
The CLKREF input (used by the Pentium III processor (CPUID=068xh), Celeron processor
(CPUID=068xh), and future 0.13 micron socket 370 processors) requires a 1.25V source. It can be
generated from a voltage divider on the VCC2.5 or VCC3.3 sources using 1% tolerant resistors. A
4.7 F decoupling capacitor should be included on this input. See Figure 24 and Table 13 for
example CLKREF circuits. Do not use VTT as the source for this reference!
Figure 24. Examples for CLKREF Divider Circuit
Table 13. Resistor Values for CLKREF Divider (3.3V Source)
R1 (
182
301
374
499
5.7
Undershoot/Overshoot Requirements
Undershoot and overshoot specifications become more critical as the process technology for
microprocessors shrinks due to thinner gate oxide. Violating these undershoot and overshoot limits
will degrade the life expectancy of the processor.
The Pentium III processor (CPUID=068xh), Celeron processor (CPUID=068xh), and future
0.13 micron socket 370 processors have more restrictive overshoot and undershoot requirements
for system bus signals than previous processors. These requirements stipulate that a signal at the
output of the driver buffer and at the input of the receiver buffer must not exceed the maximum
absolute overshoot voltage limit or the minimum absolute undershoot voltage limit. Exceeding
either of these limits will damage the processor. There is also a time-dependent, non-linear
overshoot and undershoot requirement that depends on the amplitude and duration of the
overshoot/undershoot. See the appropriate processor datasheet for more details on the processor
overshoot/undershoot specifications.
®
Intel
815 Chipset Platform Design Guide
Vcc2.5
Y33
150
4.7 µF
150
, 1%
R2 (
, 1%
110
182
221
301
PGA370
Vcc3.3
CLKREF
R1
R2
CLKREF Voltage (V)
1.243
1.243
1.226
1.242
System Bus Design Guidelines
PGA370
CLKREF
Y33
4.7 µF
sys_bus_CLKREF_divider
57
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