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PGA370 Processor Design Guidelines
2.2.5
CLKREF Circuit Implementation
The CLKREF input requires a 1.25V source. It can be generated from a voltage divider on the
Vcc2.5 or Vcc3.3 sources utilizing 1% tolerance resistors. A 4.7 uF decoupling capacitor should
be included on this input. See
Vtt as the source for this reference!
Figure 2-4. Examples for CLKREF Divider Circuit
Table 2-9. Example Resistor Values for CLKREF Divider Circuit (3.3V Source)
R1 (Ω)
182
301
374
499
2.2.6
Undershoot/Overshoot Requirements
®
The Intel
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 a maximum
absolute overshoot voltage limit (2.1V) and a minimum absolute undershoot voltage limit (-
0.35V). Exceeding these limits will cause damage to the Intel
also a time dependent, non-linear overshoot and undershoot requirement that is dependent on the
amplitude and duration of the overshoot/undershoot. See the Intel
datasheet for more details on overshoot/undershoot specifications.
2-10
Figure 2-4
PGA370
Vcc2.5
C L K R E F
Y 3 3
150 Ω
150 Ω
4.7uF
R2 (Ω)
110
182
221
301
®
Pentium
III processor has more restrictive overshoot and undershoot requirements for
and
Table 2-9
for example CLKREF circuits. Do not use
Vcc3.3
R 1
R 2
CLKREF Voltage (V)
1.243
1.243
1.226
1.242
®
Pentium
®
Intel
PGA370
C L K R E F
Y 3 3
4.7uF
®
III processor. There is
®
III processor
Pentium
®
810A3 Chipset Design Guide
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