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®
®
Intel
Pentium
M Processor and Intel
I/O Controller Hub 3 (Intel
Note: The temperature dependency of crystal frequency is a parabolic relationship (ppm / degree
squared). The effect of the changing crystal's frequency when operating at 0° C (25° C below room
temperature) is the same when operating at 50° C (25° C above room temperature).
9.6.4
RTC Layout Considerations
Since the RTC circuit is very sensitive and requires high accurate oscillation, reasonable care must
be taken during layout and routing of the RTC circuit. Some recommendations are:
•
Reduce trace capacitance by minimizing the RTC trace length. ICH3-S requires a trace length
less than 1 inch on each branch (from crystal's terminal to RTCXn pin). Route the RTC circuit
short to simplify the trace length measurement and increase accuracy when calculating trace
capacitances. Trace capacitance depends on the trace width and dielectric constant of the
board's material. On FR-4 board material, a 5 mils trace has approximately 2 pF per inch.
•
Reduce trace signal coupling by avoiding routing of adjacent PCI signals close to RTCX1,
RTCX2, and VBIAS.
9.6.5
RTC External Battery Connection
The RTC requires an external battery connection to maintain its functionality and its RAM while
the ICH3-S is not powered by the system.
Example batteries are: Duracell 2032, 2025, or 2016 (or equivalent), which may give many years
of operation.
Batteries are rated by storage capacity. The battery life may be calculated by dividing the capacity
by the average current required. For example, when the battery storage capacity is 170 mAh
(assumed usable), and the average current required is 3 µA, the battery life may be at least:
The voltage of the battery may affect the RTC accuracy. In general, when the battery voltage
decays, the RTC accuracy also decreases. The battery voltage of the RTC must be greater than 2 V
at all times to ensure the accuracy of the RTC clock.
Connect the battery to the ICH3-S via an isolation diode circuit. The diode circuit allows the
ICH3-S RTC-well to be powered by the battery when the system power is not available, and by the
system power when it is available. To do this, the diodes are set to be reverse biased when the
system power is not available.
power supply should be used in a server system to provide continuous power to the RTC when
available to significantly increase the RTC battery life
9.6.6
VBIAS DC Voltage and Noise Measurements
VBIAS is a DC voltage level that is necessary for biasing the RTC oscillator circuit. This DC
voltage level is filtered out from the RTC oscillation signal by the RC Network of R2 and C3 (see
Figure
116); therefore, it is a self-adjusted voltage. Board designers should not manually bias the
voltage level on VBIAS. Checking VBIAS level is used for testing purposes only to determine the
right bias condition of the RTC circuit.
166
®
E7501 Chipset Platform
®
ICH3-S)
170,000 µAh / 3 µA = 56,666 h = 6.4 years
Figure 116
is an example of a diode circuit. As noted, a standby
Design Guide
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