Rtc External Battery Connection - Intel Quark SoC X1000 Design Manual

RTC Design Guidelines—Intel
The following example illustrates the use of the practical values C1, C2 in the case that
theoretical values cannot guarantee the accuracy of the RTC in low temperature
condition:
Example:
According to a required 12 pF load capacitance of a typical crystal that is used with the
SoC, the calculated value of C1 = C2 is 10 pF at room temperature (25 °C) to yield an
32.768 kHz oscillation.
At 0 °C the frequency stability of crystal gives –23 ppm (assumed that the circuit has 0
ppm at 25 °C). This makes the RTC circuit oscillate at 32.767246 kHz instead of 32.768
kHz.
If the values of C1, C2 are chosen to be 6.8 pF instead of 10 pF, this will make the RTC
oscillate at higher frequency at room temperature (+23 ppm), but this configuration of
C1, C2 makes the circuit oscillate closer to 32.768 kHz at 0 °C. The 6.8 pF value of C1
and C2 is the practical value.
Note: The temperature dependency of crystal frequency is a parabolic relationship (ppm /
degree squared). The effect of 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). See crystal datasheet for more details.
12.4

RTC External Battery Connection

The RTC requires an external battery connection to maintain its functionality and its
RAM while SoC is not powered by the system.
Example batteries are: Duracell 2032, 2025, or 2016 (or equivalent), which can give
many years of operation. Batteries are rated by storage capacity. The battery life can
be calculated by dividing the capacity by the average current required. For example, if
the battery storage capacity is 170 mAh (assumed usable) and the average current
required is 6 µA, the battery life will be at least:
170,000 µAh / 6µA = 28,333 h = 3.2 years.
The voltage of the battery can affect the RTC accuracy. In general, when the battery
voltage decays, the RTC accuracy also decreases.
The battery must be connected to SoC via an isolation Schottky diode circuit. The
Schottky diode circuit allows SoC RTC well to be powered by the battery when the
system power is not available, but 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.
Figure 43
June 2014
Order Number: 330258-002US
®
Quark™ SoC X1000
is an example of a diode circuit that is used.
®
Intel Quark™ SoC X1000
PDG
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