SH3100 Datasheet, PDF (56/80 Pages) Semtech Corporation – Supervisory IC with I2C Interface and PWM

POWER MANAGEMENT
Functional Descriptions (continued)
Plot of Crystal Padding Capacitance versus Adjustment Code
uBuddy V3. Crystal Padding Capacitance vs Code
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25
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5
0
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Code
Note: The MSB of the 5-bit control word is inverted to ensure
that mid range capacitance is used as default for an unpro-
grammed device with all zeros in the control register.
SH3100
Internal 32.768 kHz Oscillator
The SH3100 is capable of functioning with or without an
external 32.768kHz crystal. The chip includes an internal
oscillator which is fuse calibrated on test to provide a
32.768 kHz clock source as an integrated alternative to
the crystal oscillator. accuracy is better than ±3% over
temperature and supply voltage, and initial accuracy at
25°C is better than ±1%. The internal oscillator starts
within 100μs of power being supplied to the chip and
acts as the low frequency system clock for quick chip
initiation.
This means that if a short reset duration has been
programmed, the high frequency output clock can
be ready very soon after a completely cold start. If a
crystal is present, the crystal oscillator starts up within
approximately two seconds and then takes over from
the internal oscillator as the system clock. The internal
oscillator is then shut down to save power. If no crystal is
present, then the internal 32.768kHz oscillator continues
to run as the system clock, and the crystal oscillator
circuitry is shut down to save power, if it has not started up
within 10 seconds of power-up. Once in normal operation,
it is possible for the microcontroller to override the clock
selection by I²C access, although this would normally only
be required for test purposes.
The main reasons for requiring a 32.768kHz crystal are
to obtain a very accurate RTC or high frequency clock.
If neither of these requirements is critical then it is
acceptable to save costs by removing the crystal and just
using the internal oscillator.
Note: If no crystal is present, XIN should be tied to ground
to avoid noise pickup on XIN being misinterpreted as crystal
oscillations.
© 2006 Semtech Corp.
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