LM3S5G51 Datasheet, PDF (292/1266 Pages) Texas Instruments – Stellaris® LM3S5G51 Microcontroller

Hibernation Module
6.3.2
■ Hibernation RTC Counter (HIBRTCC)
■ Hibernation RTC Match 0 (HIBRTCM0)
■ Hibernation RTC Match 1 (HIBRTCM1)
■ Hibernation RTC Load (HIBRTCLD)
■ Hibernation RTC Trim (HIBRTCT)
■ Hibernation Data (HIBDATA)
Back-to-back reads from Hibernation module registers have no timing restrictions. Reads are
performed at the full peripheral clock rate.
Hibernation Clock Source
In systems where the Hibernation module is used to put the microcontroller into hibernation, the
module must be clocked by an external source that is independent from the main system clock,
even if the RTC feature is not used. An external oscillator or crystal is used for this purpose. To use
a crystal, a 4.194304-MHz crystal is connected to the XOSC0 and XOSC1 pins. This clock signal is
divided by 128 internally to produce a 32.768-kHz Hibernation clock reference. Alternatively, a
32.768-kHz oscillator can be connected to the XOSC0 pin, leaving XOSC1 unconnected. Care must
be taken that the voltage amplitude of the 32.768-kHz oscillator is less than V_BAT, otherwise, the
Hibernation module may draw power from the oscillator and not V_BAT during hibernation. See Figure
6-2 on page 293 and Figure 6-3 on page 293.
The Hibernation clock source is enabled by setting the CLK32EN bit of the HIBCTL register. The
type of clock source is selected by clearing the CLKSEL bit for a 4.194304-MHz crystal and setting
the CLKSEL bit for a 32.768-kHz oscillator. If a crystal is used for the clock source, the software
must leave a delay of t_{HIBOSC_START} after writing to the CLK32EN bit and before any other accesses
to the Hibernation module registers. The delay allows the crystal to power up and stabilize. If an
oscillator is used for the clock source, no delay is needed.
292
January 23, 2012
Texas Instruments-Production Data