LM3S3Z26 Datasheet, PDF (275/877 Pages) Texas Instruments – Stellaris® LM3S3Z26 Microcontroller

Stellaris^® LM3S3Z26 Microcontroller
6.3.5
6.3.6
6.3.7
The Hibernation module can be configured to detect a low battery condition by setting the LOWBATEN
bit of the HIBCTL register. In this configuration, the LOWBAT bit of the Hibernation Raw Interrupt
Status (HIBRIS) register is set when the battery level is low. If the VABORT bit in the HIBCTL register
is also set, then the module is prevented from entering Hibernate mode when a low battery is
detected. The module can also be configured to generate an interrupt for the low-battery condition
(see “Interrupts and Status” on page 276).
Note that the Hibernation module draws power from whichever source (V_BAT or V_DD) has the higher
voltage. Therefore, it is important to design the circuit to ensure that V_DD is higher that V_BAT under
nominal conditions or else the Hibernation module draws power from the battery even when V_DD is
available.
Real-Time Clock
The Hibernation module includes a 32-bit counter that increments once per second with the proper
configuration (see “Hibernation Clock Source” on page 272). The 32.768-kHz clock signal, either
directly from the 32.768-kHz oscillator or from the 4.194304-MHz crystal divided by 128, is fed into
a predivider register that counts down the 32.768-kHz clock ticks to achieve a once per second
clock rate for the RTC. The rate can be adjusted to compensate for inaccuracies in the clock source
by using the predivider trim register, HIBRTCT. This register has a nominal value of 0x7FFF, and
is used for one second out of every 64 seconds to divide the input clock. This configuration allows
the software to make fine corrections to the clock rate by adjusting the predivider trim register up
or down from 0x7FFF. The predivider trim should be adjusted up from 0x7FFF in order to slow down
the RTC rate and down from 0x7FFF in order to speed up the RTC rate.
The Hibernation module includes two 32-bit match registers that are compared to the value of the
RTC counter. The match registers can be used to wake the processor from Hibernate mode or to
generate an interrupt to the processor if it is not in hibernation.
The RTC must be enabled with the RTCEN bit of the HIBCTL register. The value of the RTC can be
set at any time by writing to the HIBRTCLD register. The predivider trim can be adjusted by reading
and writing the HIBRTCT register. The predivider uses this register once every 64 seconds to adjust
the clock rate. The two match registers can be set by writing to the HIBRTCM0 and HIBRTCM1
registers. The RTC can be configured to generate interrupts by using the interrupt registers (see
“Interrupts and Status” on page 276). As long as the RTC is enabled and a valid V_BAT is present, the
RTC continues counting, regardless of whether V_DD is present or if the part is in hibernation.
Battery-Backed Memory
The Hibernation module contains 64 32-bit words of memory that are powered from the battery or
auxiliary power supply and therefore retained during hibernation. The processor software can save
state information in this memory prior to hibernation and recover the state upon waking. The
battery-backed memory can be accessed through the HIBDATA registers. If both V_DD and V_BAT are
removed, the contents of the HIBDATA registers are not retained.
Power Control Using HIB
Important: The Hibernation Module requires special system implementation considerations when
using HIB to control power, as it is intended to power-down all other sections of the
microcontroller. All system signals and power supplies that connect to the chip must
be driven to 0 V_DC or powered down with the same regulator controlled by HIB.
The Hibernation module controls power to the microcontroller through the use of the HIB pin which
is intended to be connected to the enable signal of the external regulator(s) providing 3.3 V to the
microcontroller and other circuits. When the HIB signal is asserted by the Hibernation module, the
January 21, 2012
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