7.3.4
Battery Management
Important:
System-level factors may affect the accuracy of the low-battery detect circuit. The
designer should consider battery type, discharge characteristics, and a test load during
battery voltage measurements.
The Hibernation module can be independently powered by a battery or an auxiliary power source
using the
VBAT
pin. The module can monitor the voltage level of the battery and detect when the
voltage drops below V
LOWBAT
. The voltage threshold can be between 1.9 V and 2.5 V and is
configured using the
VBATSEL
field in the
HIBCTL
register. The module can also be configured so
that it does not go into Hibernate mode if the battery voltage drops below this threshold. In addition,
battery voltage is monitored while in hibernation, and the microcontroller can be configured to wake
from hibernation if the battery voltage goes below the threshold using the
BATWKEN
bit in the
HIBCTL
register.
The Hibernation module is designed to detect a low-battery condition and set the
LOWBAT
bit of the
Hibernation Raw Interrupt Status (HIBRIS)
register when this occurs. 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 548).
7.3.5
Real-Time Clock
The RTC module is designed to keep wall time. The RTC can operate in seconds counter mode or
calendar mode. A 32.768 kHz clock source along with a 15-bit predivider reduces the clock to 1 Hz.
The 1 Hz clock is used to increment the 32-bit counter and keep track of seconds. In calendar mode,
registers are provided which support the tracking of date, month, year and day-of-week. A match
register can be configured to interrupt or wake the system from hibernate. In addition, a software
trim register is implemented to allow the user to compensate for oscillator inaccuracies using software.
7.3.5.1
RTC Counter - Seconds/Subseconds Mode
The clock signal to the RTC is provided by either of the 32.768-kHz clock sources available to the
Hibernation module. The
Hibernation RTC Counter (HIBRTCC)
register displays the seconds
value. The
Hibernation RTC Sub Seconds register (HIBRTCSS)
is provided for additional time
resolution of an application requiring less than one-second divisions.
The RTC is enabled by setting the
RTCEN
bit of the
HIBCTL
register. The
RTCEN
bit is also used
along with the
CALEN
bit in the
Hibernation Calendar Control (HIBCALCTL)
register to enable
the calender. Thus, if the calendar is enabled, the RTC registers,
HIBRTCC
,
HIBRTCSS
,
HIBRTCM0
and
HIBRTCLD
, cannot be used. The RTC counter and sub-seconds counters begin counting
immediately once
RTCEN
is set. Both counters count up. The RTC continues counting as long as
the RTC is enabled and a valid V
BAT
is present, regardless of whether V
DD
is present or if the device
is in hibernation.
The
HIBRTCC
register is set by writing the
Hibernation RTC Load (HIBRTCLD)
register. A write
to the
HIBRTCLD
register clears the 15-bit sub-seconds counter field,
RTCSSC
, in the
HIBRTCSS
register. To ensure a valid read of the RTC value, the
HIBRTCC
register should be read first, followed
by a read of the
RTCSSC
field in the
HIBRTCSS
register and then a re-read of the
HIBRTCC
register.
If the two values for the
HIBRTCC
are equal, the read is valid. By following this procedure, errors
in the application caused by the
HIBRTCC
register rolling over by a count of 1 during a read of the
RTCSSC
field are prevented. The RTC can be configured to generate an alarm by setting the
RTCAL0
bit in the
HIBIM
register. When an RTC match occurs, an interrupt is generated and displayed in
the
HIBRIS
register. Refer to “RTC Match - Seconds/Subseconds Mode” on page 540 for more
information.
539
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller