Datasheet

6.3.2 Clock Source
The Hibernation module must be clocked by an external source, even if the RTC feature is not used.
An external oscillator or crystal can be 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 the 32.768-kHz clock reference. For an alternate clock source, a 32.768-kHz oscillator can
be connected to the XOSC0 pin. See Figure 6-2 on page 245 and Figure 6-3 on page 246. Note that
these diagrams only show the connection to the Hibernation pins and not to the full system. See
“Hibernation Module” on page 757 for specific values.
The clock source is enabled by setting the CLK32EN bit of the HIBCTL register. The type of clock
source is selected by setting the CLKSEL bit to 0 for a 4.194304-MHz clock source, and to 1 for a
32.768-kHz clock source. If the bit is set to 0, the 4.194304-MHz input clock is divided by 128,
resulting in a 32.768-kHz clock source. If a crystal is used for the clock source, the software must
leave a delay of t
XOSC_SETTLE
after setting 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.
Figure 6-2. Clock Source Using Crystal
Open drain
external wake
up circuit
3 V
Battery
GND
C
2
C
1
R
L
X
1
VBAT
EN
Input
Voltage
Regulator
or Switch
XOSC1
XOSC0
VDD
HIB
WAKE
OUTIN
Stellaris Microcontroller
R
PU
Note: X
1
= Crystal frequency is f
XOSC_XTAL
.
C
1,2
= Capacitor value derived from crystal vendor load capacitance specifications.
R
L
= Load resistor is R
XOSC_LOAD
.
R
PU
= Pull-up resistor (1 M½).
See “Hibernation Module” on page 757 for specific parameter values.
245June 18, 2012
Texas Instruments-Production Data
Stellaris
®
LM3S8962 Microcontroller
NRND: Not recommended for new designs.