LM3S5656 Datasheet, PDF (293/1140 Pages) Texas Instruments – Stellaris® LM3S5656 Microcontroller

Stellaris^® LM3S5656 Microcontroller
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 294.
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.
Figure 6-2. Using a Crystal as the Hibernation Clock Source
Input
Voltage
Regulator
or Switch
IN
OUT
EN
Stellaris® Microcontroller
VDD
XOSC0
X₁
R_L
XOSC1
C₁
C₂
R_PU1
Open drain
external wake
up circuit
HIB
WAKE
VBAT
R_PU2
GND
3V
Battery
Note:
X₁ = 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_PU1 = Pull-up resistor 1 (value and voltage source (V_BAT or Input Voltage) determined by regulator
or switch enable input characteristics).
R_PU2 = Pull-up resistor 2 is 200 kΩ
See “Hibernation Clock Source Specifications” on page 1073 for specific parameter values.
January 21, 2012
293
Texas Instruments-Production Data