6.3.2 Hibernation Clock Source

The HIB module can be clocked by one of three different clock sources:

• A 32.768-kHz oscillator
• An external 32.768-kHz clock source
• An internal low frequency oscillator (HIB LFIOSC)

Table 6-1 summarizes the encodings for the bits in the HIBCTL register that are required for each clock source to be enabled. Note that CLK32EN must be set for any Hibernation clock source to be valid. The Hibernation module is not enabled until the CLK32EN bit is set. The HIB clock source is the source of the RTC Oscillator (RTCOSC), which can be selected as the system clock source by programming a 0x4 in the OSCSRC field of the Run and Sleep Mode Configuration (RSCLKCFG) register in the System Control Module. See Section 4 for more information.

To use an external crystal, a 32.768-kHz crystal is connected to the XOSC0 and XOSC1 pins. 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 and Figure 6-3.

Alternatively, a low frequency oscillator source (HIB LFIOSC) present in the Hibernation module can be a clock source. (The frequency can have wide variations; see the HIB clock specifications in the device-specific data sheet for more details.) The intent of this source is to provide an internal low-power clock source to enable the use of the asynchronous pin wakes and memory storage without the requirement of an external crystal. To enable the HIB LFIOSC to be the clock source for the Hibernation module, both the OSCSEL bit and the CLK32EN bit in the Hibernation Control (HIBCTL) register must be set.

The Hibernation module is enabled by setting the CLK32EN bit of the HIBCTL register. The CLK32EN bit must be set before accessing any other Hibernation module register. The type of clock source used for the HIB module is selected by setting the OSCSEL and OSCBYP bit of the HIBCTL register. If the internal low frequency precision oscillator is used as the clock source, the OSCSEL bit should be set to a 1 at the same time the CLK32EN bit is set. 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 external oscillator is used for the clock source, no delay is needed. When using an external clock source, the OSCBYP bit in the HIBCTL register should be set. When using a crystal clock source, the GNDX pin should be connected to digital ground along with the crystal load capacitors, as shown in Figure 6-2. When using an external clock source, the GNDX pin should be connected to digital ground.

Figure 6-2 Using a Crystal as the Hibernation Clock Source with a Single Battery Source

Figure 6-3 Using a Dedicated Oscillator as the Hibernation Clock Source with VDD3ON Mode