Nokia 2608 Service Manual page 99

RM-376
System Module and User Interface
SLEEP Crystal Circuit for 32.768kHz
The 32.768 kHz cryst al oscillator is the primary SLEEP clock source when TCXO clocks are disabled to save
power.
Figure 11 Sleep CLK Block Diagram
The 32.768 kHz crystal oscillator signal is generated by an external crystal (plus two shunt capacitors), which is
supplemented by a PM internal inverter and buffer.
The crystal oscillator continues to run as long as a valid supply is present, even when the PM is powered down.
This provides a continuous and accurate 32.768 kHz source. The oscillator halts when power from the external
supply and main battery are removed.
The PM includes a circuit that continually monitors the crystal oscillator signal. If the crystal stops oscillating, the
PM automatically switches to the RC oscillator and sends an MSM interrupt. Narrow pulses at the SLEEP_CLK
output may occur during this switchover. The crystal oscillator dissipates little power, adjusting its bias current
to the minimum required to maintain oscillation.
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