Nokia 1508 Manual page 9

1508(RM-388)
Baseband Description and Troubleshooting
Power Up
The mobile terminal uses the power key and a charger to power up.
Power Key
When the power key is pressed, the PMU enters the power-up sequence. Pressing the
power key causes the POWER_ON pin to VBAT. The POWER_ON signal is not part of
the keypad matrix. The power key is connected to the PM. This means that when the
power key is pressed, an interrupt will be generated to the CBP in order to power on the
CBP. The CBP reads the PMU IC's interrupt register and notifies that it is a
KPADPWR_N interrupt. The CBP reads the status of the POWER_ON signal using the
PMU's (I2C) control bus. If the POWER_ON signal stays High for a certain time, the CBP
accepts this as a valid power-on state and continues with the software baseband
initialization. If the power key does not indicate a valid power-on situation, the CBP
powers off the baseband.
Charger
Charging is controlled by start-up charging circuitry in order to detect and start charging
in case the main battery is not full and the PMU has no supply.
Power Off
While the PS_HOLD signal from the CBP is high, and the PMU IC is in power-on
status. The PMU continually monitors three events that could trigger a power-off
sequence:
The CBP drives the PS_HOLD signal low responding to the pressing of the
keypad power button.
Battery voltage drops below power off threshold (Battery Voltage < 3.2 V).
The PMU IC die temperature exceeds its "severe" over-temperature threshold.
Clock Distribution
The CBP derives its internal clock from two clock inputs, VC-TCXO and SLEEP_CLK.
The main clock signal for the baseband is generated from TCXO.
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