Siemens SXG 75 Service Repair Documentation page 41

As each regulator is enabled, detector circuits confirm that it powers up properly before triggering a
wait interval. After the wait interval expires, the next regulator is enabled. This process continues
until all the default-on regulators have powered up successfully. Once all the default-on regulators
are on another wait interval is observed before the PON_RESET_N signal is driven high.
For a successful power-on sequence, the MSM6250 must then initialise and drive the PS_HOLD
signal high within a given time. If it does so, the power remains latched on. This sequence is shown
in the diagram below:
Power supply On/Off sequencing
8.5.2 Power-Off Sequence
The PM6650 is in an ON state while the PS_HOLD signal from the MSM6250 is high. Under this
condition, the PM6650 continually monitors events that could trigger a power-off sequence:
• The MSM drives the PS_HOLD signal low in response to the handset user pressing the keypad
power button.
• The PM6650 die temperature exceeds the over-temperature threshold.
The most common power-off sequence begins with the keypad power button being pressed while
the MSM6250 is powered on and operational. The PM6650 PHONE_ON signal is connected to the
handset keypad power button and is pulled up internally. The MSM6250 monitors the state of the
key through interrupts from the PM6650. When an interrupt is triggered it initiates the following
power-down sequence:
• The user presses and holds the END key for at least the minimum interval. The power-down
sequence is aborted if not pressed long enough.
• Any information that needs to be saved is written to Flash ROM.
• The PM6650 is prevented from powering up again.
• The power-off sequence waits until the user releases the END key, otherwise the PM6650 IC will
power-up again.
• The MSM6250 drives the PS_HOLD signal low to cause the PM6650 IC to power-down the
handset.
• The PM6650 IC drives PON_RESET_N low to reset the MSM and other external devices, and
disables the TCXO regulator. The PM6650 VBACKUP pin is internally connected to its VCOIN pin to
provide backup power to SRAM.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 41 of 73