Power distribution, control and watchdogs
All power (except backup battery power) is drawn from the BL-5B Li-Ion battery. Current flows through ZOCUS
current sense resistor, used for current measurement, and thus for remaining operating time estimation.
The discrete regulators consist of a SMPS to power the UPP_WD2 core, a SMPS to power the LCD backlightning
and the keyboard LEDs, and two LDOs to deliver VCAM and VLCD respectively.
1KMA board contains one power ASIC, UEMEK and discrete regulators needed for generating the different
operating voltages. The discrete regulators consist of an SMPS to power UPP_WD2 voltage core. In addition,
there is a SMPS in 1KMA generating the operating voltage for display module backlighting. In 1KMA, the keyboard
backlight is powered with a charge pump regulator.
A “soft watchdog” in UPP_WD2 is used to reset the chip in case software gets stuck for any reason.
Power-up sequence (reset mode)
RESET mode can be entered in four ways: by inserting the battery or charger, by RTC alarm or by pressing the
power key (the VCXO is powered by UEMEK). After a 220 ms delay, regulators are configured and UEMEK enters
PWR_ON mode and system reset (PURX) is released.
During the system start-up, in RESET state, the regulators are enabled, and each regulator charges the capacitors
at the output with the maximum current (short circuit current) it can deliver. This results in battery voltage
dropping during start-up. When a battery with a voltage level just above the hardware cutoff limit is inserted,
the system may not start due to excessive voltage dropping. Dropping below 2.8 V for longer than 5 µs forces
the system to PWR_OFF state.
Controlled powering off
Controlled powering off is done when the user requests it by pressing the power-key or when the battery voltage
falls too low.
When pressing the power key, the power off sequence is activated in UEMEK and software. Basically, the power
key causes UEMEK interrupt to UPP_WD2 and the software sets watchdog time value to zero. As this happens,
the power-up reset (PURX) is forced low and all regulators are disabled.
If the battery voltage falls below the very last software cutoff level, the software will power off the system by
letting the UEMEK’s watchdog elapse.
If thermal shutdown limit in the UEMEK regulator block is exceeded, the system is powered off and PURX is
forced low.
Uncontrolled powering off
Uncontrolled powering off happens when the battery is suddenly removed or if an over-temperature condition
is detected in the regulator block while in RESET mode. UEMEK’s state machine notices battery removal after
the battery voltage has been below VCOFF- for 5 µs and enters PWR_OFF mode. Then PURX is set low and all
UEMEK’s regulators are disabled immediately. Discrete regulators are disabled as VBAT supply goes low.
Watchdogs
There are three watchdogs in UEMEK.
The first one is for controlling system power-on and power-down sequences. It is used for powering the system
off in a controlled manner. The initial time for this watchdog after reset is 32 s and it can not be disabled. The
time can be set using a register.
The second one is for security block and is used during IMEI code setting.
The third one is a power key watchdog. It is used to power off the system in case software is stuck and the user
presses the power key. This watchdog is software configurable.
RM-51
System module and user interface
Nokia Customer Care
9238460 (Issue 1)
Company Confidential
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