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3. TECHNICAL BRIEF
3.2.7 External Reset Handling
The chip reset can be controlled by an external RESET_N ball. If this ball is pulled low, the chip will be reset.
All PMU registers are reset during the external reset including LSIM control bits. The PMU statemachines are
also not reset from the external reset. An SW or watchdog reset will not reset the PMU registers. A SW and
Watchdog reset is seen on the reset_n pad to allow the reset of external devices. Basically there are three
reset sources, first the reset signal controlled by the PMU (reset_pmu_n_o), second the reset signal controlled
by the SCU (resetout_o) and third the external reset (RESET_N). The SCU reset is triggered by SW (for example
due to a SW reset or watchdog reset). The PMU reset is controlled by the PMU state machine. The output of
the reset handling block is the reset_postscu_n_o signal. This signal controls for example the C subsystem
and releases reset for the controller. During normal start up, the PMU releases the reset_pmu_n_o signal after
entering the SYSTEM ON state. At this time the resetout_o signal is high, the RESET_N pad is not pulled low
and therefore the reset_postscu_n_o signal follows the reset_pmu_n_o signal. That means the C reset will
be released and the C starts operation. If the SW triggers an external reset via the SCU, signal resetout_o will
be forced to low for a certain time and RESET_N will be forced to low by the open drain driver. At the same
time the feedback to the SCU will be masked to not reset the baseband. The RESET_N pad is in the VDDRTC
domain but the internal pull up is connected to the VDD_VDIG1 (1.8V) domain. That allows the pad to be
used as reset for external devices running in the VDD1V8 domain. The RESET_N pad can also be used to
monitor the chip internal reset condition during startup.
The open drain driver is a weak driver, that means it can be forced to high during debug from external
pushing some current into the pad. In testmode signal reset_pmu_n_o is high, that means the chip reset is
fully controlled from external.
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3. TECHNICAL BRIEF
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