STM32W108C8

System modules
STM32W108C8
In normal operation an application may request one of two low power modes through
program execution:
● Idle Sleep is achieved by executing a WFI instruction whilst the SLEEPDEEP bit in the
Cortex System Control register (SCS_SCR) is clear. This puts the CPU into an idle
state where execution is suspended until an interrupt occurs. This is indicated by the
state at the bottom of the diagram. Power is maintained to the core logic of the
STM32W108C8 during the Idle Sleeping state.
● Deep sleep is achieved by executing a WFI instruction with the SLEEPDEEP bit in
SCS_SCR set. This triggers the state transitions around the main loop of the diagram,
resulting in powering down the STM32W108C8's core logic, and leaving only the
always-on domain powered. Wake up is triggered when one of the pre-determined
events occurs.
If a deep sleep is requested the STM32W108C8 first enters a pre-deep sleep state. This
state prevents any section of the chip from being powered off or reset until the SWJ goes
idle (by clearing CSYSPWRUPREQ). This pre-deep sleep state ensures debug operations
are not interrupted.
In the deep sleep state the STM32W108C8 waits for a wake up event which will return it to
the running state. In powering up the core logic the ARM® Cortex-M3 is put through a reset
cycle and ST software restores the stack and application state to the point where deep sleep
was invoked.
6.5.3 Further options for deep sleep
By default, the low-frequency internal RC oscillator (OSCRC) is running during deep sleep
(known as deep sleep 1).
To conserve power, OSCRC can be turned off during deep sleep. This mode is known as
deep sleep 2. Since the OSCRC is disabled, the sleep timer and watchdog timer do not
function and cannot wake the chip unless the low-frequency 32.768 kHz crystal oscillator is
used. Non-timer based wake sources continue to function. Once a wake event occurs, the
OSCRC restarts and becomes enabled.
6.5.4 Use of debugger with sleep modes
The debugger communicates with the STM32W108C8 using the SWJ.
When the debugger is connected, the CDBGPWRUPREQ bit in the debug port in the SWJ
is set, the STM32W108C8 will only enter deep sleep 0 (the Emulated Deep Sleep state).
The CDBGPWRUPREQ bit indicates that a debug tool is connected to the chip and
therefore there may be debug state in the system debug components. To maintain the state
in the system debug components only deep sleep 0 may be used, since deep sleep 0 will
not cause a power cycle or reset of the core domain. The CSYSPWRUPREQ bit in the
debug port in the SWJ indicates that a debugger wants to access memory actively in the
STM32W108C8. Therefore, whenever the CSYSPWRUPREQ bit is set while the
STM32W108C8 is awake, the STM32W108C8 cannot enter deep sleep until this bit is
cleared. This ensures the STM32W108C8 does not disrupt debug communication into
memory.
Clearing both CSYSPWRUPREQ and CDBGPWRUPREQ allows the STM32W108C8 to
achieve a true deep sleep state (deep sleep 1 or 2). Both of these signals also operate as
wake sources, so that when a debugger connects to the STM32W108C8 and begins
accessing the chip, the STM32W108C8 automatically comes out of deep sleep. When the
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