ST STM32L4+ Series Reference Manual page 192

Power control (PWR)
1. Start SMPS converter (if not always enabled by HW).
2. Check that SMPS converter output is at the correct level, like 1.25 V ≤ VDD12 < 1.32 V.
3. Connect VDD12 to external SMPS converter through the switch.
•
Range 2 to SMPS Range 2 low and high:
1. Start SMPS (if not always enabled by HW).
2. Check that SMPS output is at the correct level like1.05 V ≤ VDD12 < 1.32 V.
3. Connect VDD12 to external SMPS converter through the switch.
–
1. Set register FLASH_CFGR bit LVEN to 1.
2. Adjust the number of wait states in the FLASH_ACR (up to max frequency of Range 1
refer to
3. Increase the system frequency up to the maximum allowed value for voltage Range 1
(like 80 MHz when 1.08V < VDD12 < 1.14 V or 120 MHz when VDD12 > 1.14 V).
•
SMPS Range 1 to Range 1 or SMPS Range 2 low and high to Range 2:
1. If in Range 1, reduce the system frequency to a value lower or equal to 26 MHz.
2. Adjust number of wait states according new frequency target corresponding to voltage
range (LATENCY bits in the FLASH_ACR).
3. Set register FLASH_CFGR bit LVEN to 0.
4. Disconnect VDD12 by opening the switch.
5. Stop SMPS (if required and not kept always enabled).
5.2 Power supply supervisor
5.2.1 Power-on reset (POR) / power-down reset (PDR) / brown-out reset
(BOR)
The device has an integrated power-on reset (POR) / power-down reset (PDR), coupled
with a brown-out reset (BOR) circuitry. The BOR is active in all power modes except
Shutdown mode, and cannot be disabled.
Five BOR thresholds can be selected through option bytes.
During power-on, the BOR keeps the device under reset until the supply voltage V
reaches the specified V
device reset is generated. When V
released and the system can start.
For more details on the brown-out reset thresholds, refer to the electrical characteristics
section in the datasheet.
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If 1.08 V ≤ VDD12 (like SMPS Range 2 high), then the following steps can be
applied:
Section 3.3.3: Read access
threshold. When V
BORx
latency).
drops below the selected threshold, a
DD
is above the V
DD BORx
RM0432 Rev 6
upper limit, the device reset is
RM0432
DD