ST STM32F405 Reference Manual page 123

RM0090
Entering Over-drive mode
It is recommended to enter Over-drive mode when the application is not running critical
tasks and when the system clock source is either HSI or HSE. To optimize the configuration
time, enable the Over-drive mode during the PLL lock phase.
To enter Over-drive mode, follow the sequence below:
1. Select HSI or HSE as system clock.
2. Configure RCC_PLLCFGR register and set PLLON bit of RCC_CR register.
3. Set ODEN bit of PWR_CR register to enable the Over-drive mode and wait for the
ODRDY flag to be set in the PWR_CSR register.
4. Set the ODSW bit in the PWR_CR register to switch the voltage regulator from Normal
mode to Over-drive mode. The System will be stalled during the switch but the PLL
clock system will be still running during locking phase.
5. Wait for the ODSWRDY flag in the PWR_CSR to be set.
6. Select the required Flash latency as well as AHB and APB prescalers.
7. Wait for PLL lock.
8. Switch the system clock to the PLL.
9. Enable the peripherals that are not generated by the System PLL (I2S clock, LCD-TFT
clock, SAI1 clock, USB_48MHz clock....).
Note: The PLLI2S and PLLSAI can be configured at the same time as the system PLL.
During the Over-drive switch activation, no peripheral clocks should be enabled. The
peripheral clocks must be enabled once the Over-drive mode is activated.
Entering Stop mode disables the Over-drive mode, as well as the PLL. The application
software has to configure again the Over-drive mode and the PLL after exiting from Stop
mode.
Exiting from Over-drive mode
It is recommended to exit from Over-drive mode when the application is not running critical
tasks and when the system clock source is either HSI or HSE.There are two sequences that
allow exiting from over-drive mode:
•
By resetting simultaneously the ODEN and ODSW bits bit in the PWR_CR register
(sequence 1)
•
By resetting first the ODSW bit to switch the voltage regulator to Normal mode and then
resetting the ODEN bit to disable the Over-drive mode (sequence 2).
Example of sequence 1:
1. Select HSI or HSE as system clock source.
2. Disable the peripheral clocks that are not generated by the System PLL (I2S clock,
LCD-TFT clock, SAI1 clock, USB_48MHz clock,....)
3. Reset simultaneously the ODEN and the ODSW bits in the PWR_CR register to switch
back the voltage regulator to Normal mode and disable the Over-drive mode.
4. Wait for the ODWRDY flag of PWR_CSR to be reset.
RM0090 Rev 18
Power controller (PWR)
123/1749
149