- Last edited 5 months ago ago
WWDG internal peripheral
1 Article purpose
The purpose of this article is to:
- briefly introduce the WWDG peripheral and its main features,
- indicate the peripheral instances assignment at boot time and their assignment at runtime (including whether instances can be allocated to secure contexts),
- list the software frameworks and drivers managing the peripheral,
- explain how to configure the peripheral.
2 Peripheral overview
The WWDG peripheral is a watchdog unit that can be used to protect the Cortex®-M4 based coprocessor firmware from endless loops or to monitor some real-time activities. This peripheral is clocked by the bus on which it is connected, thus it is frozen as soon as the system goes to Stop or Standby low power mode (except if the Stop emulation mode is enabled via DBGSTOP bit in DBGMCU_CR register). This block has an early interrupt feature that allows to get an interrupt (on GIC or NVIC) one cycle before reaching the final reset: this can allow to trigger a recovery mechanism on Cortex®-M4 or on Cortex®-A7.
On WWDG expiration, a MCU reset is generated, reseting Cortex®-M4 sub-system and the WWDG itself. This MCU reset also generates an interrupt on GIC thanks to EXTI. This allows Cortex®-A7 to detect Cortex®-M4 crashed and to recover it by stopping associated services, reloading Cortex®-M4 firmware and restarting Cortex®-M4.
Refer to the STM32MP15 reference manuals for the complete list of features, and to the software frameworks and drivers, introduced below, to see which features are implemented.
3 Peripheral usage
This chapter is applicable in the scope of the OpenSTLinux BSP running on the Arm® Cortex®-A processor(s), and the STM32CubeMPU Package running on the Arm® Cortex®-M processor.
3.1 Boot time assignment
The WWDG peripheral is not used at boot time.
3.2 Runtime assignment
3.2.1 On STM32MP15x lines 
Click on the right to expand the legend...
Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:
- ☐ means that the peripheral can be assigned to the given runtime context.
- ☑ means that the peripheral is assigned by default to the given runtime context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
- ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
- ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.
Refer to How to assign an internal peripheral to an execution context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possiblities might be described in STM32MP15 reference manuals.
4 Software frameworks and drivers
Below are listed the software frameworks and drivers managing the WWDG peripheral for the embedded software components listed in the above tables.
As there is only one WWDG counter cycle between the WWDG early interrupt and the WWDG reset generation, ST preconizes to allocate the WWDG early interrupt to Cortex®-M4 for a better reactivity and not to Cortex®-A7.
5 How to assign and configure the peripheral
The peripheral assignment can be done via the STM32CubeMX graphical tool (and manually completed if needed).
This tool also helps to configure the peripheral:
- partial device trees (pin control and clock tree) generation for the OpenSTLinux software components,
- HAL initialization code generation for the STM32CubeMPU Package.
The configuration is applied by the firmware running in the context in which the peripheral is assigned.