Reset is the one of the first step whenever any microcontroller power’s up. We need to understand the STM8 reset process so we can utilize the proper initialization process of the STM8 microcontroller. One more thing this will help to understand is when the microcontroller reset due to some fault or some external interference. By understanding the reset process will help you to master this process and to capture any faults you may encounter.
Types of Reset Resources in STM8
According to the STM8 reference manual there are 9 kinds of reset sources in the STM8 microcontrollers. Here is the total list of the sources.
- External reset through the NRST pin
- Power-on reset (POR)
- Brown-out Reset (BOR)
- Independent watchdog reset (IWDG)
- Window watchdog reset (WWDG)
- Software reset
- SWIM reset
- Illegal opcode reset
- EMC reset: generated if critical registers are corrupted or badly loaded
STM8 Internal Reset Circuit
If you look into the STM8 reset circuit from the STM8 reference manual it looks like the diagram below grabbed from the reference manual. The reset pin is called the NRST Pin.
External reset through the NRST pin
The NRST pin is a special pin on the microcontroller that can be used to reset the device. The NRST pin has an inbuilt RPU weak pull-up resistor and functions as both an input and an open-drain output. When a low signal is sent to the NRST pin for a certain amount of time, it causes the microcontroller to reset. This reset can happen even if the microcontroller is in a HALT state. The NRST pin can also be used to send signals to other devices to reset them.
Power-on reset (POR) and brown-out reset (BOR)
The POR (Power-On Reset) is responsible to keeps the device in a reset state. This keeps it in reset state until the power supply voltages reach a stable level (VDD and
VDDIO) Once the voltages are stable then the BOR (Brown-Out Reset) takes over and the POR is disabled. The BOR keeps the device reset until the power supply voltage reaches the normal operating range.
If the power supply voltage drops below a certain level, the BOR will trigger a reset, and the POR will be reactivated for the next power-up cycle. To prevent false resets, the BOR uses a hysteresis circuit to ensure that the voltage changes are significant before triggering a reset. Even when the device is in a low-power mode, the BOR remains active to monitor the power supply voltage.
Software Reset
Software reset is a process to reset the microcontroller through internal program. There could be many condition where we need the microcontroller to be start again from the beginning within the internal software process. This could be done by using the Flag bit in the Register WWDG_CR and the flag bit is called the T6 bit of that WWDG_CR Register.
SWIM reset
This is a special pin used to program the microcontroller via STLINK and if the device is connected to this pin it can request the microcontroller to be in reset state through the software reset via this SWIN Reset request.
An external device connected to the SWIM interface can request the SWIM block to
generate an MCU reset.
