[Design Application] Detailed explanation of reset source of EFM8SB2 series MCU

The EFM8 Sleepy Bee 2 series is one of Silicon Labs' most energy-efficient 8-bit MCUs, making it ideal for systems with high power requirements. Its 120uA/MHz operating power consumption and 0.5uA sleep current can greatly extend the battery life.

The EFM8SB2 series has rich integrated resources. It not only provides up to 8KB of flash memory space, but also provides a variety of peripheral interfaces, including I2C, UART, SPI, and the chip integrates a 12-bit ADC and comparator, which can be used for low voltage detection.

In addition to the above rich resources, EFM8SB2 also has a variety of reset sources that can reset the MCU. First of all, any MCU has a reset source. As the name implies, reset is to make the MCU start from the initial state. This article will focus on the reset sources of EFM8SB2:

The reset circuit puts the registers into a predefined default state. When the MCU enters this reset state, the following process occurs:

• Stops the execution of the core program.

• All registers are reset to the predefined values ​​noted in the register descriptions.

• The external I/O port pins will be placed in a known state.

• Interrupts and timers will be disabled.

The contents of the RAM are not affected during reset; previously stored data remain unchanged until power is removed. Port I/Os are reset to 1 in open-drain mode. The chip's internal weak pull-ups are enabled during and after reset (can be re-disabled by software), and for power-on reset, the RSTb pin is driven low until the MCU exits reset. After exiting reset, the program counter (PC) is reset and the system clock defaults to the internal oscillator. The watchdog timer is enabled and software begins program execution from location 0x0000.

After the MCU is reset, the value of the RSTSRC register can be read by software to determine the reset source. For example, if RSTSRC = 0x10 is read, it means that the last reset was triggered by software.

The reset sources of the EFM8SB2 series MCU include the following:

• Electrical reset

• Hardware external reset pin RSTb reset

• Comparator reset

• Software triggered reset (RSTSRC | = 0x10;)

• Power Monitor Reset (Monitors VDD Power)

• Watchdog Timer Reset

• Missing Clock Detector Reset

• Flash error reset

• RTC0 alarm or oscillator failure

In addition, wake-up and reset are two different states, and wake-up is relative to sleep. For the EFM8SB2 series MCU, there are three low-power modes, namely idle, suspend, and sleep. For example, after entering the suspend mode through software setting (PMU0CF|= 0x40;), the MCU enters a state of waiting for wake-up, and the status of the registers and port I/O remains unchanged. At this time, if a port match event occurs, the MCU will wake up immediately, and the software will start executing from the next statement entering the suspend state, while reset starts execution from 0x0000. This is the biggest difference between the two.

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