The application of embedded systems is expanding rapidly, and they often operate in environments that are prone to interference or harsh conditions, which can affect their stability and reliability. A key factor in maintaining system stability is the proper implementation of reset mechanisms. Designing an effective reset circuit and using reset operations strategically can significantly enhance the reliability and performance of the entire system. This paper explores the practical applications and design considerations of reset in embedded systems, based on real-world project experience, and highlights its critical role in ensuring system stability.
During the operation of an embedded system, if a program experiences a crash, jumps unexpectedly, or behaves erratically, a hardware-specific interface can be triggered—either manually or automatically—to restore the software to a predefined segment of code. This process is known as a reset. The signal sent to the hardware’s specific interface during this process is called a reset signal. To counteract instability caused by internal factors such as clock source instability or external factors like radio frequency interference, both the software and hardware of the embedded system must be carefully designed and protected. Reset operations serve as an essential safeguard in this regard. However, it's also important to note that improper reset procedures can themselves introduce instability into the system, so careful attention must be given to the design and implementation.
This article draws on the author’s personal experience to illustrate the importance of reset in embedded systems. By utilizing reset mechanisms intelligently, the system can achieve greater stability and reliability.
1. Reset Methods and Approaches
In an embedded system, the reset process typically involves two main components: the reset of the processor itself and the reset of peripheral devices (external function modules), as illustrated in Figure 1.
[Image: Reset application stability in embedded systems]
Generally, embedded system resets fall into two categories: hardware reset and software reset. A hardware reset involves triggering a reset signal through physical means, such as a dedicated reset pin. When a reset signal is applied to the RST pin for a certain duration (depending on the system's machine cycle and processor specifications), the CPU samples the signal and initiates the reset sequence. Common types of hardware reset include power-on reset, button reset, voltage monitor reset, and watchdog reset. These signals can be integrated into the system’s logic design and connected to the RST terminal.
A software reset, on the other hand, is initiated through the software framework to reinitialize the system. Resets can also be categorized based on whether they are internal or external to the processor, resulting in on-chip and off-chip resets.
From a signal level perspective, hardware resets can be either high-level or low-level. A high-level reset is active when the signal is high and completes the reset on the falling edge of the pulse. A low-level reset is active when the signal is low and completes the reset on the rising edge. While the choice depends on the system, most embedded systems use low-level reset due to lower current consumption in TTL circuits.
2. Implementation and Stability Design of Power-On Reset
2.1 Power-On Reset (POR)
Power-On Reset (POR) is a fundamental process that ensures the embedded system starts up correctly. It involves applying a reset signal to the RST pin until the power supply stabilizes, after which the reset is released. During POR, the system undergoes a complete initialization, resetting all logic cells and modules within the processor and initializing internal functions such as memory controllers, interrupt handlers, and I/O configurations.
Many microcontroller chips have built-in POR circuits. For example, the MCU shown in Figure 2(a) has an internal POR that generates a reset signal upon power-up, as illustrated in Figure 2(b). Some processors require an external RC delay circuit to generate the necessary reset pulse width, which depends on the chip’s requirements. An excessively large capacitor may result in a long reset time, while a too small capacitor might not provide enough time for the system to stabilize.
2.2 Power-On Reset Failure and Mitigation Strategies
In practice, power-on resets can fail due to various reasons, such as power fluctuations, interference, or misoperations. A short voltage drop might prevent the system from restarting properly if the reset signal cannot meet the POR condition. Similarly, a brief power supply switch may not allow sufficient time for the reset to take effect, leading to system crashes.
To address these issues, designers often adjust the reset threshold to ensure it falls within the normal operating range of the processor. Extending the reset signal duration allows the system to remain in reset long enough for the power to stabilize before releasing the reset. Additionally, a diode discharge circuit can be added to an RC circuit to quickly discharge the capacitor during a sudden power drop, improving the reliability of the POR mechanism.
2.3 Case Study: Microcontroller Startup Instability in Fast Switching Systems
In a ZigBee IoT project, a problem was observed where the ZigBee module would rapidly shut down and restart, causing instability during power-up. After analyzing the system, it was found that the MCU failed to initialize properly when the system was powered on and off quickly. The software logs revealed that the MCU did not enter the required initialization routine, leading to inconsistent behavior.
By introducing a delay between power cycles, the system could start up reliably. This issue is common in embedded systems, where insufficient reset timing can lead to unstable operation after power cycling. Proper reset design is therefore essential for ensuring consistent and reliable system behavior.
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