Introduction
In embedded development, current backflow is an easily overlooked issue that can lead to serious consequences. This chapter will delve into the causes, impacts, and how to identify and prevent this problem in practical engineering.
Current backflow issues rarely occur in the era of 5V microcontrollers, primarily due to the high voltage tolerance of the 5V microcontroller IO ports and the excellent internal structure design for IO protection. In the era of 3.3V microcontrollers, such issues have a certain degree of sporadic occurrence, but they are still rare. Despite the reduced voltage tolerance of the microcontroller IO ports, the internal protection design remains robust. Additionally, the time from power-on to program execution is very short; after proper initialization, the IO ports can match well with peripheral levels, thus avoiding current backflow. However, with the current MPUs, the processor scale is relatively large, with many IO ports, and the system power usually has several different voltage rails, along with strict power-on sequencing requirements. Coupled with the multi-stage startup of the processor, the startup time is significantly longer. If the levels are not handled carefully during the time from power-up to IO port initialization, current backflow may occur.
Current backflow is a very subtle issue, categorized as a “chronic disease”, which is difficult to detect immediately during product usage. Issues may only arise after long-term operation in the application field, and the manifestations of these issues are very diverse. Even if a solution is provided, the effect is not immediate; it requires long-term operation to verify the effectiveness of the improvement plan.
If a product experiences a failure after running in the field for an extended period, and the same model cannot be reproduced in the laboratory, it can be considered to investigate whether there is current backflow in the circuit design, assuming the basic localization is a hardware issue. Based on years of application experience, the following faults/issues can be investigated for current backflow:
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Intermittent restarts or crashes after running in the field for several months; the fault disappears after replacing the core board;
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Intermittent network anomalies or abnormal IO control of devices after running in the field for several months; normal operation after replacing the core board;
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The system fails to boot after running in the field for several months; normal operation after replacing the core board; the faulty board still cannot start the system after replacing the baseboard in the laboratory.
Current backflow is also a common issue. Searching for “IO current backflow” online yields many related cases and articles on solutions, as shown in Figure 1.
Current Backflow and IO Current BackflowCurrent backflow refers to the phenomenon where the direction of current flow is opposite to the conventional flow direction. In a circuit, the current typically flows from the positive terminal of the power supply, through the load, and into the negative terminal of the power supply. However, in certain situations, such as when the supply voltage is too high, protective measures are insufficient, or circuit design errors occur, the current may flow in reverse, returning from the negative terminal to the positive terminal. This phenomenon is termed current backflow.
IO current backflow refers to the phenomenon where current flows back from the power line or signal line into the chip or circuit in electronic devices. This is usually due to improper configuration of the power or signal lines, or design errors in the circuit.
The potential consequences of current backflow exhibit uncertainty, diversity, and concealment in symptoms, often making them difficult to comprehend. Mild cases may lead to signal distortion or instability, affecting the performance and stability of the device. Severe cases may result in one or more IOs malfunctioning or damaging IO pins, causing corresponding peripheral control anomalies, and may even lead to communication interface issues. In more severe cases, the system may crash, resulting in inexplicable faults, with the most severe cases burning out the CPU, necessitating repair or disposal.
Follow us for the next analysis: Why do the above phenomena occur due to current backflow?
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