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Have you noticed that most microcontrollers operate at 3.3V instead of 3V or 3.5V? Why is that? If you know the reason, please tell us in the comments.
Here is the transcript of the video:
Every number in electronics is not just thrown out randomly.
3.3V is derived from multiple factors, involving level compatibility, as well as the evolution of semiconductor processes, and even considering commercial compatibility issues.
The 3.3V power supply voltage was standardized and began to be used after the 1990s. Before that, it was all 5V. To understand the origin of 3.3V, one must first clarify how we transitioned from 5V to 3.3V, as well as the origin of 5V.
5V was defined as the logic level standard for TTL logic gates, which were commonly used in early digital circuits to perform many circuit functions. The operating voltage that allowed transistors to switch between saturation and cutoff was 5V. Therefore, 5V was defined as the TTL logic level standard. Before the 1990s, to maintain level compatibility, and because semiconductor processes were still relatively backward at that time, the main logic power supply and interface voltage remained at the 5V level standard. For example, the well-known 51 microcontroller matched the 5V logic level standard. Although 5V has strong noise immunity, it operates at a higher voltage and consumes more power.
After the 1990s, with the evolution of semiconductor processes, the number of transistors on chips increased exponentially. To reduce the overall power consumption of chips, a lower voltage standard was needed. Semiconductor processes evolved to 0.6um, 0.5um, and 0.35um. In semiconductor manufacturing, it was discovered that as the process technology improved, the thickness of the MOS gate oxide layer decreased. For instance, at 0.35um, the gate oxide thickness reduced to about 7nm, and the maximum source-drain voltage it could withstand was about 4V. Subtracting a 10% safety margin from this voltage gives us 3.6V. Additionally, the power supply network on the board generally ensures a ±10% margin, which leads us to 3.3V. The first CPU to use a 3.3V operating voltage was Intel’s Pentium P54C.
With the continuous evolution of semiconductor processes, in addition to 5V and 3.3V, there are also 2.5V, 1.8V, and 1.2V logic voltage supply standards. Typically, the faster the circuit runs and the lower the power consumption, the lower the logic level needs to be.
To clarify the origin of 3.3V, I consulted several PhDs and postdocs in microelectronics. As mentioned earlier, the origin of 3.3V must be derived from multiple factors and certainly was not thought up arbitrarily.
It is important to note that it is not true that chips can only operate at 3.3V; there is a ±10% margin, meaning they can function normally at voltages as low as 3V and as high as 3.6V.
Lastly, I want to add a very important point regarding applications: when can 3.3V be compatible with 5V? Taking the STM32 chip as an example, the logic levels for output and input at various pins are 3.3V. To enhance the chip’s compatibility, some pins can tolerate 5V, meaning that although the operating voltage is 3.3V, if the input is at 5V, it will still be recognized as a high level. The datasheet indicates which interfaces can tolerate 5V; those marked with “FT” (Five Tolerate) indicate that this pin can normally recognize a 5V logic level signal without damaging the chip due to the logic level voltage exceeding its 3.3V standard. Here, tolerance means normal recognition without damage. If the pin is set to analog input mode, it cannot accept 5V.
In hardware, many standards or experiences have been continuously used. I wonder if, like me, you also think about why certain designs are made and why specific values are chosen during usage. The exact origin of the 3.3V value may not affect our application design, but sometimes digging into the past might reveal interesting stories behind electronics.
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We are Nimo, the founders of Darwin. We only talk about technology and not about dating. Darwin is an online education platform aimed at serving professionals in the electronics industry, providing skill training videos covering popular topics in various subfields, such as embedded systems, FPGA, artificial intelligence, etc. We tailor layered learning content for different groups, such as commonly used knowledge points, disassembly evaluations, electrical competitions/intelligent vehicles/postgraduate entrance examinations, etc. Welcome to follow us.
