We know that general IC components usually have only two power pins, one is Vcc or Vdd, and the other is Gnd or Vss. However, modern MCUs often have multiple power pins. What is the reason for this?
Taking a certain 100-pin MCU as an example, we can see that it has 5 sets of VDD/VSS, and it requires each set of VDD/VSS to be connected. In fact, these multiple VDDs are internally connected to each other. Since this is the case, why don’t chip manufacturers leave only one set? Or can we connect only one set externally? If this is possible, it would save pins and reduce external wiring.
If only one set of VDD is connected externally, the current path inside the chip is relatively long, as shown by the red lines in the diagram below.
For high-frequency circuits, the VDD current will change abruptly at high frequencies (as the frequency increases, the required current also increases). At this time, the inductive reactance generated along the path will hinder the change of current. The longer the path, the greater the inductive reactance, thus the obstruction to the current will become more pronounced, leading to voltage changes in the chip network, which can affect the normal operation of the MCU system. For low-frequency or DC circuits, the long path has less impact. Therefore, early low-frequency MCUs had only one VDD. Multiple VDDs can reduce inductance effects (the total inductance decreases when inductors are connected in parallel, and a shorter path also reduces inductance).
You can imagine the power supply process of the above chip as the process of irrigating a piece of land. If there is only one water inlet, it will definitely not be as good as having multiple inlets for water.
In addition, a few more points:
1) The analog peripherals of the MCU, such as ADC, usually have a separate power supply VDDA. This is because ADCs require as clean a power supply as possible to ensure the accuracy of the conversion results. A separate line can avoid interference from noise on other power supplies.
2) Various voltage requirements. Sometimes, different parts of the chip operate at different voltages. A typical example is a low-voltage core and high-voltage I/O. The core uses a lower voltage to reduce power consumption, while the I/O operates at a higher voltage to better connect with external circuits. In this case, multiple power supplies need to be separated, which is common in complex SOC chips.
3) Compared to a single VDD, having multiple VDDs reduces the current passing through each VDD pin, so that the pins do not have to bear excessive current, enhancing reliability.
Author: wuyage, Source: TopSemic Embedded
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