Why Do Microcontrollers Have Multiple VDD Pins?

We all know that typical 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, as shown in the figure below, we can see that it has 5 sets of VDD/VSS, and each set of VDD/VSS must be connected. In fact, these multiple VDDs are internally connected together, so why don’t chip manufacturers just leave one set? Or, if multiple sets are left, 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’s path inside the chip is relatively long, as shown by the red lines in the figure below.

For circuits with high frequencies, the VDD current will have abrupt changes at high frequencies (since the higher the frequency, the more current is needed), at which point the inductive reactance generated along the path will hinder the current’s change. The longer the path, the greater the inductive reactance, thus the hindrance to current will be 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, while multiple VDDs can reduce inductive effects (inductance in parallel reduces total inductance, and shorter paths also mean lower inductance).

We can imagine the chip’s power supply process as the irrigation of a field; having only one water inlet is definitely not as effective as having multiple inlets.

In addition, a few more points:

1) Analog peripherals of MCUs, such as ADCs, usually have a separate power supply, VDDA, because ADCs require as clean a power supply as possible to ensure the accuracy of conversion results, and a separate line can avoid interference from noise from other power supplies.

2) Different 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 I/O operates at a higher voltage for better connection with external circuits. In this case, multiple power supplies are also needed, commonly seen in complex SOC chips.

3) Compared to a single VDD, multiple VDDs reduce the current flowing through each VDD pin, thus preventing any single pin from having to bear excessive current, enhancing reliability.