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I have seen many microcontrollers having different and multiple power rails.

For example, 5V, 3V3 & 1V8 are the power rails in the same microcontroller.

Why is there a necessity of three different power rails in a single microcontroller?

Can someone tell me.

And another thing I observed is that, the microcontroller datasheet recommends that each power rail should have decoupling capacitors. That's fine. But the datasheet recommends a bigger capacitor and smaller capacitor.

For example, for the 3V3 rail, the datasheet recommends a 4.7 uF capacitor close the pin and another 100 nF behind the 4.7 uF capacitor? Why do we need different capacitors if its only for decoupling? Why can't we use 2 numbers of 4.7 uF capacitor instead of 1x 4.7 uF capacitor and 1x 100 nF capacitor?

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    \$\begingroup\$ Can you add a link to the specific datasheet you are referring to? The general answer can probably be given with the information you have provided, but for more accurate insights the datasheet is necessary. \$\endgroup\$ Commented Oct 19, 2021 at 6:59
  • \$\begingroup\$ Thank you for the comment. But I don't have the specific datasheet. I have used a few NXP and Microchip microcontroller and most of them seem to have more than 1 power rails and in some cases they have 3 power rails. I am not sure why its necessary to have more power rails. \$\endgroup\$
    – user220456
    Commented Oct 19, 2021 at 7:16
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    \$\begingroup\$ @Newbie well you should look at some of the datasheets what the rails are used for. We can only guess. \$\endgroup\$
    – Justme
    Commented Oct 19, 2021 at 7:20
  • \$\begingroup\$ It really depends on the internal tech. Some are powered by, say, 3.3V and have an internal core running at 1.2V. Maybe they have a convenience LDO integrated so it seems they are only at 3.3V but it generate it's own core supply. As for the capacitor different capacitors are effective at different frequencies \$\endgroup\$ Commented Oct 19, 2021 at 7:30

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An MCU that has USB functionality couid have three supply voltages. 5V for powering up from USB 5V, internal 3.3V regulator to use 3.3V as the IO pin voltage for communicating with other devices, and internal 1.8V regulator for powering the MCU core.

Different capacitors are used because the work at different frequencies. A 100nF cap is more effective at higher frequencies than 4.7uF, and 4.7uF cap is more effective at lower frequencies as it can store more energy. The internal regulators in the MCU can have some capacitance requirement for stability like any other discrete LDO voltage regulators. So a single 100nF or single 4.7uF would have a poor decoupling or stability performance at the frequency range of interest.

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  • \$\begingroup\$ Thank you for the answer. When you say, 100nF cap is more effective at high frequencies, you are mentioning about the ability of that capacitor to bypass the AC noise and disturbances to the ground, Am I correct? \$\endgroup\$
    – user220456
    Commented Oct 19, 2021 at 7:18
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    \$\begingroup\$ I would also add that in case the MCU has an integrated ADC, usually it's powered from a dedicated pin, will require special decoupling, and maybe even an external reference. \$\endgroup\$ Commented Oct 19, 2021 at 7:18
  • \$\begingroup\$ @Newbie Basically yes. \$\endgroup\$
    – Justme
    Commented Oct 19, 2021 at 7:24
  • \$\begingroup\$ @Newbie There is this thing called "reality" that gets in our way. In theory all capacitors are ideal, and you will not need the 100nF if you have the 4.7µF. But real things have properties like series impedances and frequency dependencies and so on. Grab a good book on electronics and read. \$\endgroup\$ Commented Oct 19, 2021 at 14:56

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