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Are there any Microcontroller's that do not require any external clock source for working

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Full Member level 6
May 10, 2020
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I've seen an MCU (don't remember the exact part number at the moment), that doesn't have XTAL pins.

Like, it was an 8-pin MCU. Like, it didn't have any clock input pins on the device.

Just want to understand, if anyone has worked with such devices? In that case, how do those devices, start to work? Like, without clock, and with only power, how does the device start working?

I understand that there are internal oscillators that would help. But just not able to understand how the device would start up? Like, how would it be for programming and other stuff?

Even when MCUs have pins for external oscillator or crystal connection, a lot of them have internal RC oscillator that you could connect to the rest of the embedded systems or even outside that chip to the rest of your board.

A lot of STMicroelectronics® MCU chips provide this feature.

Many MCUs have internal clocks, most also allow external clocking as well.
For the programming sequence, the bytes are loaded by setting bits and voltages on the pins, there is no need for the clock type or frequency to be known. The exception is when a serial bootloader is present so the clock and serial speeds have to be known beforehand but even then some mechanism has to exist to install the bootloader.


Key in this discussion is accuracy of internal clock as many applications
are concerned with time and frequency related tasks, like measuring or
generating pulses or other clocked devices, communications, eg. serial

Key is what those specs are over T and V as RC based oscillators are
not exactly considered "precision". Many are 10% kinds of numbers,
although newer devices +/- 2%. Onchip oscillators are typically
trimmed during final test. Some even have cal routines to improve,
under program control.

Typical table :


As you can see locking to USB in those applications can provide fairly accurate clocking.

Most, if not all, have PLLs to generate the higher freqs necessary by fast processors.

Regards, Dana.

Internal XO may be limited by available frequency options, even overtone operation may fall short by modern clock frequency standards? Above 50Mhz or so you may need a more complex clock source (like PLL upscaling from a 10Mhz ref source).


Unusual if a datasheet doesn't have sections dedicated to power-up behaviour and onboard oscillator, and surely expected waveforms. I suppose you ask in abstract sense and not about internal start-up circuits that are sometimes used in voltage references or to kick-start OA current sources and biasing, etc.

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