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I've seen a microcontroller (don't remember the exact part number at the moment,) that doesn't have XTAL pins.
It was an 8-pin MCU - it didn't have any clock input pins on the device.
How do those devices work without a 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?
Many (most?) MCU's have an internal oscillator that can be used as the clock.
Typically there will be a configuration that is held in non-volatile memory to set the clocking mode. The actual method will depend upon the specific microcontroller.
The downside to using the internal clock is that it is not very accurate, maybe +/- 1-2% and so cannot be used for applications that require high-accuracy such as for time of day. It may not even be accurate enough for reliable use as a clock for async communications (commonly referred to as RS232).
The AVR line of microcontrollers have an internal clock available. On many versions of AVR the speed of the internal clock is selectable from a few choices to optimize power.
The Microchip PIC range of chips (or at least some of them) can use their own internal oscillator, instead of an external clock.
Just look for internal oscillator in the documentation or features on an MCU. There are drawbacks and positives to using an internal oscillator
Chances are, your favorite MCU has an internal RC oscillator. There are numerous microcontroller families from all major manufacturers that include this module, including those from Texas Instruments, STMicroelectronics, and Microchip. There are also accompanying application notes online from virtually all manufacturers on how to calibrate the internal oscillator of their MCUs. https://www.allaboutcircuits.com/technical-articles/the-good-and-the-bad-of-mcu-internal-oscillators/
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?
The device operates the same way, the clock source is pulled from an internal clock source. A processor will not operate without a clock. To select the clock usually a JTAG is used to program the MCU and tell it to use an internal or external source. Upon startup the MCU reads the configuration from ROM and tells it which clock to use. Sometimes the internal clock is used and then it switches to an external clock. The difference is external clocks can be more precise so if you need more precision in timing then
In general, the programming device could be providing the clock needed to configure clock source if the clock is missing. That is not always the case though.
Many microcontrollers have an internal oscillator, with the specification explained on the datasheet. If more accuracy (and often higher clock frequency) is required, you'd use an external clock source.
The Atmega microcontrollers, such as the Microchip Atmega328p used in Arduino can use their internal oscillator. In the case of Atmega328 the clock source is set in "fuse" bits during the programming with an AVR ISP programmer.
Most of the microcontrollers have internal clock. The internal clock is usually resistor capacitor based (RC Oscillator) which is not very accurate (only 1% to 2%. External Oscillator are crystal based and are very accurate. PIC microcontrollers and many AVRs have internal clock. You can check the datasheet of the MCU for Oscillator details.
Yes, there are many MCUs with internal oscillators that require no external clock sources.
Most MCUs have several clock sources, and a clock source has to be explicitly selected to be something else than a default. The default is one of the internal oscillators. There may be more than one, in fact.
In some MCUs, the clock configuration is permanent and comes from a special "config word" written into the MCU by the programmer device. This "config word" is known by many names, e.g. "fuse word", and it may reside in the code address space, or outside of it.
In other MCUs, the configuration is volatile and is programmed into the clock control registers by the firmware as it starts up. The bootloader may change the clock configuration, and then once the firmware runs it may make further changes as needed.
MCUs with complex clocking have "clock watchdogs" that use the internal oscillator to detect presence of an external clock signal. When the external clock fails for any reason, the MCU can automatically switch to the internal clock as a fallback. The firmware can respond to it in an interrupt handler, and can take necessary recovery/failsafe actions.
Almost all Microcontrollers have a internal clock source. Some micro-controllers have features to use a external clock. If you are not concerned about any clock timing error in nano second scale, you can just use internal clock. Almost all real time operating systems use a timer based on internal clock of a microcontroller for their timing, scheduling and task functionality.A internal clock is usually accurate in millisecond scale.
All peripherals need a clock source to function and generate signals at accurate intervals. ADC, SPI, PWM, General purpose timer and others use the internal clock. They derive their clock from the main clock source of the microcontroller which is usually written as XTAL.
More info about XTAL and external clock EXTAL here:
