frequencymicrocontrolleroscillatortimer
I am a beginner with respect to microcontrollers, so I just want to ask is there two option for choosing the frequency of the microcontroller: to be internal or external?
If there is internal frequency available in the controller, then we don't need to use external crystal oscillator?
As others have said, accurate frequency and frequency stability are reasons to use a external ceramic resonator or crystal. A resonator is several times more accurate than the internal R-C oscillator and good enough for UART communication. A crystal is much more accurate, and necessary if you are doing some other types of communication like CAN, USB, or ethernet.
Another reason for a external crystal is choice of frequency. Crystals come in a wide range of frequencies whereas the internal oscillator is usually one frequency with maybe a choice of 4x PLL enabled. Some newer 24 bit core PICs have both a multiplier and divider in the clock chain so you can hit a wide choice of frequencies from the single internal oscillator frequency.
There are of course various applications that inherently require accurate frequency or timing other than communications. Time is the property in electronics that we can measure most accurately cheaply, so sometimes the problem is transformed into one of measuring time or producing pulses with accurate timing.
Then there are applications which require some long term synchronization with other blocks. A 1% oscillator would be off by over 14 minutes per day if used as the basis for a real time clock. Accurate long term time may also be needed without having to know real time. For example, suppose you want a bunch of low power devices to wake up once every hour to exchange data for a few seconds and then go back to sleep. A 50ppm crystal (very easy to get) will be off no more than 180ms in a hour. A 1% R-C oscillator could be off by 36 seconds though. That would add significant on-time and therefore power requirements to the devices that only needed to communicate for a couple of seconds every hour.
Yes, the 16F84 does need an external oscillator. It is a very old PIC.
However almost all of the newer PICs have an Internal RC Oscillator that can be selected, which will be mentioned in the datasheet.
I would really consider getting hold of a newer PIC, something like a 16F690, or 16F1824/16F1828. These are far more current, and can do anything the 16F84 can do and much more.
If you want to use your 16F84 though, either use an external clock (e.g. from 555 timer or oscillator based on e.g. an inverting gate with RC or crystal) or crystal as specified in the datasheet, or if you don't have an external clock or crystal use the RC option.
Best Answer
It depends on what you want to use the processor for, the manufacturer, etc. RC internal oscillators are "only" good for 1-2% accuracy, there are some processors that have tuning adjustment registers that allow you to shift the frequency, so with the right software/calibration, etc. you can get better results.
For (asynchronous) serial port type communications, the RC inaccuracy can become critical at high baud speeds (generally 230kbps and above, depending on processor and baud rate clock flexibility)
With RC oscillators you typically get 1 or 2 frequencies unless the MCU offers PLL capabilities in which case there can be a wide range. This can be used to advantage to dynamically scale the clock speed for battery powered situations where you can't go to 'sleep' due to complicated monitoring situations, but don't have to be running flat-out.
A crystal can get you any frequency you would want within the MCU operating limits, however you need to verify that the loading is correct so that the oscillator starts up correctly and is accurate and stable over time and temperature.
The RC internal oscillator is generally guaranteed to work by the manufacturer with no hardware design issues.