It is said that 8051 has an on-chip oscillator but requires external clock to run it. What may be the internal circuit for on-chip oscillator, how it runs after connecting crystal oscillator? Also what is the purpose of external capacitor with crystal oscillator?
In fact, I would suspect that most 8051 applications use just a crystal and do not use an external clock Depends on the manufacturer and IC. (refer to the respective datasheet).
The original 8051 architecture was designed such that crystal or RC oscillator or an external clock generation circuit can be used as clock input to 8051.
The specific device Datasheet will contain details of how long to allow for the oscillations to become stable... The reference text books explaining the 8051 architecture don't contain any such topic or even a Note mentioning this. And the OP is a newbie, may be a student who has just started studying from the book (he has posted the link).
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In the TTL oscillator circuit, the terminals of the component pick up the ac noise, which is amplified and fedback to the input.
Meant RC Oscillator and not TTL. Note was about the RC oscillators which are AC circuits.
An oscillator obviously need an amplitude larger than 1. But depending on design, it needn't be close to 1. The TTL inverter have an amplification that is way much more than 1.
oscillators have gain slightly more than 1. The amplitude must be definitely more than 1 (if units is Volts)
"The original 8051 architecture was designed such that crystal, or RC oscillator, or an external clock generation circuit can be used as clock input to 8051"
Yes. Exactly.
So it is incorrect to say that it requires an external clock to run it.
Sure, an external clock can be used, but it is not required.
That was my point.
"RC oscillators ... are AC circuits"
As is a crystal oscillator - the crystal provides the resonant element, as does the RC in an RC oscillator.