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?
Lomas Subedi - You have misunderstood.
The books say - "The 8051 has an on-chip clock generation circuit" and not 8051 has on-chip oscillator . The 8051 based architecture chips have on-chip clock generation circuit.
It simply means that there is a Mod-12 counter. The externally applied clock signal (which may be given from a function generator, crystal oscillator or RC oscillator, though crystal oscillator is preferred) will be divide by 12 and then applied to the CPU.
NOTE: The external oscillators (are ac circuits & not digital in nature) may not generate pure square wave(digital signal), i.e. the generated signal may have noise(ac component). Hence a counter is used. The output of this counter will be a square wave signal with frequency = osc. freq/12
The 12-clock feature of the original 8051 isn't to remove any noise. The crystal have two pins - you drive on one pin, and read back on the other pin. The read-back is a weak analog signal that will be processed to both generate the drive signal for the first crystal pin and generate the square wave for internal clocking.
That the chip have 12-clock logic is because it implements a state machine, where some of the 12 cycles are used for setting address pins, some for fetching instruction, some for decoding, ... By only using one phase of the input signal, it doesn't matter so much that the oscillator circuit will not manage a perfect 50% duty cycle.
More advanced processor cores do more things concurrently, where you have many instructions processed at the same time, but at different phases. So one instruction may be fetched while the next one is decoded and a third one is being executed. But it is a much more advanced design to have a pipeline where multiple instructions flows through the processor. The 8051 is very old, and made with a minimum of transistors. So one instruction is handled at a time, and the processor needs a large number of clock cycles to sequence all the "book keeping" for that single instruction.
Since the processor have two oscillator pins - one output for driving the crystal, and one input for reading back - it is possible to skip the processor pin that drives a crystal. Instead, an external oscillator module can be connected to the crystal input pin.
Many (most?) modern 8051 derivatives take less than 12 clocks per instruction.
Again, see the datasheet for the particular chip for specific details
NO, NO!! the '51 has an internal oscillator that requires an external crystal to function. there is no requirement for "a function generator, crystal oscillator or RC oscillator". some modern derivatives can even run (while not as precisely clocked) without a crystal.
The traditional 8051 is very old, and made with a minimum of transistors. So one instruction is handled at a time, and the processor needs a large number of clock cycles to sequence all the "book keeping" for that single instruction. this is not true for most modern derivatives
Erik
"this is not true for most modern derivatives"
Which is why I started my post with the sentence "The 12-clock feature of the original 8051 isn't to remove any noise." (emphasis on "original" added now)
Today, transistors are almost free. With a design where the inner transistors are very small and just the I/O transistors are scaled large for protection and current drive capability, the transistors consumes very little space and requires very little power. So no longer a need for long sequence machines just to process a single instruction.
Smaller transistors also means less capacitance and shorter travel distance for the signals, so there are less need for wait states for data to travel around the chips.