I read about different parts of micro-processor like ALU, registers etc. all different digital parts. Are there any analog parts inside the processor?
This is a complex question, because what actually makes a part "digital" can have multiple definitions.
Fundamentally, reality is analog (at least at the scales which most microprocessors operate at). Therefore, you can make a coherent argument that there are not actually any digital microprocessors. "Digital" is a theoretical mechanism for simplifying the expression of analog systems where the analog voltages therein are (as much as possible) constrained to two states, each of which represent a boolean value.
This simplification makes it much easier for our puny human brains to contemplate complex systems, and much easier for people to write software to evaluate the behaviour of said complex systems.
However, if you are asking if any components inside most microprocessors operate outside this simplified view, the answer is generally no.
- Some microprocessors have integrated ADCs (analog-digital converters), which by definition must operate at least partially outside of the digital simplification.
- Some microprocessors also have DACs (Digital-analog converters), which are much the same as ADCs.
- Some microprocessors have analog comparators that can be configured to act upon input analog signals.
- Schmitt trigger input buffers are also partially analog.
Basically, at this point, the question is more, assuming you're asking about whether components inside a MCU operate outside of the digital simplification, the question then becomes "How do you define a microprocessor"? Fundamentally, the *CPU core( of almost all microprocessors is purely digital.
However, many, many microprocessors integrate on-die peripherals like the ones mentioned above that are very much "analog" devices, so you must ask if you are defining the entire integrated-circuit as the "microprocessor", or just the actual processing core, which may only be a small part of the actual processor's IC die.
I've reverse-engineered old microprocessor chips and there are some things that are more analog than digital. For instance, many chips (e.g. Z-80) implement register storage with a pair of inverters connected in a loop. To write a new value to the bit, a larger transistor "overpowers" the smaller inverter transistor, forcing the desired value into the bit. It's not just 1's and 0's, but the relative currents that matter here.
Reading a register value can be more analog than digital. For example, the 8085 uses a moderately complex differential amplifier to read register values.
Another analog circuit in some processors is the substrate bias generator, basically a charge pump to generate the desired substrate voltage without requiring another supply.
The 8008 microprocessor has an on-chip reset circuit that detects power-on. This circuit is analog, using a diode drop to set the voltage thresholds. (Most processors have a reset pin, but pins were scarce on the 8008 because Intel insisted on an 18-pin package.)
Pass-transistor latches are very common in microprocessors. This type of latch depends on the charge stored on the capacitance of an unconnected gate. You can think of it as digital, but it's more analog than regular gates.
The analog vs digital distinction becomes an issue when you're simulating a chip. If you can simulate everything digitally it's easier, but for some chips you need a more physics-based simulation that keeps track of the amount of charge moved around rather than just 1's and 0's. An example is when a circuit has transistors of different sizes and you need to determine the "winner".
A modern example of analog circuitry in a microprocessor is Intel's hardware random number generation. This circuit uses a metastable latch to generate bits from thermal noise. (The resulting bits are genuinely random, not pseudo-random.) An analog bias circuit ensures metastability.
Is Micro-processor completely Digital?
it depends on your definition of "digital": any digital signal is actually represented in analog form.
traditionally, we don't pay much attention to that as it has limited practical implications.
Are there any analog parts inside the processor?
Yes. there are analog input circuitry; there are analog comparators; they are opamps; there are pgas, there are programmable current sources; there are adc, and they are dac, to list a few.
As mentioned above, virtually all contemporary microprocessors and other "digital" ICs provide the users a digital abstraction. There is automated and tool-based design by the chip designers at the register transfer level. But ultimately the design gets down to not just analog circuitry (e.g., a transistor and a capacitor) but down to physics. Very dense circuits differentiate between a "1" and a "0" essentially by counting electrons.