What is really the difference between FPGA and ASIC?

What's the difference between FPGA and MCU like AVR?

• 1) Is FPGA only for chip designer? 2) Can FPGA simulate a AVR chip? or it can simulate a capacitance?

• Answer:

  FPGA: An FPGA or a Field Programmable Gate Array, can be essentially be thought to be a collection of logic gates that you program into any configuration you like. So you can dynamically program the FPGA  to represent any logic that will fit on it. Thus representing many different chips using the same FPGA. MCU: A microcontroller can be thought of as a single instance of a logic circuit. It is essentially just a collection of logic gates wired according to some specific circuit diagram. More specifically it consists of a central processor and some peripherals connected to it. 1. Are FPGAs only for chip designers? Yes and No. FPGAs are incredibly useful for chip designers who can build working prototypes of their chips and use these to test all the stated functionality and detect faults. This can potentially save millions of dollars in a bad tapeout. Nonetheless they can also be used to develop standalone embedded systems. One example I know of is processing hardware for satellites. Infact I had a chance to some of these  on Astrosat [http://repository.ias.ac.in/58987/%5D. 2. Can an FPGA simulate an AVR? Can it simulate a capacitance? The short answer is: 'Yes it can!' Infact here's a link to a project that does just that: http://hackaday.com/2009/11/19/avr8-virtual-processor-on-fpga/ As for the answer for capacitance, the answer is much trickier. Like any logic circuit an FPGA will have it own maximum speed of operation (Speed Grade) that is limited by capacitance. So there are capacitive delays in the logic circuits. Unfortunately these are almost never the delays that you are designing for, when making a chip. So it is very hard to simulate for capacitance values seen in an ASIC design. Nonetheless you can always simulate a slower version of your design on an FPGA. Note however that by slower I mean of the order of hundreds of Mhz which is still much faster than an AVR