Kai Klaas wrote:
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"when you hit morse key, then AM carrier is modulated by some tone, sinus for smallest bandwith."

That is not correct. In CW communication, there is no modulation of the carrier at all. It is simply turned on and off as the key is pressed. The "beep" tone that you hear on the receiving side is produced entirely within the receiver, by beating the received signal against a local oscillator, which (after filtering) produces a tone whose frequency is the difference between the frequencies of the received signal and the local oscillator.

To decode the Morse code with a microprocessor, the audio "beep" tone is first converted to a binary signal with some sort of tone detector circuit, like the NE567. The binary signal is then fed into the microprocessor for conversion to plain text. The conversion algorithm will depend on whether you are trying to receive perfect machine-generated code at a known speed, or human-generated code at variable speeds. Reading machine-generated code is trivial; human-generated code can be very complex depending on the accuracy you are trying to achieve. In either case, the technique is similar to a software UART where the "serial bit stream" is the signal coming from the tone detector circuit.

It is also possible to perform the tone detection in software and eliminate most of the tone detector circuitry, but that's probably more complex than the original poster was thinking about, and the little 8051 might not have enough speed for that.

The various ham radio publications used to carry contruction articles for this kind of stuff all the time, but I don't know if they still do - I've been away from the hobby for many years. You might be able to find some relevant info at www.arrl.org