Can anyone help me please!!!

Do you know how to read external clock use arduino mega 2560. Ex: Read encoder

What external clock? (clock=device or do you mean the "beat") What interface / protocol?

Without any details about your external device there is no chance to help you.

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How is "Can anyone help me please!!!" a meaningful subject line?

Do you know how to read external clock

What external clock?

Read encoder

Is an encoder a clock?

Looks like a cross-post of: Read encoder by arduino due