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.
How is "Can anyone help me please!!!" a meaningful subject line?
Do you know how to read external clock
What external clock?
Is an encoder a clock?
Looks like a cross-post of: Read encoder by arduino due