Data processing unit having multiple-purpose port used as a resonator connection port in first mode and as a data i/o port in second mode

A data processing unit provided with at least first and second ports and fabricated on a single semiconductor chip, said first and second ports being coupled to an external resonance element in an internal oscillation mode, said first port being supplied with an external clock signal in a transmission mode, comprising:

• (a) system clock supplying means having two input nodes coupled to said first and second ports, respectively, and an output node coupled to a system clock line, said system clock supplying means being operative in one of said internal oscillation mode and said transmission mode;
• (b) a port controlling circuit responsive to a mode selecting signal and operative to produce a mode control signal representative of said internal oscillation mode or said transmission mode;
• (c) a data signal propagation line coupled to said port controlling circuit for propagating a data bit;
• and
• (d) a switching circuit responsive to said mode control signal representative of said internal oscillation mode to allow said system clock supplying means to produce a system clock in cooperation with said external resonance element coupled to said first and second ports, said system clock being supplied to said system clock line said switching circuit further being responsive to said mode control signal representative of said transmission mode to allow said system clock supplying means to transmit said external clock signal appearing at said first port to said system clock line, said switching circuit further providing an electrical path for said data bit between said data signal propagation line and said second port in response to said mode control signal representative of said transmission mode.