Portable handheld device and main control station use the serial to communicate. The interface chip of traditional universal asynchronous receiver transmitter has the characteristic of more pins, larger volume and more complex interface. Therefore, using a field programmable gate array chip and the VHDL programme, the asynchronous receiving unit, asynchronous transmitting unit and dual port RAM are designed, then the serial communication of portable handheld device is realized and the circuit area and the power consumption are reduced.