2FSK Modem Based on the Microcontroller AT89C51

In order to simplify the device of modulation and demodulation of 2FSK, a 2FSK modem based on the microcontroller AT89C51 was designed. This modem is mainly composed of an AT89C51 microcontroller, a simple two order passive low-pass filter, and a shaping circuit. Modulation is achieved on the basis of the principle of pulse width modulation (PWM). By executing PWM signal generating program, a digital base band signal is converted into the corresponding PWM signal after it is input into an I/O port of the microcontroller. Then the PWM signal is outputted from another I/O port and sent into the two order passive low-pass filter. On the output side of the filter, the 2FSK signal can be obtained. For demodulation, the 2FSK signal shaped by the shaping circuit is input into microcontroller from its I/O port. Then the widths of positive and negative half-waves are measured by the microcontroller internal timer. By comparing the value of each width with the appropriate threshold value, the binary digital signal is gotten and outputted from another I/O port of microcontroller. This modem makes full use of the resources of the microcontroller. As a result, it is simple, easy to use, economical and reliable. Furthermore, it can generate and demodulate the 2FSK signal with high good accuracy.