2 Modulation Technique

The Universal Synchronous and Asynchronous Receiver and Transmitter (USART) peripheral receives the input data provided in the serial terminal. The modulation process starts when there are available data in the USART Receive Buffer.

The TCF is configured in NCO Fixed Duty-Cycle mode and generates the modulated data signal. This operating mode offers great flexibility as the frequency of the generated signal is proportional to the increment value and, therefore, can be established precisely. Frequency shifting also happens without abrupt phase discontinuities of the output waveform.

The TCE is configured in Normal mode and generates an overflow interrupt once every millisecond. This periodic interrupt modulates the data byte by altering the increment value of the TCF and effectively switching the frequency of the output waveform to either the “mark” or the “space” corresponding value based on the state of each data bit.

The frame format of the modulated signal generated by the TCF is similar to the USART frame format. For every data byte, a total of 11 bits are modulated and transmitted. The output state of the TCF is initially logic low as the peripheral is disabled when there is no data to transmit.

As soon as the modulation process starts, the TCF is enabled, and a “mark” followed by a “space” are generated to emulate the Start bit. The data byte is modulated next, followed by another “mark” to emulate the Stop bit, before the TCF is disabled. The framing format will act as a synchronization method at the other end of the communication line, where the demodulation happens.