The circuit is a digital communication system to transmit low frequency analog signals such as voice. Digital Voice Transmitter uses techniques like Pulse Code Modulation (PCM) for encoding analog voice signal and Amplitude Shift Keying (ASK) for modulation. To simulate circuit in this project, initially activate Mixed Mode simulator from the Schematic Editor window. Simulation can be performed by selecting Run Transient analysis (Oscillograph) or Analysis -> Transient Analysis from Simulation menu.
For simulation, an AC signal source is used as voice signal generator. It generates sine wave of 2.5V peak to peak voltage and 1 Hz frequency. This signal is pulse code modulated using ADC module of HC11 to get an eight level PCM coded data for transmission. Encoded parallel data is converted to serial with the help of SCI module. Signal sampling rate depends on baud rate of SCI module. The microcontroller is coded for 28Hz sampling frequency by placing 0x35 in BAUD register.
A diode (D1) – voltage source (E) circuit clamps input voice signal to positive side. ADC samples and converts the signal to 8 bit digital data depending on instantaneous value of the signal.
The converted data is placed at SCDR for transmission through TXD pin. The serial data available at TXD pin is amplitude shift keyed with an oscillator frequency of 1 KHz.
Output waveforms can be observed in waveform viewer after simulation. The source code written either in C or Assembly language can be viewed from the
code editor window.
The Algorithm is as shown:
Step 1: Start
Step 2: Initialize ADC
Step 3: Initialize SCI
Step 4: Wait for Conversion Complete Flag (CCF) of ADC to set
Step 5: Copy ADR1 to SCDR
Step 6: Enable transmission
Step 7: Wait for transmission complete flag to set