High Level Design
E-Directory
The E-Directory has 5 basic modes whose functions and "responses" are described below:
Entry Mode:
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Search Mode:
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Scroll Mode:
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Delete Mode:
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Dial:
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Tone Generator
In a dual tone telephone system, the dial tone corresponding to each digit is in fact a superposition of two sine waves with different frequencies. Once the user chooses a phone number from the e-directory to dial, the microcontroller will generate a "sequence" of DTMF pulses that correspond to the sequence of phone number digits. Since the length of phone number varies according to the destination of the call, e.g. long distance call consists of 10 digits including the area code for the central office, our e-phone directory allows the user to dial up to 20 digits for the phone number. The combination of frequencies corresponding to each digit is given below:
Click on the numbers to see the waveform |
DTMF Decoder
In order to check if the microcontroller has generated the correct dual tone signal, a DTMF decoder chip was used to decode the signal. The decoder chip has 4 output pins that indicate the decoded digit. To map these 4 output value to phone numbers, these 4 pins will be connected to another MCU which is interfaced with another LCD to displayed the decoded number.