The system is based on μC/OS-II embedded real-time operating system and CallerID (caller ID) technology, and is composed of modules such as ringing stream detection, caller decoding, DTMF decoding, analog on-hook, and number storage. The introduction of the μC/OS-II embedded real-time operating system greatly facilitates the management of modules and the management of number storage. The system uses the obtained caller number to compare with the number in the stored blacklist, and determines whether the caller rings the line or controls the analog hook circuit to hang up. The system multiplexes the telephone keypad control and displays it in LCD Chinese, which can conveniently manage the numbers in the blacklist.
System hardware circuit design
The fixed-line call firewall is based on the caller ID (CID) design. The decoded caller number is compared with the number in the blacklist to simulate hanging up or ringing. The system is set up by multiplexing the phone and the keyboard, and the number is input through the telephone keypad to be stored in the EEPROM.
Bell flow detection circuit
China's mainland caller ID is in FSK format. The caller number is sent between the first ringing and the second ringing. In order to avoid the impact of the first ringing on the user before the caller number is obtained, it is necessary to cut off the ringing before the ringing circuit of the phone operates, so the ringing detection is required to be effective quickly. The ringing current is a sine wave with a RMS value of 25 Hz and 15 volts. A voltage comparator is used. When the ringing current voltage rises above 60V, the comparator outputs a high level, which is inverted by the inverter. Trigger the external interrupt of the microcontroller. Figure 3 shows the ringing detection circuit:
Figure 3 ringing detection circuit
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