PIC16F628 is used for the circuit which displays a day of the week in a large-size 7 segment LED matrix. Also, an order lighting-up control is done to suppress the consumption of electric power of the LED.
Receiver circuitIn the case of Japan, the radio of standard time is transmitted as long wave radio from the transmitting station in OOTAKADOYA mountain (40kHz) of Fukushima Prefecture, and HAGANE mountains (60kHz) of Nagasaki Prefecture.
Reception of this radio is performed by U4226B. The parts attached outside are a receiving coil, 40kHz crystal oscillators, and some resistors and capacitors. U4226B is designed for radio-controlled clock applications in the frequency range of 40 to 80 kHz. Refer to the data sheet for detailed specification.
Received signal amplifying circuit
The output signal TCO( Time Code Output ) of U4226B is the pulse of 0V to 5V. This signal is amplified by the transistors in order to make sharp the rising edge and the falling edge of the pulse.
LED1 is for the monitors of a TCO signal. The time information sent by radio consists of 60 pulses, and one pulse is sent in 1 second. Although the width of each pulse changes with information sent, blink of LED1 is performed for every second. Thereby, you can check whether radio has received normally. A receiver and a display processor can be installed in the place left by cable splicing. In this case, noise may be induced to a cable. The receiver circuit and the display processing circuit are electrically separated by the photo coupler (PC0) in order to lessen the influence.
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The capacitor C21 connected to the base of Q3 is for removing the noise induced to the cable. If the capacity of this capacitor is large, pulse shape will deteriorate.
A TCO signal is inputted into RA4 port of PIC16F877. Unlike other ports, this port is an open drain type. That is, a pull-up (it pulls up to +5V) is not carried out inside PIC. Therefore, the pull-up is carried out by R4.
Clock oscillating circuit
The time of this clock is displayed by the time information sent on radio. The right time cannot be displayed if radio stops. Therefore, the highly precise crystal oscillator is used for the clock of PIC. Oscillation frequency is 12.8MHz. The execution time of one step of PIC becomes 4/12.8 = 0.3124 microseconds. When radio is received normally, the accuracy of this oscillator does not become a problem. Two or more equipment, such as a cesium beam type atom frequency standard machine, is used for the Japanese standard clock. The accuracy is announced as 10-13. Since it is 31,536,000 seconds for one year, it is the accuracy that the error for 1 second will arise in 317,098 years. This system is not used so long. Therefore, this is the clock which does not go wrong at all.
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