Method for obtaining second reference signal using CMOS pointer-type quartz electronic clock integrated circuit

In electronic production and testing, two reference second signals are often needed: one is a second pulse (1Hz) generated per second. It is mainly used for counting, timing (time accumulation) and time control; the other is a standard gated time base signal with a width of 1 second. It is mainly used for frequency measurement gates in the process of converting any analog quantity into digital quantity (A/D). This article introduces the method of obtaining the second reference signal using a CMOS pointer quartz electronic clock integrated circuit, which can be used in middle school laboratories, electronic device production and adjustment (general production workshop) measuring instruments, etc.

The principle of quartz electronic clock IC is basically the same. Usually, DIP dual in-line package is used (oscillation, shaping, frequency division, narrow pulse formation and power output circuits are all integrated on one chip). Now take the LC7650 quartz electronic clock IC as an example to illustrate its application circuit as shown in Figure 1. Two lN4148 diodes are connected in series with a 1kΩ resistor to divide the 5V power supply. Using the characteristic that the forward voltage drop of the diode is about 0.75V, 13~1.7V voltage is taken out and added to the ⑧ and ⑦ pins of LC7650 as the power supply voltage (VDO=1.2~2V). The resonant frequency of the quartz crystal is 32768Hz (the 10MΩ resistor connected in parallel can improve the working stability of the quartz crystal, and the frequency stability of this circuit can reach 1O-6). The accuracy of the crystal oscillator frequency mainly comes from the accuracy of the quartz crystal resonant frequency; by adjusting the 20-300pF and 5/20pF oscillation capacitors, the manufacturing error of the quartz crystal can be compensated so that the oscillator can oscillate accurately at 32768Hz. This frequency signal is shaped by the circuit inside the chip and divided by 16 levels, and transformed into a low-frequency signal with a frequency of 0.5Hz (32768Hz/216=0.5Hz). It is then transformed into a low-frequency signal with a frequency of 0.5Hz (32768Hz/216=0.5Hz) by the monostable, bistable and logic gate narrow pulse forming circuit, and positive and negative narrow pulses are alternately output from pins ③ and ④. The period is 2s and the pulse width is 31.25ms, as shown in Figures 2 (a) and (b).

Generally, there are two requirements for the second reference signal: one is that the pulse period is 1 second; the other is that the pulse must have sufficient amplitude to meet the driving requirements of digital circuits such as CMOS and TTL. Since the output pulse amplitude of LC7650's ③ and ④ pins is less than the power supply voltage (1.3-1.7V), the output pulse needs to be logically processed and the amplitude converted to meet the use requirements. Figure 3 uses two diodes to form an OR gate. As can be seen from Figures 2 (a) and (b), within any 1 second, one end of ③ and ④ pins always outputs a positive pulse, so the switch tube 9014 can be turned on, and a positive second pulse with a period of 1s (pulse width 31.25ms) and an amplitude of about 5V is obtained from the OUT end. Its waveform is shown in Figure 2 (c). Similarly, Figure 4 can obtain a negative second pulse, and its waveform is shown in Figure 2 (d). To obtain a 1 second gated time base signal, Figure 5 can be used, that is, the two NAND gates in CD4011 are connected to form an RS trigger, and the 9014 is driven to output a symmetrical square wave with a width of 1 second and an amplitude of about 5V, as shown in Figure 2 (e).