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First, the design purpose

1. Familiar with the pin arrangement of integrated circuits.

2. Master the logic function and usage of each chip.

3. Understand the structure of bread board and its wiring method.

4. Understand the composition and working principle of digital clock.

5. Familiar with the design and production of digital clock.

Second, the design requirements

1. Design index

Time takes 24 hours as a cycle;

Displays hours, minutes and seconds;

There is a time adjustment function, which can adjust the time and minutes respectively to make it correct to the standard time;

The timing process has the function of telling time. When the time reaches 5 seconds before the hour, the buzzer will tell time.

In order to ensure the stability and accuracy of timing, the crystal oscillator must provide a clock time reference signal.

2. Design requirements

Draw the circuit schematic diagram (or analog circuit diagram);

Selection of components and parameters;

Circuit simulation and debugging;

PCB file generation and printout.

3. The production needs to be assembled and debugged by itself, and problems can be found and solved.

4. Write a design report, write the whole process of design and production, and attach relevant information and drawings.

Third, the design principle and its block diagram

1. Composition of digital clock

A digital clock is actually a counting circuit that counts the standard frequency (1HZ). Since the starting time of counting cannot be consistent with the standard time (such as Beijing time), it is necessary to add a time correction circuit to the circuit, and the standard 1HZ time signal must be accurate and stable. Generally, a quartz crystal oscillator circuit is used to form a digital clock. Figure 3- 1 shows the overall block diagram of the digital clock.

Figure 3- 1 digital clock composition block diagram

(1) crystal oscillator circuit

The crystal oscillator circuit provides a stable and accurate square wave signal with the frequency of 32768 Hz for the digital clock, which can ensure the accuracy and stability of the digital clock. Crystal oscillator circuits are used for analog and digital electronic clocks.

(2) Frequency divider circuit

The frequency divider circuit divides the high-frequency square wave signal of 32768Hz by 32,768 () times to obtain a square wave signal of 1hz for the second counter to count. The frequency divider is actually a counter.

(3) Time counting circuit

The timing circuit consists of a two-bit binary counter, a fractional decimal counter and a time-bit decimal counter circuit, wherein the two-bit binary counter and the fractional decimal counter are 12 counters according to the design requirements.

(4) decoding drive circuit

The decoding drive circuit converts the 842 1BCD code output by the counter into the logic state required by the digital tube, and provides enough working current to ensure the normal operation of the digital tube.

5] Digital tube

Digital tubes usually include light emitting diode (LED) digital tubes and liquid crystal (LCD) digital tubes. This design provides LED digital tube.

2. The working principle of digital clock

1) crystal oscillator circuit

Crystal oscillator is the core of digital clock, which ensures the accuracy and stability of the clock.

The circuit shown in Figure 3-2 is a digital crystal oscillator circuit with square wave output, which is composed of CMOS NOT gates. In this circuit, the CMOS NOT-gate U 1, crystal, capacitor and resistor constitute the crystal oscillator circuit, and U2 realizes the shaping function, converting the sine wave-like waveform output by the oscillator into an ideal square wave. The output feedback resistor R 1 provides bias for the NOT gate, which makes the circuit work in the amplification region, that is, the NOT gate functions like a high-gain inverting amplifier. Capacitors C 1 and C2 form a resonant network with the crystal, control the oscillation frequency, and provide a phase shift of 180 degrees, thus forming a positive feedback network with the NAND gate and realizing the function of the oscillator. Because the crystal has high frequency stability and accuracy, the stability and accuracy of the output frequency are guaranteed.

The frequency of XTAL crystal is 32768HZ. This component is specially designed for digital clock circuit, and its frequency is low, which is beneficial to reduce the number of frequency divider stages.

It can be found from relevant manuals that C 1 and C2 are both 30pF. When higher frequency accuracy and stability are needed, a correction capacitor can also be connected and temperature compensation measures can be taken.

Because the input impedance of CMOS circuit is extremely high, the feedback resistance R 1 can be selected as10mΩ. Higher feedback resistance is beneficial to improve the stability of oscillation frequency.

Non-gate circuit can choose 74HC00.

Figure 3-2 COMS crystal oscillator

2) frequency divider circuit

Usually, the crystal oscillator output frequency of digital clock is high, so in order to get the second signal input of 1 Hz, it is necessary to divide the output signal of oscillator.

Usually, the circuit to realize the frequency divider is a counter circuit, which is generally realized by multi-level binary counters. For example, the frequency division multiple of 32768Hz oscillation signal pair 1 Hz is 32768(2 15), that is, the counter that realizes this frequency division function is equivalent to a 15-pole binary counter. Commonly used binary counters are 74hc393 and so on.

In this experiment, the frequency division circuit is composed of CD4060. CD4060 can achieve the highest frequency division in digital integrated circuits, and it also contains the NOT gate required by oscillator circuit, which is more convenient to use.

The CD 4060 is a binary counter with a count of 14, which can divide the signal of 32768Hz into 2 Hz. Its internal block diagram is shown in Figure 3-3. As can be seen from the figure, the clock input of CD 4060 has two NOT gates in series, so it can directly realize the functions of oscillation and frequency division.

Figure 3-3 Internal Block Diagram of CD 4046

3) Timing device

A timing unit sometimes has several parts, such as counting, minute counting and second counting.

The hour counting unit is generally a decimal counter, and its output is in the form of two-bit 842 1 BCD code; The unit of minute and second counting is hexadecimal counter, and its output is also 842 1 BCD code.

Generally, 10 decimal counter 74HC390 is used to realize the counting function of time counting unit. In order to reduce the number of devices used, 74hc390 can be selected, and its internal logic block diagram is shown in Figure 2.3. The device is a dual-channel 2-5- 10 asynchronous counter, and each counter provides an asynchronous clearing terminal (active high).

Figure 3-4 Internal Logic Block Diagram of 74hc390 (1/2)

The second digit counting unit is a decimal counter, so there is no need for decimal conversion, just connect QA with CPB (effective falling edge). CPA (falling invalid) is connected to the input signal of 1 Hz second, and Q3 can be connected to the CPA of decimal counting unit as the carry-up signal.

The second decimal counting unit is a hexadecimal counter, which needs decimal conversion. The circuit connection method of converting decimal counter into hexadecimal counter is shown in Figure 3-5, in which Q2 can be used as an uplink carry signal to connect with CPA of a multi-bit counting unit.

Figure 3-5 10 -6 counter conversion circuit

The circuit structures of fractional bit and fractional decimal counting units are exactly the same as those of fractional bit and fractional decimal counting units respectively, except that Q3 of fractional bit counting unit should be connected to CPA of fractional decimal counting unit as an uplink carry signal, and Q2 of fractional decimal counting unit should be connected to CPA of time decimal counting unit as an uplink carry signal.

The circuit structure of the hour bit counting unit is still the same as that of the second or hour bit counting unit, but it is required that the whole hour bit counting unit should be a decimal counter instead of an integer multiple of 10, so it is necessary to combine the hour bit counting unit and the decimal bit counting unit into a whole for decimal conversion. The circuit for realizing decimal counting function with a piece of 74HC390 is shown in Figure 3-6.

In addition, in the circuit shown in Figure 3-6, the remainder binary counting unit can be used to convert the 2 Hz output signal of the frequency divider into a 1 Hz signal.

Figure 3-6 12 binary counter circuit

4) decoding drive and display unit

The counter accumulates time and outputs it in the form of 842 1BCD code. The selective display decoding circuit converts the output number of the counter into the output logic and a certain current required by the digital display device. CD45 1 1 is selected as the display decoding circuit, and LED digital tube is selected as the display unit circuit.

5) Timing power supply circuit

The time when there is an error when reconnecting the power supply or walking needs to be corrected. Usually, the method to correct the time is to cut off the normal counting channel, then manually trigger the counting or add the square wave signal with higher frequency to the input end of the counting unit to be corrected, and then turn to the normal timing state.

According to the requirements, digital clocks should have minute correction and time correction functions. Therefore, we should cut off the direct counting path of division and time, and adopt a circuit that can switch between normal timing signal and correction signal at any time. Figure 3-7 shows the timing correction circuit with basic RS flip-flop.

Figure 3-7 Correction Circuit with Jitter Elimination Circuit

6) the hour circuit

Generally speaking, the clock should have the function of a time-telling circuit, that is, the digital clock will automatically tell the time a few seconds before the hour to remind you. Its function is to emit continuous or rhythmic audio sound waves, and it can also be a real-time voice prompt.

According to the requirements, the circuit should start to tell the time within 10 second before the hour, that is, when the time is between 59 minutes and 59 minutes and 59 seconds, the time-telling circuit will send out the time-telling control signal. 74HC30 is used for the time-telling circuit, and buzzer is used for the electro-acoustic device.

Fourth, components

1. Equipment needed for the experiment

5V power supply.

Bread board 1.

oscilloscope

Multimeter.

Tweezers 1.

1 scissors.

The network cable is 2m/person.

* * * Six 8-segment digital tubes.

CD45 1 1 6 integrated blocks.

CD4060 integrated block 1 block.

The 74HC390 integrated block is 3 pieces.

74HC5 1 integrated block 1 block.

74HC00 integrated block 5 yuan.

74HC30 integrated block 1 block.

Five10mΩ resistors.

500 Ω resistance 14.

2 30p capacitors.

1 32.768k clock crystal.

Buzzer.

2. Chip internal structure diagram and pin diagram

Figure 4- 1 7400 Four-2 Input NAND Gate Figure 4-2 CD45 1 1BCD Seven-Segment Decoder/Driver

Figure 4-3 CD4060BD Figure 4-4 74HC390D

Figure 4-5 74HC5 1D Figure 4-6 74HC30

3. The internal structure of bread board.

There are five groups of columns on the right side of bread board, and five groups below. On the left of bread board, there are four groups. In each group, X and Y columns (0- 15, 16-40, 4 1-55, ABCDE, FGHIJ and E and F) are not connected.

V. Circuit diagram of functional block

1.CD45 1 1 and LED digital tube are connected to form a CD45 1 1 driving circuit. The digital tube can be displayed from 0 to 9 to check the quality of the digital tube, as shown in Figure 5- 1.

Figure 5- 1 45 1 1 driver circuit

2. An LED digital tube, a CD45 1 1, a 74HC390 and a 74HC00 are connected to form a decimal counter, and the circuit displays from 0 to 9 under the action of a crystal oscillator, as shown in Figure 5-2.

Figure 5-2 74390 Decimal Counter

3. An LED nixie tube, a CD45 1 1, a 74HC390, a 74HC00 and a crystal oscillator are connected to form a hexadecimal counter, and the nixie tube displays from 0 to 6, as shown in Figure 5-3.

Figure 5-3 74390 hexadecimal counter

4. Compose a hexadecimal circuit with a hexadecimal circuit and a decimal circuit. The circuit can display from 0 to 59, as shown in Figure 5-4.

Figure 5-4 sexagesimal Circuit

5. Use two sexagesimal circuits to synthesize a double sexagesimal circuit, and there is a carry between two sexagesimal circuits, as shown in Figure 5-5.

Figure 5-5 Double sexagesimal Circuit

6. Connect the CD4060, resistor and crystal oscillator to the frequency-dividing crystal oscillator circuit, as shown in Figure 5-6.

Figure 5-6 Frequency Divider-Crystal Oscillator Circuit

7. Connect 74HC5 1D and 74HC00 with resistors to form a time calibration circuit, as shown in Figure 5-7.

Figure 5-7 Time Calibration Circuit

8. Connect 74HC30 and buzzer to form a time-telling circuit. See attached figure 5-8.

Figure 5-8 Hours Time Telling Circuit

9. Figure 5-9 is the general diagram of the circuit with two sexagesimal and a decimal system connected into hours, minutes and seconds.

Figure 5-9 Connection Diagram of Hour, Minute and Seconds

VI. See attached figure 6- 1 for the layout of the general wiring assembly.

Seven. See attached figure 7- 1 for the chip connection diagram.

Eight. abstract

Problems and solutions in the design of 1.

1) In the process of detecting the situation in bread board, there is no connection where it should be connected. After inspection, it was found that there was no vertical contact between the multimeter tip and the inside of bread board.

2) During testing the driving circuit of CD45 1 1, it was found that the digital tube could not display normally. After inspection, it was found that the main reasons were poor contact, including poor contact of wires and poor contact of chips. During the experiment, several diodes of the digital tube sometimes appear and sometimes disappear. Test the digital tube with 5V power supply, with one end grounded and the other end in contact with each diode. If it is found that the diode can display normally, then use a multimeter ohmmeter to test whether each wire is in good contact. During the test, it was found that several wires can sometimes be connected, and sometimes they can't. After reconnecting the wires with poor contact, it is found that it can be displayed normally. Secondly, due to the poor contact of the chip, several pins that should be connected were not connected with a multimeter, but the detected wires were in good condition. The solution is to pull out the chip of CD45 1 1, readjust its pins according to the situation of bread board hole, and then insert the chip into bread board evenly. Later, it was found that it could be displayed normally. In this experiment, a bad LED digital tube was also found.

3) In the process of connecting the crystal oscillator, the crystal oscillator cannot start to vibrate. After eliminating the problem of poor contact between the wire and the chip, compare the circuit diagram again and find that the 12 pin is not grounded.

4) In the process of connecting hexadecimal, it is found that the circuit can only jump 4 or 5, and later it is found that it is caused by the wrong pin connecting NAND gate, and it can be displayed normally after correction.

5) In the process of connecting the correction circuit, the time and minutes can be corrected normally, but the second is affected. Especially when the time is one minute, the second jumps from 40 to 59 and then back to 40, and there is no carry between minutes and seconds. The circuit can be displayed normally in the process of carrying hours, minutes and seconds, so the problem of poor contact between the chip and the connecting wire can be eliminated. After inspection, there is no error in the wiring of the calibration circuit. Then charge the QA, QB, QC and QD pins at 10 second with the DC voltage range of the multimeter, and find that the QA pin has voltage, but there is no voltage at that time. Then, the carry ends of seconds to minutes and minutes to minutes are detected, and it is found that the carry ends of seconds to minutes are not unplugged.

6) In the process of making the time-telling circuit, it is found that the buzzer starts to tell the time at 57 minutes and 59 seconds. Later, through the detection circuit, it was found that the 74HC30 chip was connected as a chip with 16 pin, so the wiring was misplaced, and it could still report the time normally after reconnecting.

7) When connecting the frequency dividing circuit, disconnect the QD of the hour bit and the 1 pin of the hour bit, then connect the 1 pin of the hour bit to the 3-pin of the crystal oscillator, and connect the 3-pin of the hour bit to the 1 pin of the second bit. The connected circuit diagram is not working properly. When the hour number jumps from 0 to 9, the hour number can only display 0. In this circuit, three pins can only display 0. Therefore, a logic circuit with 12 binary connection 74HC390 can be left for frequency division. Therefore, pin 2 and pin 6 of the decimal CD45 1 1 should be grounded, pin 7 should be connected to pin 5 of 74HC390, pin 3 and pin 4 of 74HC390 should be disconnected, and then pin 4 should be connected to pin 9, among which

2. Design experience

During the design of this digital clock, I became more familiar with the structure of the chip and mastered the working principle and specific usage of each chip.

In the connection of hexadecimal, decimal, hexadecimal carry and decimal, it is required to be familiar with the function of logic circuit and chip pin, so that when the circuit fails, the error can be accurately found and corrected in time.

When designing a circuit, the simulation is often done first, and then the physical diagram is connected, but sometimes the simulation and circuit connection are not completely consistent. For example, in the connection schematic diagram of simulation, there is often no pin 16 or 14 connected to high level, and pin 7 or 8 connected to low level, so it is often easy to miss in the actual circuit connection. Another example is the 74HC390 chip, which is itself a decimal counter. The feedback line must be connected in the analog circuit to display normally, but not necessarily in the actual circuit, so there is still some difference between the analog diagram and the circuit connection diagram.

The main reasons for errors in designing circuit connection diagram are poor contact between wiring and chip and wiring errors.

3. Suggestions for design

The design of this digital clock focuses on simulation and wiring. Although I can connect the circuit diagram and display it normally, I am not very familiar with the principle of the circuit itself. Generally speaking, through this design experiment, the practical ability of the experiment is further enhanced.