Topic: Design and manufacture of digital clock.

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design objective

Familiar with the pin arrangement of integrated circuits.

Master the logic function and usage of each chip.

Understand bread board structure and its wiring method.

Understand the composition and working principle of digital clock.

Familiar with the design and manufacture of digital clock.

design requirements

1. Design index

Time takes 24 hours as a cycle;

Display 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.

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 time signal of 1HZ must be accurate and stable. Usually, a digital clock consists of a quartz crystal oscillator circuit. Figure 3- 1 shows the general structure of a digital clock.

Figure 3- 1 digital clock composition block diagram

(1) crystal oscillator circuit

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

(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 time counting circuit consists of a binary counter, a binary counter and a time decimal counter, wherein the binary counter and the binary counter are 60-decimal counters, and the time decimal counter and the time 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, CMOS NOT-gate U 1, crystal, capacitor and resistor constitute the crystal oscillator circuit, and U2 realizes the shaping function, converting the approximate sine wave 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 function of the NOT gate is similar to that of a high gain inverting amplifier. Capacitors C 1 and C2 form a resonant network with the crystal to control the oscillation frequency, and at the same time 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 low frequency is beneficial to reduce the number of frequency dividers.

It can be found from relevant manuals that both C 1 and C2 are 30pF. When higher frequency accuracy and stability are needed, the correction capacitor can 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. In order to get the second signal input of 1Hz, it is necessary to divide the output signal of the 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 the 32768Hz oscillation signal pair 1hz is 32768(2 15), that is, the counter that realizes this frequency division function is equivalent to 15 binary counters. 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 also contains the NOT gate required by oscillator circuits, which is more convenient to use.

CD4060 is a binary counter with a count of 14, which can divide the signal of 32768HZ into 2HZ. Its internal block diagram is shown in Figure 3-3. As can be seen from the figure, the clock input of CD4060 has two series-connected NOT gates, so it can directly realize the functions of oscillation and frequency division.

Figure 3-3 Internal Block Diagram of CD 4046

3) Timing device

Time units sometimes include counting, minute counting and second counting.

The hour counting unit is generally a 12 binary counter, and its output is in the form of two-bit 842 1BCD code; Minute counting and second counting units are hexadecimal counters, and its output is also 842 1BCD code.

Generally, 10 base counter 74HC390 is used to realize the counting function of time counting unit. In order to reduce the number of devices, 74HC390 is selected, and its internal logic block diagram is shown in Figure 2.3. This device is a dual 2-5- 10 asynchronous counter, and each counter is equipped with an asynchronous zero clearing terminal (active at high level).

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

The second bit counting unit is 10 decimal counter, so decimal conversion is not needed. Just connect QA to CPB (falling edge is valid). CPA (invalid falling edge) is connected with 1HZ input signal, and Q3 can be connected with CPA of ten-digit counting unit as the carry-up signal.

The second decimal counter is a hexadecimal counter and needs to be converted into a hexadecimal counter. The circuit connection method for converting 10 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 counting unit, with several bits.

Figure 3-5 10 -6 counter conversion circuit

The circuit structures of decimal counting unit and decimal counting unit are completely the same as those of binary counting unit and binary counting unit respectively, except that Q3 of decimal counting unit should be connected to CPA of decimal counting unit as an uplink carry signal, and Q2 of decimal counting unit should be connected to CPA of current bit 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 unit bit counting unit, but it is required that the whole hour counting unit should be a binary counter of 12 instead of an integer multiple of 10. Therefore, it is necessary to combine units and decimal counting units into a whole to convert 12. Using 1 chip 74HC390 to realize 12 binary counting.

In addition, in the circuit shown in Figure 3-6, the remainder binary counting unit can be used to convert the 2HZ output signal of the frequency divider into a 1HZ 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 1 BCD 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. The display decoding circuit is CD45 1 1+0, and the display unit circuit is LED digital tube.

5) Timing power supply circuit

There is an error in the time when the power supply needs to be reconnected or when walking. 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 after correction.

According to the requirements, digital clocks should have minute correction and time correction functions. Therefore, the direct counting channel of minutes and hours should be cut off, and the circuit that can switch between the normal timing signal and the correction signal at any time should be used for access. Figure 3-7 shows a time correction circuit with a basic RS flip-flop.

Figure 3-7 Correction Circuit with Jitter Elimination Circuit

6) the hour circuit

Generally, clocks and watches should have the function of striking time circuit, that is, a digital clock will automatically strike the hour a few seconds before the hour as a reminder. Its function is to emit continuous or rhythmic audio sound waves, and more complicated, 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 from 59 minutes and 50 seconds to 59 minutes and 59 seconds, the time-telling circuit should inform the time control signal. Time-telling circuit shall be 74HC30, and buzzer shall be electro-acoustic device.

ingredient

1. Equipment needed for the experiment

5V power supply.

Bread board 1.

oscilloscope

Multimeter.

Tweezers 1.

Scissors 1.

The network cable is 2 meters per 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.

Two 30p capacitors.

32.768k clock crystal 1.

doorbell

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 side of bread board, there are four groups, in which columns X and Y (0- 15, 16-40, 4 1-55, ABCDE, FGHIJ and E and F) are not connected.

Circuit diagrams of several functional blocks

Connect a CD45 1 1 and an LED digital tube to form a CD45 1 1 drive circuit, and the digital tube can display 0-9 to check the quality of the digital tube, as shown in Figure 5- 1.

Figure 5- 1 45 1 1 driver circuit

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

Figure 5-2 74390 Decimal Counter

A hexadecimal counter is composed of an LED digital tube, a CD45 1 1, a 74HC390, a 74HC00 and a crystal oscillator. The digital tube displays from 0 to 6, as shown in Figure 5-3.

Figure 5-3 74390 hexadecimal counter

Hexadecimal circuit is composed of hexadecimal circuit and decimal circuit. The circuit can display from 0 to 59, as shown in Figure 5-4.

Figure 5-4 sexagesimal Circuit

Two sexagesimal circuits are used 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

Using CD4060, the resistor and the crystal oscillator are connected to form a sub-frequency crystal oscillator circuit, as shown in Figure 5-6.

Figure 5-6 Frequency Divider-Crystal Oscillator Circuit

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

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

Figure 5-8 Hours Time Telling Circuit

Figure 5-9 is a general diagram of a circuit that can carry hours, minutes and seconds by connecting two sexagesimal and a decimal system.

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

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

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

Eight. abstract

Problems encountered in the design process and their solutions.

In the process of detecting the situation in bread board, there is no connection where it should be connected. Later, it was found that the multimeter nib was not in vertical contact with the inside of bread board.

In the process of testing the driving circuit of CD45 1 1, it was found that the digital tube could not display normally. It is found that the main reasons are poor contact, including poor contact of wires and poor contact of chips. In the course of the experiment, several diodes of the digital tube appear and disappear. The digital tube is tested with a 5V power supply, with one end grounded and the other end touching each diode. If it is found that the diode can display normally, then use a multimeter to check whether each wire is in good contact. In the process of testing, it is found that several wires can sometimes be connected and sometimes they can't be connected. After reconnecting the wires with poor contact, it is found that it can be displayed normally. Secondly, due to the problem of poor chip contact, several pins that should be connected were not connected with the multimeter ohm file, but the detected wires were in good condition. The solution is to pull out the chip 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, we also found a bad LED digital tube and two bad CD 45 1 1, which can be displayed normally after replacement.

In the process of connecting the crystal oscillator, the crystal oscillator cannot be started. After eliminating the problem of poor contact between the wire and the chip, check the circuit diagram again and find that the 12 pin is not grounded.

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

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 display normally in the process of carrying hours, minutes and seconds, so the problem of poor contact between the chip and the wiring can be eliminated. After inspection, there is no error in the wiring of the correction circuit. Then charge the QA, QB, QC and QD pins of 10 second with the DC voltage range of the multimeter, and find that there is voltage at the QA pin, but there is no voltage at the QA pin. Then, detect the carry end of seconds to minutes, and find that the carry end of seconds to minutes is not pulled out.

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 the time can be reported normally after reconnection.

When connecting the frequency dividing circuit, disconnect the QD of the hour hand and the 1 pin of the hour hand, then connect the 1 pin of the hour hand to the 3-pin of the crystal oscillator, and connect the 3-pin of the hour hand to the 1 pin of the second hand. The connected circuit diagram is not working properly. When the hour bit jumps from 0 to 9, the hour bit can only display a 0, and this circuit adopts 3-pin frequency division. Therefore, a logic circuit with 12 binary connection 74HC390 can be left for frequency division. Therefore, pin11of the decimal digit CD451is grounded, pin 7 is connected to pin 5 of 74HC390, pin 3 and pin 4 of 74HC390 are disconnected, and then pin 4 is 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 physical diagram is often connected after simulation, but sometimes the simulation and circuit connection are not completely consistent. For example, in the simulated connection schematic diagram, there are often no high-level 16 or 14 pins and low-level 7 or 8 pins, so it is often easy to miss in the actual circuit connection. Another example is the 74HC390 chip, which is itself a decimal counter and must be connected to the analog circuit.

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.