1. Design purposes

1. Be familiar with the pin arrangement of integrated circuits.

2. Understand the logical functions and usage of each chip.

3.

Understand the breadboard structure and its wiring methods.

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

5.

Be familiar with the design and production of digital clocks.

2. Design requirements

1. The design index time is based on a cycle of 24 hours; it displays hours, minutes and seconds;

It has a time correction function, which can independently correct the hours and minutes to the standard time; the timing process has Time reporting function, a buzzer will sound 5 seconds before the time reaches the hour;

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

2. Design requirements

Draw the circuit schematic diagram (or simulated circuit diagram); select components and parameters; circuit simulation and debugging;

PCB file generation and printout.

3． Production requirements Self-assembly and debugging, and ability to find and solve problems.

4. Write a design report

Write out the entire process of design and production, attach relevant information and drawings, and provide insights.

3. Design principles and block diagram

1. The composition of a digital clock

The 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), a time correction circuit needs to be added to the circuit. At the same time, the standard 1HZ time signal must be accurate and stable. Digital clocks are usually constructed using quartz crystal oscillator circuits. Figure

3-1 shows the general block diagram of a digital clock.

⑴Crystal oscillator circuit

The crystal oscillator circuit provides the digital clock with a stable and accurate 32768Hz square wave signal, which can ensure the accuracy and stability of the digital clock. Whether it is an analog electronic clock or a digital display electronic clock, a crystal oscillator circuit is used.

⑵Frequency divider circuit

The frequency divider circuit divides the 32768Hz high-frequency square wave signal 32768 times to obtain a 1Hz square wave signal for counting by the seconds counter. . The frequency divider is actually a counter.

⑶Time counter circuit

The time counting circuit is composed of second units and seconds tens counters, minute units and minute tens counters, and timely units and time tens counter circuits. Among them, the second ones digit and second tens digit counters, minute ones digits and minute tens digit counters are hexadecimal counters, and according to the design requirements, the hour ones digit and hour tens digit counters are hexadecimal counters.

⑷Decoding drive circuit

The decoding drive circuit converts the 8421BCD code output by the counter into the logic state required by the digital tube, and provides sufficient operating current to ensure the normal operation of the digital tube. .

⑸Nigital tube

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

2. The working principle of digital clock

1) Crystal oscillator circuit

The crystal oscillator is the core of the 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 a square wave output formed by a CMOS NOT gate. In this circuit, the CMOS NOT gate U1, a crystal, a capacitor and a resistor form a crystal oscillator circuit. , U2 implements the shaping function, converting the waveform output by the oscillator that is similar to a sine wave into a more ideal square wave. The output feedback circuit

Resistor R1 provides a bias for the NOT gate, allowing the circuit to operate in the amplification region, that is, the function of the NOT gate is similar to a high-gain inverting amplifier. Capacitors C1, C2 and the crystal form a resonant network to complete the control function of the oscillation frequency and provide a 180-degree phase shift, so that the AND gate forms a positive feedback network and realizes the function of the oscillator. Because the crystal has high frequency stability and accuracy, the output frequency is guaranteed to be stable and accurate.

The frequency of crystal XTAL is selected as 32768HZ. This component is specially designed for digital clock circuits, and its frequency is lower, which is beneficial to reducing the number of frequency divider stages.

From the relevant manual, it can be found that C1 and C2 are both 30pF. When higher frequency accuracy and stability are required, correction capacitors can also be connected and temperature compensation measures can be taken.

Since the input impedance of the CMOS circuit is extremely high, the feedback resistor R1 can be selected as 10MΩ. Higher feedback resistance is beneficial to improve the stability of oscillation frequency.

74HC00 is optional for the NOT gate circuit.

2) Frequency divider circuit

Usually, the output frequency of the crystal oscillator of a digital clock is relatively high. In order to obtain a 1Hz second signal input, the output signal of the oscillator needs to be divided. frequency.

Usually the circuit that implements the frequency divider is a counter circuit, which is generally implemented using a multi-stage binary counter. For example, the frequency division multiple for dividing an oscillation signal of 32768Hz into 1HZ is 32768 (215), that is, the counter that implements this frequency division function is equivalent to a 15-pole binary counter. Commonly used binary counters include 74HC393 and so on.

In this experiment, CD4060 is used to form a frequency dividing circuit. CD4060 has the highest number of frequency divisions among digital integrated circuits, and CD4060 also contains the NOT gate required for the oscillation circuit, making it more convenient to use.

The CD4060 counts as a 14-level binary counter, which can divide the 32768HZ signal into 2HZ. Its internal block diagram is shown in Figure 3-3. As can be seen from the figure, the clock input terminal of the CD4060 Two series-connected NOT gates can directly realize the functions of oscillation and frequency division.

3) Time counting unit

The time counting unit has several parts such as time counting, minute counting and second counting.

The hour counting unit is generally a hexadecimal counter, and its output is in the form of a two-digit 8421BCD code; the minute counting and second counting units are hexadecimal counters, and their output is also in the form of 8421BCD code.

Generally, the decimal counter 74HC390 is used to realize the counting function of the 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. This device is a dual 2-5-10 asynchronous counter, and each counter provides an asynchronous clearing terminal (active high level).

The second units counting unit is a decimal counter, no need for base conversion, just connect QA to CPB (falling edge valid). CPA (falling invalid) is connected to the 1HZ second input signal, and Q3 can be used as an upward carry signal to be connected to the CPA of the tens counting unit.

The second ten-digit counting unit is a hexadecimal counter and requires hexadecimal conversion. The circuit connection method for converting a decimal counter into a hexadecimal counter is shown in Figure 3-5, in which Q2 can be used as an upward carry signal to be connected to the CPA of the units-digit counting unit.

The circuit structure of the units and tens counting units is exactly the same as that of the units and tens counting units respectively, except that Q3 of the units counting unit should be the same as the upward carry signal. The CPA of the bit counting unit is connected, and the Q2 of the tens bit counting unit should be connected to the CPA of the units counting unit as an upward carry signal.

The circuit structure of the hour ones counting unit is still the same as that of the seconds or ones counting unit, but it is required that the entire hour counting unit should be a decimal counter, not an integer multiple of 10, so the ones digit and Only when the ten-digit counting unit is combined into a whole can the hexadecimal conversion be performed. The circuit using a piece of 74HC390 to realize the hexadecimal counting function is shown in Figure 3-6.

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

4) Decoding driver and display unit

The counter realizes the accumulation of time and outputs it in the form of 8421BCD code. A display decoding circuit is selected to convert the output digital of the counter into a digital display device. For the required output logic and certain current, CD4511 is selected as the display decoding circuit, and LED digital tube is selected as the display unit circuit.

5) Time correction power circuit

When the power is turned on again or when an error occurs in the travel time, the time needs to be corrected. Usually, the method of correcting the time is: first cut off the normal counting path, and then perform manual trigger counting or add a higher frequency square wave signal to the input end of the counting unit that needs to be corrected. After correction, switch to normal Timing status is enough.

According to the requirements, the digital clock should have branch correction and time correction functions. Therefore, the direct counting path of the units digit and the time units digit should be cut off, and a circuit that can switch between the normal timing signal and the correction signal at any time should be used. Access it. Figure 3-7 shows a time correction circuit with a basic RS flip-flop.

6) Hourly time chime circuit

General clocks should have the hourly chime circuit function. That is, a few seconds before the hour hits the hour, the digital clock will automatically chime the time as a reminder. Its mode of action is to emit continuous or rhythmic audio sound waves, and more complex ones can also be real-time voice prompts.

According to the requirements, the circuit should start to chime the hour within 10 seconds before the hour, that is, when the time is between 59 minutes and 50 seconds and 59 minutes and 59 seconds, the time chime circuit will chime the time control signal. Choose 74HC30 for the timekeeping circuit and choose the buzzer as an electroacoustic device.

IV. Components

Equipment required for the experiment

5V power supply. 1 breadboard. Oscilloscope. multimeter. 1 pair of tweezers.

1 pair of scissors. The network cable is 2 meters per person. ***6 negative eight-segment digital tubes.

CD4511 integrated block 6 pieces.

CD4060 integrated block 1 piece. 74HC390 integrated block 3 pieces. 74HC51 integrated block 1 piece.

5 pieces of 74HC00 manifold. 1 piece of 74HC30 manifold. 5 10MΩ resistors.

14 500Ω resistors. 2 30p capacitors. 32.768k clock crystal 1 piece.

Buzzer.

Five function block circuit diagrams

1. A CD4511 and an LED digital tube are connected to form a CD4511 drive circuit. The digital tube can display from 0---9, and then Check the quality of the digital tube.

4511 drive circuit

2. Use an LED digital tube, a CD4511, a 74HC390, and a 74HC00 to connect into a decimal counter. Under the action of the crystal oscillator, the circuit changes the digital tube from 0 to 9 shows.

74390 decimal counter

3. Use an LED digital tube, a CD4511, a 74HC390, a 74HC00 and a crystal oscillator to form a hexadecimal counter. The digital tube ranges from 0-6 show.

74390 Hexadecimal Counter

4. Use a hexadecimal circuit and a decimal connected to form a sexagesimal circuit. The circuit can display from 0-59.

Sexagesimal circuit

5. Use two sexagesimal circuits to synthesize a double sexagesimal circuit, with a carry between the two sexagesimals.

Double sexagesimal circuit

6. Use CD4060, resistor and crystal oscillator to form a frequency division - crystal oscillator circuit.

Frequency division - crystal oscillator circuit

7. Use 74HC51D and 74HC00 and resistors to connect them to form a timing circuit.

Time correction circuit

8. Use 74HC30 and a buzzer to connect to the hourly time clock circuit.

Hourly time chime circuit.

9. Use two sexagesimal numbers and one hexadecimal number to connect to form a circuit that carries hours, minutes and seconds. .

Carry concatenation of hours, minutes and seconds.

6. General wiring component layout.

7. Chip connection.

8. Summary

1. Problems encountered during the design process and their solutions.

1) During the process of testing the condition of the breadboard, there was a situation where the connection should be connected but not. After inspection, it was found that the tip of the multimeter was not in vertical contact with the inside of the breadboard.

2) During the process of testing the CD4511 drive circuit, it was found that the digital tube could not display normally. After inspection, it was found that it was mainly due to poor contact, including poor wire contact and poor chip contact. During the experiment, several sections of diodes on the digital tube appeared and disappeared. Use a 5V power supply to test the digital tube. One end is grounded and the other end is in contact with each section of the diode. It is found that the diode can display normally. Then use the ohm setting of the multimeter to check whether each wire is in good contact. During the detection process, it is found that several wires sometimes It can be connected, but sometimes it cannot be connected. After reconnecting the wire with poor contact, I found that it can display normally. Secondly, due to the problem of poor chip contact, I used the ohm setting of the multimeter to detect that several pins were not connected where they should be connected. However, the detected wires were in good condition. The solution was to pull out the CD4511 chip and check the holes in the breadboard. Re-adjust the pins so that they are facing the holes, and then insert the chip into the breadboard evenly. Afterwards, it was found that it can display normally. In this experiment, a broken LED digital tube and two broken CD4511 were also found. , can display normally after replacement.

3) During the process of connecting the crystal oscillator, the crystal oscillator cannot start. After eliminating the problem of poor contact between the wire and the chip, we checked the circuit diagram again and found that the problem was caused by pin 12 not being connected to ground.

4) During the process of connecting the hexadecimal system, it was found that the circuit could only jump between 4 and 5. Later, it was found that the pin connected to the NAND gate was connected to the wrong pin. This was corrected. can be displayed normally.

5) During the process of connecting the correction circuit, the hours and minutes can be corrected normally, but the seconds are affected, especially when the hours and minutes are larger than the seconds, the seconds jump randomly and the time is not corrected. , the seconds jump from 40 to 59, and then jump back to 40. There is no carry between minutes and seconds. The circuit can display normally during the carry process of hours, minutes and seconds, so the problem of poor contact between the chip and the wiring can be eliminated. After checking, there is no error in the connection of the calibration circuit. Then, the DC voltage range of the multimeter is used to test the QA, QB, QC and QD pins of the ten seconds digit. It is found that the QA pin has voltage sometimes and sometimes no voltage. Then the second to minute and minute are detected. At the carry end, it was found that the carry from seconds to minutes was not unplugged.

6) During the process of making the timekeeping circuit, it was found that the buzzer started to time at 57 minutes and 59 seconds. Later, after testing the circuit, it was found that the 74HC30 chip was connected as a 16-pin chip. , even the wiring was misplaced, but after rewiring, it can tell time normally.

7) When connecting the frequency dividing circuit, disconnect the QD of the hour's ones digit and the 1 pin of the hour's tens digit, then connect the 1 pin of the hour's tens digit to the 3 pin of the crystal oscillator, and the 3rd pin of the hour's tens digit. The pin is connected to pin 1 of the ones digit of seconds. The connected circuit diagram does not work properly. The tens digit of the hour jumps from 0-9, and the ones digit of the hour can only display a 0. In this circuit, the frequency division of pin 3 is used twice. , so it cannot be displayed normally, so we need to connect the hexadecimal system to a logic circuit of 74HC390 and free it up for frequency division. Therefore, pins 12 and 6 of the ten-digit CD4511 are grounded, and pin 7 is changed to be connected to pin 5 of 74HC390. Pins, disconnect pins 3 and 4 of 74HC390, and then connect pin 4 to pin 9. The free pin 3 of 74HC390 can be used for 2Hz frequency division. After frequency division, it becomes 1Hz, and the entire circuit is now normal. digital clock counting.

2. Design experience

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

In the connection method of connecting hexadecimal, decimal, hexadecimal carry and hexadecimal, it is required to be familiar with the functions of the logic circuit and the pins of the chip, so that you can accurately find out when the circuit goes wrong. The error was identified and corrected promptly.

In designing circuits, simulation is often done first and then the physical diagram is connected. However, sometimes the simulation and circuit connections are not completely consistent. For example, in the simulated connection diagram, pin 16 is often not connected to the high level. Or pin 14 and pin 7 or pin 8 connected to low level, so it is often easy to miss in the actual circuit connection. Another example is the 74HC390 chip, which itself is a decimal counter. In the simulation circuit, the feedback line must be connected for normal display, but there is no need to connect it in the actual circuit. Therefore, there is still a certain difference between the simulation diagram and the circuit connection diagram.

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

3. Suggestions for this design

This digital clock design focuses on simulation and wiring. Although the circuit diagram can be connected and displayed normally, the circuit itself The principle is not very familiar. In general, this design experiment further enhanced the hands-on ability of the experiment.

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