Digital Electronic Technology Course Design Report

1. Design Purpose

A digital clock is a device that uses digital circuit technology to realize hours, minutes and seconds. Compared with mechanical clocks, they are more accurate and intuitive, have no mechanical devices, and have a longer service life, so they are widely used.

In principle, a digital clock is a typical digital circuit, which includes combinational logic circuits and sequential circuits.

Therefore, the purpose of our design and production of digital clocks this time is to understand the principles of digital clocks and learn to make digital clocks. And through the production of digital clocks, we can further understand the various small and medium-sized clocks used in production. The role and practical methods of large-scale integrated circuits. And because digital clocks include combinational logic circuits and sequential circuits, you can further learn and master the principles and usage methods of various combinational logic circuits and sequential circuits.

2. Design requirements

(1) Design indicators

① Time is based on 12 hours as a cycle;

② Display hours, minutes and seconds;

③ It has a time correction function, which can adjust the hours and minutes separately to the standard time;

④ The timing process has a time reporting function. When the time reaches 10 seconds before the hour, Buzzer time reporting;

⑤ In order to ensure the stability and accuracy of timing, the crystal oscillator must provide the hand time reference signal.

(2) Design requirements

① Draw the circuit schematic diagram (or simulation circuit diagram);

② Selection of components and parameters;

④ 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. Principle 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.

(a) Block diagram of digital clock

2. Crystal 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. Generally, there are two types of digital crystal oscillator circuits with a square wave output. One type is composed of TTL gate circuits; the other is a circuit composed of CMOS NOT gates. This design uses the latter one. As shown in Figure (b), a crystal oscillator circuit is composed of a CMOS inverter U1, a crystal, a capacitor, and a resistor. U2 implements the shaping function and converts the oscillator output waveform that is similar to a sine wave into a more ideal square wave. The output feedback 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.

(b) CMOS crystal oscillator (simulation circuit)

3. Time counting circuit

Generally, decimal counters such as 74HC290, 74HC390, etc. are used to realize the counting function of the time counting unit. 74HC390 was selected in this design. It can be seen from its internal logic block diagram that it is a dual 2-5-10 asynchronous counter, and each counter has 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 2.4, in which Q2 can be used as an upward carry signal to be connected to the CPA of the units counting unit.

Decimal to hexadecimal conversion circuit

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 bit counting unit should be connected to the CPA of the tens counting unit as an upward carry signal, and Q2 of the tens counting unit should be connected to the CPA of the ones 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 hexadecimal 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 that uses a piece of 74HC390 to realize the hexadecimal counting function is shown in Figure (d).

(d) Dodecimal circuit

In addition, in the circuit shown in Figure (d), there is still a binary counting unit, which can be used as a frequency divider 2HZ output signal conversion For 1HZ signal purpose.

4． Decoding driver and display unit circuit

Select CD4511 as the display decoding circuit; select LED digital tube as the display unit circuit. The input binary signal is translated into decimal numbers by CD4511, and then displayed by the digital tube. The LED digital tubes here are connected using the positive and negative method.

The counter realizes the accumulation of time and transmits it to the CD4511 chip in the form of 8421BCD code. The 4511 chip then converts the BCD code into a decimal number and sends it to the digital tube for display.

5. Time correction circuit

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. It is a time or branch time circuit implemented using COMS NOR gates. The In1 terminal is connected to the low-bit carry signal; the In2 terminal is connected to the correction signal. The correction signal can be directly taken from the 1HZ or 2HZ generated by the frequency divider (not too high or too low) signal; the output terminal is connected to the minute or hour units timing input terminal. When the switch is turned down, because the output of the phase sum of the correction signal and 0 is 0, and the other end of the switch is connected to high level, the normal input signal can pass through the AND or gate smoothly, so the timing circuit is in a normal timing state; when the switch When it is turned upward, the situation is just opposite to the above, and the time adjustment circuit is in the time adjustment state.

In actual use, because the circuit switch has a jitter problem, an RS flip-flop is generally connected to form a switch debounce circuit, so the entire circuit is as shown in Figure (f).

(f) Correction circuit with anti-bounce circuit

6. Hourly time chime circuit

The circuit should start the hourly chime 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.

When the time ranges from 59 minutes and 50 seconds to 59 minutes and 59 seconds, the minutes and tens digits, minute units and seconds digits remain unchanged, which are 5, 9 and 5 respectively, so the minutes can be changed The QC and QA of the tens digit of the counter, the QD and QA of the ones digit, and the QC and QA of the tens digit of the seconds counter are phased together to generate a timekeeping control signal.

The time reporting circuit can be composed of 74HC30. The 74HC30 is an 8-input NAND gate.

4. Components

1. 1 piece of four-breadboard (No. A45)

2. 1 pair of tweezers

3. 1 pair of scissors

4. ***6 negative eight-segment digital tubes

5. Network cable 2 meters/person

6. 6 pieces of CD4511 integrated blocks

7. CD4060 integrated block 1 piece

8. 74HC390 integrated block 3 pieces

9. 74HC51 integrated block 1 piece

10. 74HC00 integrated block 4 pieces

11. 1 piece of 74HC30 manifold

12. 5 pieces of 10MΩ resistors

13. 14 pieces of 500Ω resistors

14. 2 pieces of 30p capacitors

15. 1 32.768k clock crystal

16. 10 buzzers (per shift)

1) Chip connection diagram

1)74HC00D 2)CD4511

3)74HC390D 4)74HC51D

2. Introduction to Breadboard

A breadboard consists of five parts, one vertical and four horizontal. The breadboard itself is a solderless board.

The breadboard style is:

Breadboard precautions:

1. There are usually banana sockets next to the breadboard for input voltage, signals and grounding.

2. The connected black lines in the picture above indicate that the jacks are connected.

3． When pulling the wires, try to keep the wires as close to the breadboard as possible, and keep the wires at right angles to avoid crossing or crossing components.

4． After the breadboard has been used for a long time, the copper wires connecting the jacks may sometimes fall off. In this case, mark the row of jacks. and no longer used.

5. Circuit diagram of each functional block

In principle, a digital clock is a typical digital circuit that can be composed of many small and medium-sized integrated circuits, so it can be divided into many independent circuits .

(1) Hexadecimal circuit

It is composed of 74HC390, 7400, digital tube and 4511. The circuit is shown in Figure 1.

(2) Decimal circuit

It is composed of 74HC390, 7400, digital tube and 4511. The circuit is shown in Figure 2.

(3) Sexagesimal circuit

It consists of two digital tubes, two 4511s, a 74HC390 and a 7400 chip. The circuit is shown in Figure 3.

(4) Double sexagesimal circuit

It is composed of two sexagesimals. It connects the input signal of the units digits to the Qc of the tens digits of seconds to generate a carry. The circuit diagram is shown in Figure 4.

(5) Time counting circuit

It consists of 1 twelve-digit circuit and 2 six-digit circuit. Since there is already a double-sixty circuit on it, just combine it with the twelve-digit circuit Just connect the circuit. See Figure 5 for the detailed circuit.

(6) Correction circuit

It is composed of 74CH51D, 74HC00D and resistors. The correction circuit has two parts: sub-correction and time correction. The circuit is shown in Figure 6.

(7) Crystal oscillator circuit

It consists of a crystal, two 30pF capacitors, a 4060, and a 10M resistor. Pin 3 of the chip outputs a 2Hz square wave signal. The circuit As shown in Figure 7.

(8) Hourly time chime circuit

It is composed of 74HC30D and buzzer. When the time is from 59:50 to 59:59, the buzzer will chime the time. The circuit is as shown in Figure 8.

6. Simplified layout of main wiring components

The entire digital clock is composed of a time counting circuit, a crystal oscillator circuit, a correction circuit, and an hourly timekeeping circuit.

The correction circuit is used to replace the carry between hours, minutes and seconds in the time counting circuit. When the time correction circuit is in the normal input signal, the time counting circuit will time normally, but when it is corrected, it will not A carry to time is generated, and the correction of the minute and time is separate, and the correction circuit is also an independent circuit.

The signal input of the circuit is generated by the crystal oscillator circuit and input to each circuit.

The simplified diagram is shown in Figure 9.

7. Overall chip connection diagram

Due to the difference between simulation and actual components, on the basis of the original simplified diagram, this diagram based on the actual chip was drawn according to the actual layout. The wiring diagram of the layout is shown in Figure 10.

8. Summary

1. Problems encountered during the experiment and their solutions

① Breadboard test

Test whether the contacts of the breadboard are connected.

② Measurement of seven-segment display and seven-segment decoder

Connect the display to CD4511. When connecting for the first time, the digital tube did not display any numbers at all. After inspection, it was found that it was digital. It was caused by the tube not being grounded. After grounding, it was found that the numbers still could not be displayed correctly. After testing with a multimeter, it was found that it was caused by some poor contact between the chip pins, so it is very important to confirm whether the chip is in good contact.

③ Connection and testing of time counting circuit

There are no big problems with hexadecimal and decimal systems. It is just an old problem with the chip pins. It can be solved by re-inserting the chip. . But in sexagesimal system, after wiring according to the diagram, it was found that the numbers on the display were always in base 100 system, not sexagesimal system. After testing, it was found that there was no problem with the connection of the circuit or the contact of the chip. Finally, when reconnecting the wiring, I found that it was caused by the wrong pin connection. After correcting the problem, the display became normal.

④ Correction circuit

Since the above process caused errors due to wrong pin connections, the correction circuit is connected completely according to the simulation diagram. During the test, the time calibration is started. , no problem occurred, but when it came to the branch branch, it was found that the second circuit of the counting circuit began to jump randomly and made errors. Therefore, there must be something wrong with the circuit. After repeated comparisons, it was found that it was caused by forgetting to remove the connection between the tens digit of seconds and the ones digit of minutes when connecting to the correction circuit. Therefore, when wiring, Be sure to remove unnecessary unnecessary threads.

2. Design experience

Through this design and production of a digital clock, I learned about the procedures for designing circuits. It also allowed me to understand the principles and design concepts of digital clocks. To design a circuit, you must first The actual wiring is done only after the simulation is successful. However, the final product may not be exactly the same as the simulation, because there are various conditions restricting the actual wiring. Moreover, circuit connections that cannot be successful in simulation can be successful in reality due to the characteristics of the chip itself. Therefore, the differences between the two should be considered during design to find the most suitable design method.

Through this study, I have a general understanding of various circuits. Therefore, it is better to walk than to sit and talk. For these circuits, I should actually operate them myself to have a deep understanding. .

3． Suggestions on design

I hope that the teacher will tell us some information and principles about the circuit before we start making it, as well as how to detect the circuit and how to detect the chip. This will help us further enter the situation and complete the design