STC89C51 clock

As shown in Figure 2-1, this system uses STC89C51RC microcontroller as the core. The clock chip provides time and date information. The temperature and humidity detection module collects temperature and humidity information. The function key input cooperates with the main control chip to realize various functions. LCD1602 is used as the display. Information and human-computer interaction interface are provided, and the buzzer can realize various prompt sounds.

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• 1 Hardware design

1.1 Display module

The display module uses LCD1602 liquid crystal screen. LCD1602 liquid crystal display is a widely used character-type liquid crystal display module. It consists of a character-type liquid crystal display (LCD), main control and drive circuit HD44780 and its extended drive circuit HD44100, as well as a small number of resistors, capacitors and structural parts assembled on a PCB board. The circuit uses pin headers to connect the LCD1602 display module to the circuit. The circuit diagram is shown in Figure 3-1.

Table 3-1 shows the pin function diagram of LCD1602. Its number corresponds to the number of the female header U3 port. Port 3 is used to adjust the contrast of the liquid crystal. The contrast is the weakest when connected to the positive power supply, and the contrast is the highest when connected to the ground. If it is too high, a "ghosting" phenomenon will occur, so a 10K variable resistor is added to adjust the contrast to make the LCD character display clearer. Port No. 5 corresponds to the R/W pin. According to the actual system function requirements, this port is directly grounded to save the microcontroller port.

Table 3‑1 LCD1602 pin function table

1.2       Temperature and humidity sensor

The temperature and humidity sensor uses the DHT11 sensor. The DHT11 digital temperature and humidity sensor is a temperature and humidity composite sensor with calibrated digital signal output. It applies dedicated digital module acquisition technology and temperature and humidity sensing technology to ensure that the product has extremely high reliability. and excellent long-term stability. The single-wire serial interface makes system integration easy and fast. The ultra-small size and extremely low power consumption make it the best choice for this type of application in harsh applications.

Table 3-2 shows the pins and functions of the DHT11 temperature and humidity sensor. Its pins correspond to the U1 pin numbers in Figure 3-2. A 4.7K resistor is pulled up on the DATA serial data bus to improve communication stability and accuracy. .

Table 3‑2 DHT11 pin function diagram

Figure 3‑2 DHT11 module circuit

1.3 DS12C887 module

The clock chip uses the DS12C887 clock chip. The DS12C887 clock calendar chip adopts a 24-pin dual-in-line package. It integrates a crystal oscillator, an oscillation circuit, a charging circuit and a rechargeable lithium battery to form a thickened integrated circuit module. When there is no external power supply can work for 10 years. It has the advantages of good microcomputer interface, high precision, simple peripheral interface, stable and reliable operation, etc., and can be widely used in various real-time situations that require higher precision.

The DS12C887 clock chip module circuit is shown in Figure 3-3, and the clock chip pins and functions are shown in Table 3-3. The MOT pin is grounded and the working mode is selected as Motorola mode, using the Motorola communication protocol. When no interrupt condition exists on the IRQ# pin, the IRQ level is in a high impedance state, so the 4.7K resistor is pulled up, and the RESET# pin is directly connected to VCC. This ensures that the internal control registers of the DS12C887 are not affected when the power is turned off.

Table 3‑3 DS12C887 pin function table[2]

Figure 3‑3 DS12C887 module circuit

1.4 Buzzer sound circuit

The buzzer sound circuit is shown in Figure 3-4. The buzzer uses a passive buzzer and uses a triode to drive the buzzer. When P15 outputs a low level, the triode is turned on, and P15 outputs pulse signals of different frequencies through the microcontroller. To drive the passive buzzer to sound different frequencies. Resistors R3 and R8 are used to limit the path current and prevent excessive current from damaging the circuit. Figure 3‑4 Buzzer drive circuit

1.5 Button input circuit

The system has four function buttons and one reset button. The circuit diagram is shown in Figure 3-5. The setting button, temperature and humidity display button, value setting button, and confirmation button are respectively connected to the IO port of a microcontroller. When the button is pressed, the circuit is turned on, and the level of the IO port connected to the button is pulled low. The microcontroller detects this low power. The flat signal determines whether the corresponding button is pressed. Among them, SW3 (temperature and humidity display button) is connected to P3.3 (INT1) of the microcontroller and uses interrupts to realize the temperature and humidity display function.