Design of multi-point temperature control heating control system using SST89E564RC single chip microcomputer

A multi-point temperature control heating control system was designed using the SST89E564RC single-chip microcomputer and a new temperature measuring device. The heating system can be controlled in real time according to the temperature settings of each point in the room, thereby improving the comfort of the room and the economy of heating.

1 System Design Goals

The overall design concept of the system is to use the SST89E564RC single-chip microcomputer as the control core. The hardware of the entire system includes the temperature detection part, the control execution part, the display and keyboard system and the minimum system basic circuit. The system uses the single-chip microcomputer to obtain the temperature sensor data and compare it with the system design value, and controls the execution system according to the comparison results. The control block diagram of the temperature control system is shown in Figure 1.

2 System Hardware Design

According to the functions that the system needs to complete, the system hardware structure is designed as shown in Figure 2.

2.1 Control Core

The system uses SST89E564RC microcontroller as the control core to perform temperature acquisition, information display and actuator control. SST89E564RC is a high-reliability, small-sector FLASH microcontroller launched by SST Corporation of the United States. It has 72 KB of Super-Flash and 1 KB of RAM embedded inside. By further expanding its RAM, it can meet the operating conditions of the embedded system operating system.

2.2 Temperature Sensor

The temperature sensor uses the digital temperature sensor DS18820 from Dallas Semiconductor. The sensor supports the "one-line bus" interface, which can easily measure multi-point temperature. It can also be programmed to set the resolution of 9 to 12 bits, with a maximum accuracy of ±0.062 5°C. The resolution setting and the alarm temperature set by the user are stored in the E2PROM and are still saved after power failure. The product supports a voltage range of 3 to 5.5 V, and its small size makes the system design more flexible and convenient. The pin arrangement of DS18820 is shown in Figure 3, where DQ is the digital signal input/output terminal; GND is the power ground; and VDD is the external power supply input terminal.

The internal structure of DSl8820 mainly consists of four parts: 64-bit photolithography ROM, temperature sensor, non-volatile temperature alarm triggers TH and TL, and configuration registers.

The 64-bit serial number in the photolithography ROM is photolithography-prepared before leaving the factory, and it can be regarded as the address serial code of the DS18820. The function of the photolithography ROM is to make each DS18820 different, so that multiple DS18820s can be connected to one bus.

The internal memory of the DS18820 temperature sensor includes 9 B high-speed temporary RAM and 1 B non-volatile electrically erasable E2PROM, which stores high temperature and low temperature triggers TH, TL and structure registers. The 7th bit (TM) of this byte is 0, the lower 5 bits are always 1, and the 6th and 5th bits (R1, R0) are used to set the resolution, as shown in Table 1.

According to the communication protocol of DS18820, the host must go through three steps to control DS18820 to complete temperature conversion: reset it before each reading and writing, send a ROM instruction after the reset is successful, and finally send a RAM instruction, so that the predetermined operation can be performed on DS18820. Reset requires the main CPU to pull down the data line for 500μs and then release it. After receiving the signal, DS18820 waits for about 16 to 60μs, and then sends a low pulse of 60 to 240μs. The main CPU receives this signal to indicate that the reset is successful. Since DS18820 uses a single line for control and data reading, the timing requirements for the operation are very strict, otherwise the correct operation of the device will not be completed due to timing mismatch.

2.3 Controlling execution

(1) Control of the combustion system of the wall-mounted boiler. The control circuit uses a pulse relay device as the overall control part of the entire system. When the temperature of all rooms reaches the set value, the wall-mounted boiler stops working. The characteristics of this relay are: when the coil receives a pulse signal, the coil is energized, the electromagnet is attracted, and the contact is driven to close and connect the circuit to be controlled. When the next signal arrives, the electromagnet is attracted, the contact is disconnected, and the controlled power supply is cut off. Therefore, it has self-locking and signal remote control functions. Due to the effect of the magnet, the position of the slider does not change after the control pulse disappears and remains stable, so the device has low power consumption and memory function.

(2) Room temperature control. Under the premise of the combustion control system working, the room temperature control uses the OK6515 self-holding pulse solenoid valve produced by Shanghai Okai Solenoid Valve Manufacturing Co., Ltd. to control the on and off of each circuit according to the return value of the room temperature measurement. The pulse solenoid valve adopts pulse and permanent magnet technology. It only needs to switch the electrode contact of the pulse by the controller to change the switch state of the solenoid valve. When the controller sends out an electric pulse, the valve core is driven to overcome the permanent magnetic force and move up and down, so that the valve disc is in a self-holding state under the action of the permanent magnet after it is in place.

2.4 Graphic LCD display module

In order to provide an intuitive user display interface, the system uses a graphic liquid crystal display module LCDl2864, which has an 8-bit standard data bus, 6 control lines and a power line, and can be directly interfaced with the CPU to display various characters and graphics. Considering the small amount of Chinese characters used in the system, an LCD without a Chinese character library is selected. The fonts of the Chinese characters used are extracted and saved in the internal FLASHROM in binary form.

3 System Software Design

The system software design is mainly based on the system program flow and the timing requirements of DS18820. In order to reduce the development difficulty and improve the development efficiency, the μC/OS-II embedded operating system was introduced into the system development and the LCD display driver was transplanted. On the other hand, in order to ensure the accuracy of the DS18820 operation timing, the DS18820 initialization and read and write code are still in assembly language.