Application of LPC21XX Series Microcontroller Based on ARM7 in BAS

1. Overview of Building Automation System

Building Automation System (BAS) is actually a central monitoring system. It centrally monitors various power equipment, air conditioning equipment, cold and heat source equipment, fire protection, anti-theft equipment, etc. in a building (or building complex), so as to achieve the goal of ensuring a comfortable environment in the building, taking full account of energy conservation and environmental protection, and making the status and utilization rate of various equipment in the building reach the best.


Figure 1 BAS components

Figure 1 is a schematic diagram of the composition of BAS, in which the central control room (data center) mainly includes three parts: central processing unit (a microcomputer, memory, tape drive and interface device), peripheral equipment (display terminal, keyboard, printer) and uninterruptible power supply.

Sensors refer to various sensitive elements, contacts and limit switches, and transmitters installed on site, which are used to detect various parameters of field equipment (temperature, humidity, pressure, liquid level, etc.) and send signals to controllers (substations, data centers, etc.).

Actuators refer to mechanisms installed on site to receive output command signals from substation controllers and control field operating equipment, such as electric valves, actuators and regulating mechanisms.

Substation controllers are direct digital controllers (DDCs) based on microprocessors. They receive signals from sensors, perform digital operations, logical judgments, and automatically output control signals to actuators after processing. Substation controllers are the core of the entire control system. They have four input/output interfaces: AI, AO, DI, and DO.

2. Overview of ARM and LPC21XX series microcontrollers

Here we use the LPC21XX microcontroller of PHILIPS based on the ARM7 core. ARM stands for Advanced RISC Microprocessor. The ARM series is a 32-bit microprocessor RISC microcontroller solution. In portable communications, digital multimedia and industrial control, the use of ARM is becoming a standard trend. According to relevant reports, in 2001, 74.6% of the sales of many RISC microprocessors were ARM microprocessors. ARM licenses its technology to many well-known semiconductor, computer and OEM manufacturers in the world, providing ARM-related technology and services. With this partnership, ARM quickly became the creator of many global RISC standard products. At present, a total of more than 70 companies have signed hardware technology license agreements with ARM, including major companies such as INTEL, IBM, LG, NEC, SONY, and PHILIPS.

The ARM7 processor is a member of the ARM general-purpose 32-bit microprocessor family. The ARM processor has excellent performance but low power consumption. The ARM structure is designed based on the principle of Reduced Instruction-Set Computer (RISC). The instruction set and related decoding mechanism are much simpler than the Complex Instruction Set Computer (CISC). The ARM7 processor uses a 3-stage pipeline to increase the speed of the processor instruction stream, which allows several operations to be performed simultaneously and allows the processing and memory systems to operate continuously. The ARM7 processor uses the von Neumann structure, and instructions and data share a 32-bit bus. Only load, store and exchange instructions can access the data in the memory.

PHILIPS's LPC21XX series microcontroller is an MCU based on a CPU containing an ARM7TDMI-S core and an embedded Flash memory. It also has two CAN channels. The 128-bit wide memory interface and unique acceleration structure enable 32-bit code to run at an operating frequency of up to 60MHZ. Because the LPC21XX series uses a very small LQFP or HVQFN package, extremely low power consumption, multiple 32-bit timers, 4-way 10-bit ADC, PWM output and up to 9 external interrupts. The operating temperature range of some industrial microcontrollers in this series can be from -40 to 85 ℃, which makes it particularly suitable for application areas such as building automation systems.

The following table lists two products that have been used as controllers in building automation systems, including 8-bit MCUs and 16/32-bit microcontrollers LPL21XX using the ARM7 core. Since there are many manufacturers designing and producing 8-bit MCUs, the on-chip resources are also very different depending on the scope of application, and the author only selected a representative chip for comparison.


Table 1: Comparison of resource performance of controllers used in BAS[page]

From the table, we can more intuitively understand the performance improvement of our building automation system brought by the 16/32-bit microcontroller using the ARM7 core, which achieves higher instruction throughput, better real-time interrupt response, and lower power consumption.

3. Technical solution of LPC21XX application in BAS

Figure 2 shows the simplified block diagram of the substation controller CPU module of the BAS using the LPC21XX series including two CAN controllers.


Figure 2 Block diagram of a BAS microcontroller using LPC21XX

The CAN controller uses a combination of logic circuits to realize the communication between the network physical layer and the data link layer. It provides an interface with the physical line of the microprocessor. Through its programming, the CPU can set its working mode, control its working state, send and receive data, and build the application layer on its basis. The CAN transceiver can also be called a CAN bus driver. It is used to provide an interface between the CAN controller and the physical bus and the differential sending and receiving functions of the CAN bus.

In Figure 2, a CAN controller is responsible for communicating with the four MCU input/output interfaces of AI, AO, DI, and DO through a CAN transceiver, and another CAN controller is responsible for communicating with the host computer through another CAN transceiver. In addition, the Ethernet interface can also be used to realize remote communication between the host computer and the MCU. The serial port is mainly used to directly download the configuration program and control the display of the LCD panel.

In addition, we have expanded the 16-bit SRAM, which has the advantages of fast reading and writing speed and no number of times; in order to ensure that there is space for storing a large RTOS (real-time operating system), and because an external memory subsystem responsible for memory image storage and program execution is required, a 16-bit FLASH is also expanded. , and also uses a power monitoring chip. When the SRAM loses power, it will trigger the transfer of SRAM data to the FLASH memory.

4. Conclusion

Until now, there is no unified and detailed national standard for domestic building control products. In the process of product design and development, we are based on the corresponding actual needs and draw on some industry specifications that have been generally recognized in intelligent building systems. At present, embedded technology and products based on ARM chips and embedded operating systems are in the ascendant, and the application prospects in the field of building automation are also very broad. In future research and development, we will further make full use of the powerful processing and control capabilities of 32-bit ARM microcontrollers, and make them more intelligent and flexible through the development of embedded RTOS to meet the ever-changing and increasing market needs.

References:

[1]. DDC datasheet http://www.dzsc.com/datasheet/DDC_1870710.html.
[2]. RISC datasheet http://www.dzsc.com/datasheet/RISC_1189725.html.
[3]. ARM7TDMI- S datasheet http://www.dzsc.com/datasheet/ARM7TDMI-S_1231795.html.
[4]. LQFP datasheet http://www.dzsc.com/datasheet/LQFP_1054170.html.