Low power consumption system design based on MCP2030 wireless activation

Introduction

After entering the 21st century, with the continuous popularization and development of social informatization, the application of embedded systems has become more and more extensive. Among them, the design and application of self-powered embedded systems have been restricted due to power consumption limitations. Generally speaking, the current consumption of a normal working embedded system is at the mA level, while it can be controlled at around μA level in the dormant state. The energy saving of three orders of magnitude is undoubtedly very tempting for the limited self-powered power supply, so such systems basically adopt the dormant activation solution to achieve energy saving and extend the working life.

There are currently three main dormant activation solutions available: event activation method, timing activation method and positioning activation method. The event activation method is mainly used in detection alarms and other occasions. The system is generally in sleep mode. If a specific parameter exceeds the limit, the system will be activated. This method is generally implemented in conjunction with the corresponding sensor, and the microprocessor also occupies the corresponding interrupt resources; the timing activation method is mainly used in periodic systems (such as the collection of three-meter data in a community). The system regularly reports the collected data according to the time interval set by the timer. The implementation of this activation method is also very convenient. You only need to add the interrupt handler of the timer in the corresponding microprocessor; the positioning activation method is mainly used in location-sensitive systems (such as valuable asset management and automatic gates in parking lots). The system installs detection equipment at specific locations. If supervisors or equipment leave or enter these specific areas, the system will be activated. There are many ways to implement the positioning activation method. This article mainly introduces a method for implementing positioning activation using wireless signals.

1 Basic Principles

The LF band signal in the wireless signal spectrum has the characteristics of strong penetration. It can penetrate non-magnetic media such as water, concrete, plastic, etc. (not limited by line of sight distance), so using the LF band to design an activation circuit is a better solution. There is an inverse proportional relationship between the frequency of the wireless signal and the wavelength, and the antenna length depends on the wavelength length. The wavelength of 500MHz RF signal is 60cm, so the antenna is very short and can be easily realized; while the wavelength of 125kHz LF signal is 2.4km, so it is definitely not practical to make such an antenna. Therefore, the LF band signal is used as the activation signal, and the receiving end no longer uses the electromagnetic field (radio) principle to work, but directly receives the magnetic field (magnetic) signal, and then uses the induced signal of the magnetic field in the coil for judgment and processing, as shown in Figure 1. The system mainly consists of two devices: the magnetic field transmitter and the receiver.

Figure 1 Working principle of magnetic field

MCP2030 is an analog front-end device developed by Microchip specifically for low-frequency wireless magnetic field communication. The device integrates 8 programmable configuration registers and 1 read-only status register. According to the register configuration, MCP2030 can output demodulated data, carrier clock and magnetic field strength RSSI. The analog receiving circuit of this device has strong sensitivity and can receive and identify 1mVpp signals and demodulate 8% weak modulation signals. In order to obtain reliable magnetic field signals, MCP2030 uses 3 groups of antennas and 3 groups of receiving and demodulating circuits. The 3 groups of antennas point to the mutually perpendicular X, Y, and Z axes respectively, so that no matter how the receiver is placed, the magnetic field signal can always be obtained, thus solving the directionality problem of the magnetic field signal. Its structural block diagram is shown in Figure 2.

Figure 2 MCP2030 block diagram

Figure 3: MCP2030 has output [page]

MCP2030 integrates wireless signal digital sequence filtering components, and the digital sequence can be set as needed. The device responds only when a specific digital sequence is received, so it can effectively avoid activation caused by other signal interference. Figure 3 shows the situation where the wireless digital sequence meets the set digital sequence. The specific digital sequence is a "2ms with 2ms without" carrier signal. At this time, LFDATA outputs an ASK modulation signal after detecting the specific sequence. If the wireless digital sequence does not meet the set digital sequence, LFDATA has no output.

MCP2030 has the significant advantage of extremely low power consumption. In order to facilitate its application in embedded systems with self-contained power supplies, three working modes are specially designed and optimized, namely sleep mode, standby mode and working mode. The sleep mode is controlled by the SPI interface command. After entering the sleep mode, except for the register, memory and SPI functional circuit, all circuits including the RF limiter will be turned off to minimize the current consumption (0.2μA). The device needs to be awakened from the sleep mode by power-on reset and any other SPI command except the sleep command. When there is no LF signal at the antenna input, the device will automatically be in standby mode, but the various circuits inside the device are powered on and ready to receive input signals. The typical current consumption in standby mode is 4μA (3 receiving antennas are working); when there is an LF signal on the LF antenna input and the internal circuit switches with the received data, the device is in low-current working mode, in which the current consumption is only 13μA.

In addition, the device also supports half-power and no-power working modes. In the no-power working mode, the device completely extracts energy from the magnetic field to work; in the half-power working mode, the device obtains energy from the magnetic field as much as possible, and is powered by the power supply when necessary. [page]

2 Design Application

Active RFID tags are an important part of RFID systems. They have the advantages of large storage capacity, long communication distance and rich functions. They can be widely used in logistics tracking, valuable asset management and other fields. The main components of its internal circuit are: controller, activation signal detection circuit, RAM/ROM, timer, UHF transceiver, power supply, etc. Among them, the activation signal detection circuit can be implemented by MCP2030, as shown in Figure 4. The ability of MCP2030 to identify and receive the set digital sequence can effectively control the working state of the tag. When the tag reaches a specific location where the RF activation transmitter is installed, MCP2030 outputs the corresponding receiving signal from the SPI interface, so that the controller exits the sleep state, and receives, analyzes and processes the data, and finally stores it in the corresponding position of RAM/ROM. When it is necessary to interact with the reader, the controller communicates through the UHF transceiver. After the controller is processed, it automatically enters the sleep state until the next magnetic field activation signal is received or the timer generates a timing interrupt.

Figure 4 Activation signal detection circuit

As shown in Figure 4, MCP2030 is connected to the controller through the SPI interface. The SPI interface definitions are LFDATA, SCCLK, and MCCS. The interface command consists of a 16-bit control word, and the command format is as follows:

D13~D15 are command types. MCP2030 determines the meaning of subsequent data based on the command type and performs corresponding operations. Among them, 0x07 is a write data command and 0x06 is a read data command. If it is a write data or read data command, then D9~D12 are register addresses, which respectively specify the register addresses to be operated by the command, D1~D8 are register data contents, and D0 is the command line verification information; if it is not a write data or read data command, the data content of D0~D12 is meaningless.

In order for MCP2030 to work properly, the device must be correctly initialized and configured when the system is powered on and reset. Here, the wireless signal digital filtering sequence is set to 2 ms with and 2 ms without, and the channel automatic selection function and demodulation signal output function are enabled. The initialization program segment is as follows:

The program for the controller to send data to MCP2030 is as follows:

The program segment for the controller to receive data from MCP2030 is as follows:

Conclusion

This article specifically introduces the application of MCP2030 in active RFID tags based on its characteristics. The device not only integrates 3-channel low-frequency receiving circuits and 3-directional magnetic field detection receiving circuits, but also has low power consumption and multiple energy-saving working modes. It is very suitable for other embedded system applications that require low-power wireless activation.

References

[1] Microchip Technology Inc. MCP2030 Datasheet, 2006.

[2] Microchip Technology Inc. Low Frequency Magnetic Transmitter Design, 20071029.