Active RFID Design Based on MSP430 and CC1100

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Active RFID Design Based on MSP430 and CC1100 [Copy link]

Radio Frequency Identification (RFID) technology is a contactless automatic identification technology that uses wireless radio frequency communication. Compared with the currently widely used barcode technology, RFID has the characteristics of large capacity, long identification distance, strong penetration ability, and strong anti-fouling. RFID technology has developed relatively mature and has been commercialized on a large scale. With the introduction of RFID portable devices, power consumption has become an issue that needs to be considered. This article will specifically explain the software and hardware implementation methods of bidirectional active tags based on the low-power design concept of MSP430F2012 and CC1100.

1. Low-power design

1.1 Overview of low power consumption

Power consumption is basically defined as the rate of energy consumption, which can be divided into two categories: transient power consumption and average power consumption. The two have different meanings, different application backgrounds and optimization strategies, and are usually generally summarized as low-power design. In actual research, they can be divided into the following according to different situations:

(1) Transient power consumption optimization: The goal is to reduce peak power consumption and solve circuit reliability problems.

(2) Average power consumption optimization: The goal is to reduce energy consumption within a given time, mainly for battery-powered portable electronic devices to extend battery life or reduce device weight.

1.2 Physical sources of power consumption

The power consumption of chip circuits mainly comes from two aspects: dynamic power consumption and static power consumption. Dynamic power consumption is mainly the charging and discharging of capacitors and short-circuit current. Static power consumption is mainly leakage current, including PN junction reverse current and subthreshold current, as well as penetration current. If the working timing and software algorithm design are defective, it will reduce the system working efficiency, prolong the working time, and directly increase the system energy consumption.

1.3 Low-power design strategy

Algorithm-level power consumption optimization: At the beginning of circuit design, it is necessary to select algorithms, and algorithms with high power efficiency should be selected as much as possible. First of all, from the perspective of the size of the logic required to implement the algorithm, the fewer the number of operations, the required bandwidth, the fewer storage operations, and the fewer port operations in the algorithm, the lower the power consumption of the circuit to which this algorithm is applied. In actual design, it is necessary to balance the overall performance and power consumption according to the requirements of the application. At the same time, the co-processing required in the algorithm must be considered. The simpler the co-processing required by the algorithm, the fewer the collaborative modules, and the lower the power consumption required to implement the algorithm. In addition, fewer temporary variables in the algorithm, the short effective time of temporary variables, and the reasonable use of loops will reduce the power consumption required by the algorithm.

System-level power consumption design and management: System-level power consumption management is mainly dynamic power consumption management. The usual practice is that when in idle state, it operates in sleep state, and only some devices are working; when an interrupt is generated, the interrupt wakes up other devices. In fact, this part requires hardware support, such as: low-power technology of the power system; the division of system hardware and software is to determine which functional modules are implemented by software with lower power consumption, and which functional modules are implemented by hardware with lower power consumption; the selection of low-power processors.

2. System hardware design

Comprehensively considering the system power consumption source and low-power design strategy, the hardware design selects microcontrollers and RF transceiver chips with low power consumption characteristics, and tries to simplify the circuit to reduce power consumption.

2.1 Selection of main chips

The structure of the MSP430 series of microcontrollers is completely based on the low-power operation of the system. The power supply adopts a low voltage of 1.8~3.6V, the power consumption in active mode is 250μA/MIPS, and the power consumption in RAM data retention mode is only 0.1μA. Since the system is in sleep or low-power state for more than 90% of the time, leakage current becomes another important factor affecting the power consumption of the system. The maximum leakage current of its I/O input port is only 50nA. In addition, there is a unique clock system design, including two different clock systems: basic clock system and frequency-locked loop (FLL and FLL+) clock system or DCO digital oscillator clock system. The clock system generates the clocks required by the CPU and various functional modules, and these clocks can be turned on or off under the control of instructions, thereby realizing the control of overall power consumption. Due to the different modules used when the system is running, that is, different working modes are adopted, the power consumption of the chip will be significantly different. There is a total of one active mode (AM) and five low-power modes (LPM0~LPM4) in the system. In addition, the MSP430 series of microcontrollers use vector interrupts, support more than ten interrupt sources, and can be nested arbitrarily. It only takes 6μs to wake up the CPU with an interrupt request. Through reasonable programming, it can not only reduce system power consumption, but also respond quickly to external requests.

The RF chip is the core part of the entire RFID card, which is directly related to the read and write distance and reliability of the tag, and also directly affects the power consumption of the entire system. CC1100 is a single-chip UHF wireless transmitter chip launched by Chipcon. It has a small size, low power consumption, and supports programmable control of 1.2~500kbps data rate. Its operating voltage range is 1.9~3.6V, and it can work in four bands of 915MHz., 868MHz., 433MHz and 315MHz. It can also be configured through the program to provide -30~10 dBm output power in all frequency bands. It has built-in address decoder, first-in first-out stack area, modulation processor, clock processor, GFSK filter, low noise amplifier, frequency synthesizer, power amplifier and other functional modules. It has two low-power working modes: shutdown mode and idle mode. In shutdown mode, the working current is less than 200nA. In this article, CC1100 works at a frequency of 433MHz, adopts FSK modulation, has a data rate of 100kbps, and a channel spacing of 200kHz.

2.2 Circuit Design

In order to simplify the system structure, this system only consists of the necessary microprocessor unit, RF transceiver unit, antenna and battery unit. The voltage regulator circuit between the battery and the device is omitted, and the system is directly powered by the battery. The static current consumption caused by the voltage regulator circuit is saved, and the battery life is further extended. In order to prevent the battery voltage from transiently decreasing due to the large current in the transmitting state, a large-capacity capacitor is used in parallel with the battery. The zero-power undervoltage reset (BOR) protection function integrated in the MSP430F2012 can perform a complete reset when the voltage is lower than the safe operating range, which well solves the random error operation problem caused by incomplete reset of the microcontroller.

3. Software Design

Trying to use software instead of hardware is also a measure often taken in low-power system design. This program development comprehensively considers the two aspects of timing scheduling and work efficiency to reduce the power consumption of the system.

3.1 Reasonable design of working timing

Since the CPU running time has a great impact on the power consumption of the system, its working time should be shortened as much as possible. Longer idle mode or power-off mode is the key to reducing the power consumption of the microcontroller system in software design. The program running flow chart is shown in Figure 2 (a) and (b). When the system is powered on and the initialization operation is completed, it immediately enters the low-power mode. Only when the system receives the correct information and generates an interrupt will it wake up the microcontroller to enter the working mode. Try to complete the processing of information or data in a short time. When the processing is completed, it immediately returns to the low-power mode and waits for the next interrupt to arrive.

3.2 Improve work efficiency
Use macro definitions to replace subroutine calls. Because when the CPU enters a subroutine, it will first push the current CPU register into the stack (RAM), and pop the CPU register out of the stack when leaving. This will cause at least two operations on the RAM, so reading the RAM will bring more power consumption than reading the Flash. Using macro definitions instead of subroutine calls will undoubtedly reduce the power consumption of the system.
Try to reduce the amount of CPU calculations, calculate some calculation results in advance, put them in the Flash, use the table lookup method to replace real-time calculations, reduce the CPU calculation workload, and effectively reduce the power consumption of the CPU; for unavoidable real-time calculations, end when the accuracy is sufficient; try to use short data types: such as using character 8-bit data instead of 16-bit integer data, try to use fractional operations and avoid floating-point operations, etc.
Let the I/O module run intermittently, and turn off the unused I/O modules or intermittently used I/O modules in time to save energy. Unused I/O pins should be set to output or input, and pulled high with pull-up resistors. If the pins are not initialized, the leakage current of the microcontroller may increase.

This article introduces the design of low-power active RFID tags based on MSP430 microcontroller in detail. It makes reasonable use of the interrupt, timing, calculation and other functions of MSP430 microcontroller. With the help of software advantages, it takes energy-limiting measures for the low-energy CC1100 module, improves the battery life and increases the reliable operation time of the system. At present, domestic RFID technology is still in its infancy. I believe that with the in-depth research of related technologies, RFID will be widely used.