Design of low power active RFID handheld device based on LPC2142

To ensure that the system can be reset reliably, a dedicated reset chip CAT809 is used to reset the system. The reset circuit is shown in Figure 4.

2.3 Power Detection Circuit

The power detection circuit uses the A/D converter inside the LPC2142. The reference voltage Vref of the A/D is obtained by dividing the power supply voltage +3.3 V through resistors, as shown in Figure 5. The theoretical value of the reference voltage is 2.533 V. Since the voltage of the lithium battery can reach up to 4.2 V (when fully charged), it exceeds the range of the A/D converter, so the range needs to be expanded. The solution is to divide the output voltage of the lithium battery (3.7 V in the figure) through two identical resistors, detect the divided voltage, and multiply the converted voltage value by 2.

2.4 Charging and electrostatic protection circuit

As shown in Figure 6, the system is powered by two power sources: a lithium battery and USB VBUS. The two power sources are connected to the input pin of the lithium battery charging chip MAX1551 to charge the battery. The /CHG pin is a charging status indicator pin. It is high impedance when not fully charged and outputs a low level when fully charged. During charging, the microprocessor determines whether the charging is complete by detecting the state of this pin and displays the charging status on the LCD. In order to prevent static electricity from damaging the microprocessor, the USB interface circuit needs to be protected from static electricity. The chip used here is SN65220.

2.5 Voltage conversion module

The handheld system has high requirements for the conversion efficiency of the power supply and the quiescent current of the power chip. The higher the conversion efficiency, the smaller the quiescent current of the chip, and the longer the battery life of the handheld system under the same conditions. The voltage conversion circuit is shown in Figure 7. Using Linear Technology's DC-DC conversion chips LTC3530 and LTC3525-5V, within the battery's power supply voltage range, its efficiency is above 80%, and up to 90%; and it has an enable pin, which is convenient for power management and can obtain the +3.3 V and +5 V power supply voltages required by the system. In order to ensure that the system can still be turned on by the power button when it is turned off, it is necessary to power the microprocessor separately, and the conversion chip LP2985 is used here.

2.6 Keyboard Circuit

The keyboard circuit uses the keyboard management chip CH452A, as shown in Figure 8. It communicates with the MPU through the I2C interface. The power button of the handheld device is implemented with a discrete button, and the 0.1μF capacitor in parallel with the button can eliminate jitter.

nRF24L01 is a single-chip wireless transceiver chip that works in the 2.4-2.5 GHz universal ISM frequency band. It has the advantages of small area, high data transmission rate, and low power consumption. It can work in frequency hopping mode and can effectively avoid interference from the surrounding environment. It communicates data with the microprocessor through the SPI interface, and the antenna adopts an inverted F-type PCB antenna that occupies less PCB space. The RF circuit diagram is shown in Figure 9.

2.8 LCD Circuit

The 2.8-inch TFTLCD model YM280T from Shenzhen Yaoyu Technology Co., Ltd. is used, which can work in 8-bus mode. The other circuits are shown in Figure 10.

The backlight circuit combines the keyboard backlight with the LCD backlight and controls them with a transistor switch to reduce power consumption. The sound prompt circuit uses a buzzer with a volume of 5 mm × 5 mm × 2 mm to meet the needs of a compact handheld device. The external expansion Flash uses the serial Flash memory AT45DB081 with few pins and small package size, and performs data communication through SSP (the SSP interface is compatible with the SPI interface).

3 Software Design

3.1 Data Packet Format

The packet format in enhanced ShockBurst mode is shown in Figure 11.

The preamble is used for synchronization and is only used in the sending mode; the flag bit is used for packet identification, only two of which are used, and the remaining 7 bits are reserved; the data is the item identification information with a width of 1 to 32 bytes to be transmitted/received; the CRC check selects a 16-bit CRC check with a generating polynomial of X16+X12+X5+X1.

3.2 Handheld device workflow

The workflow of the handheld device is shown in Figure 12. This process is for active cards that can be read and written. The information in the card needs to be read out, and then the modified information is written into the ID card after deducting a certain fee (or number of times). In order to save power consumption, if no key is pressed within 5 seconds (to be set according to actual conditions) after a key is pressed (including power on), the microprocessor will enter idle mode. When a key is pressed again, the microprocessor will be awakened by the external interrupt generated by the keyboard.

4 System Testing

The size of the PCB board made in this system is 5 cm×10 cm, which fully meets the requirement of small space occupation. All components are packaged in SMD. When testing, test the power supply first. If there is no problem with the power supply, test other parts. Every time you weld a module, check whether the welding of the power supply, ground and other pins is reliable at any time. When testing the whole machine, first check whether the power supply and ground are short-circuited. After checking, power on the whole machine to proceed to the next step of software and hardware joint debugging.

When the transmit power is set to 0 dBm in the software, the communication distance is about 80m in an open test site; in a closed corridor, the communication distance is 30-40m. In idle mode, the current measured from the battery output is 4.8 mA. The system uses a 1400 mAh lithium battery, and the standby time (in idle mode) can reach more than 10 days.

Conclusion

The active RFID handheld device designed in this paper has been applied to parking lot management systems. It has the advantages of long standby time, small size and high reliability, and has good promotion value.