Since the launch of the "Golden Card Project" in 1993, China's financial and circulation fields have gradually entered the era of electronic money, and various contact IC card readers have been widely used. It combines microelectronics and computer technology to increase the modernization of people's lives and work.
The International Standard for Contact Integrated Circuit (IC) Cards (ISO/IEC 7816) is jointly developed by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) for physical characteristics, electrical signals and transmission in the field of contact IC cards. All aspects of the agreement and other aspects have been standardized. According to this standard, the card is divided into two types, A and B, according to the power supply voltage of the supply card, that is, a class A card of 5 V voltage and a class B card of 3 V voltage [1]. Traditional contact IC card readers mostly use Class A interface devices or Class B interface devices, which can only operate on a single Class A card or Class B card, but the readers with Class AB interface devices are not widely used. . With the widespread use of contact IC cards in daily life, readers with Class AB interfaces will have a broad market. The reader designed in this paper is powered by a 5 V power supply voltage, which can realize full-featured reading and writing of Class B cards. This technology can solve the problem of simultaneous reading and writing of Class A and Class B cards in a 5 V system. .
1 system hardware design
The IC card reader in this article uses ATMEL's high-performance, low-power 8-bit AVR microprocessor ATMEGA32. The level-shifting chip uses National Semiconductor's LM1117-3.3, which can provide the card holder. 3.3 V supply voltage. The interface conversion circuit between the microprocessor and the card holder uses Texas Instruments' (TI) SN74TVC3010 device, which can simultaneously convert 10 channels of signals, which fully meets the needs of reader design. The hardware structure of the reader is shown in Figure 1. The reader is mainly composed of three parts.
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(1) Power supply. Mainly to provide a stable 5 V and 3 V supply voltage for the entire reader. The 5 V power supply is directly provided by the host computer through the USB port, and can be used as a power source for the microprocessor, the buzzer circuit, and the photoelectric indicating circuit in the reader/writer. The 5 V supply voltage can be converted to a stable 3.3 V level by the LM1117-3.3 chip, providing the supply voltage for the subsequent SN74TVC3010 interface circuit and deck. The specific circuit is shown in Figure 2.
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(2) Data exchange. The data exchange between the PC and the reader is also the data exchange with the ATMEGA32 MCU. The PC sends a command to the MCU through the host computer software to realize the reading and writing operation of the card. The card reader uses RS232 serial port to communicate with the PC. Due to the different interface levels, the MAX232 chip is used inside the card reader to realize the conversion between different levels [2].
(3) Communication between the reader/writer and the CPU card. There is a normally closed contact card holder on the card reader. This is the interface between the reader and the CPU card. It is connected to the CPU card through 8 contacts that meet the requirements of ISO 7816 [3].
The SN74TVC3010 is a bidirectional level shifting device that does not require a direction control signal during the conversion process. The SN74TVC3010 chip contains a transistor array consisting of 11 N-channel turn-on transistors. All the transistors in the array have the same electrical characteristics. Their gates are internally connected together. Therefore, any one of the transistors can be used as a reference transistor. As the pass transistor, the maximum positive voltage on the low voltage side of each turn-on transistor is limited to the voltage set by the reference transistor. Since the transistors are fabricated symmetrically and the I/O signals pass through each transistor in both directions, any port connection per bit can be used as the low voltage side [4].
In the reader designed in this paper, one end of the reference transistor is connected to a 5 V power supply through a 200 kΩ resistor, and the other end is connected to a 3.3 V power supply, as shown in FIG. When data is transferred from the deck to the microcontroller, the voltage reaches 3.3 V, the turn-on transistor turns off, and the level on the microcontroller pin is pulled to 5 V through the pull-up resistor. When data is transmitted from the microcontroller to the deck, the voltage at the card terminal is clamped at approximately 3.3 V. This can accurately achieve normal communication between the microcontroller and the card.
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2 low-level firmware design
The firmware programming of this reader is written in C language, the programming environment is IAR Embedded Workbench, and the simulation and downloading of the underlying firmware is done in AVR Studio 4. The overall structure of the firmware is shown in Figure 4.
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