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Issue #211 February 2008
RFID Payment Terminal
by Carlos Cossio
Start | System Overview | Energy Transmission | 100% Amplitude Modulation | Load Modulation | MIFARE Card | MIFARE and ISO 14443 | Pay as You Go | Hardware Design | Antenna Design Rules | Antenna Size | Directly Matched Antenna | Firmware Design | Catch the Wave! | Sources & PDF
HARDWARE DESIGN
I built the hardware design around the Luminary Micro LM3S811. It has the small footprint required for this application with a large flash memory (see Photo 2).
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| Photo 2—This is a close-up view of the RFID payment terminal’s internal circuitry. The hardware is so simple because all of the complexity dealing with the contactless interface is performed by a dedicated IC designed by NXP Semiconductors. |
Because the hardware design is intended to work as a portable handheld payment terminal, it is powered through a 9-V alkaline battery (see Figure 6). The battery voltage is stepped down to 5 V with the help of a 5-V fixed-voltage regulator to power all of the circuitry inside. The EK-LM3S811 board has its own voltage regulator to achieve 3.3 V, which is the voltage required by the LM3S811. Additionally, for longer periods of activity, it is possible to use an external 9-V power supply.
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| Figure 6—The main board is built around a Luminary Micro LM3S811 microcontroller. The schematic for the coil antenna is at the bottom. |
The LCD module is a standard 2 × 16 display, compatible with the industry-standard Hitachi LCD controller. It is connected to the LM3S811 using a 4-bit bus configuration to save I/O lines.
The keypad is a 16-key multiplexed keypad to enter numerical and alphanumerical data when requested by the device. To save I/O lines, the keypad is connected to the LM3S811 analog input ADC through a resistor ladder. The resistor network values are chosen so the impedance drop is proportionally distributed among all keys in increments of 500 W. To ensure good keypad reliability, it is a good idea to choose all the resistors as precision resistors of 1% tolerance.
Communications with the outside world are accomplished with the help of a MAX232 level shifter to enable an RS-232 interface. There is a buzzer of programmable frequency used to signal specific events such as transaction completed successfully, transaction failed, or keypad beep.
The real-time clock support of the RFID payment terminal is accomplished with a 32.768-kHz crystal oscillator using a Pierce oscillator design.
The contactless interface is implemented with the help of the MF RC531 ISO 14443 reader IC for the modulation and demodulation of the contactless signals at 13.56 MHz. Additionally, the RFID payment terminal has an innovative built-in antenna under the keypad to enable portable operation right out of the box without plugging in an external antenna.
Finally, note that debugging and firmware downloading is easy. There is a USB port connection for debugging and updating the flash if further updates or enhancements are planned in the future.
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