"With the continuous improvement of uplink and downlink data rates supported by wireless networks. Mobile phone applications in data services are emerging. More and more people are using mobile phones instead of watches, notepad and MP3. Mobile phones have become indispensable information terminals.
NFC (near field communication) technology will make all this a reality. In June 2006, Nokia, China Mobile, Philips and e-card launched China's first NFC mobile payment test in Xiamen. Using the Nokia 3220 mobile phone with embedded NFC module, users can make mobile payment at any business outlet (bus, ferry, cinema and fast food restaurant) covered by easy pass card in Xiamen.
Moreover, in the near future, through the combination of mobile phone and NFC technology, users can realize the following applications through mobile phone: Download concert time, place and program list on street posters and magazines; Play interactive Orienteering games in the park; Refresh the arrival time of the bus in real time at the station; Send text messages in the office to control the time of housekeeping waiters entering and leaving the house; Replace the existing student card and student card in the school: query the map, bus route, catering and shopping information in the intelligent public telephone booth all over the urban area; Pay at gas stations, supermarkets, banks and other places with POS machines and collect electronic invoices with mobile phones.
Based on the introduction of TI's latest multi protocol fully integrated 13.56 MHz transceiver chip trf7970a, an NFC module with active mode and passive mode is designed in this paper.
1 Hardware Design
1.1 trf7970a introduction
Trf7970a is an integrated analog front-end and data framing device for 13.56 MHz RFID / near-field communication system. The built-in programming option makes this device suitable for a wide range of applications. The first mock exam is capable of executing any of the following 3 modes: RFID/NFC reader, NFC peer, and card emulation mode. The pin distribution is shown in Figure 1.
The chip has several operating modes, which can be controlled by configuring two input pins (EN and en2) and several bits in the chip status control register (0x00), as shown in Table 1.
Figure 1 pin distribution
When en is set to high level (or at the rising edge of en2 and then confirmed by en = 1), the voltage regulator is activated and the 13.56 MHz oscillator is started. Sys when the power supply and oscillator frequency have stabilized_ The CLK output is switched from an auxiliary frequency of 60 kHz to a frequency of 13.56 MHz from the crystal oscillator. At this point, the reader is ready to communicate and perform the required tasks. The MCU can then program the chip status control register 0x00 and select the operation mode by editing the additional register.
(1) In standby mode (bit 7 of register 0x00 = 1), the reader can resume full operation in 100s.
(2) Mode 1 (RF output disabled, bit 5 = 0 of register ox00 and bit 1 = 0 active mode) is a low power mode, which enables the reader to return to full operation within 25 seconds.
(3) If reader to reader anti-collision is performed, mode 2 (only the RF receiver is valid, the activation mode of bit 1 = l of register ox00) can be used to measure the external RF field (as described in the RSSI measurement paragraph).
(4) Modes 3 and 4 (the entire RF section is valid). Bit 5 of register 0x00 = active mode of L) is the normal mode for normal transmit and receive operations.
1.2 working principle of NFC module
Figure 2 shows the NFC circuit composed of NFC antenna, trf7970a and microcontroller (MCU).
After the system is powered on, MCU first configures trf7970a chip, and can select the working mode by configuring pins en and en2 and several bits of chip status control register (the specific configuration parameters are shown in Table L). MCU transmits the configuration data to trf7970a chip through 13. NFC module mainly has two modes: active mode and passive mode.
Fig. 2 circuit diagram of NFC module
The NFC module can exchange data in active or passive mode. In the passive mode, the device that starts NFC communication (also known as NFC initiating device and main module) provides RF field during the whole communication process, in which the transmission speed is optional and sends data to another module. The other module is called NFC target module (slave module). It does not need to generate RF field, but uses load modulation technology, that is, it can transmit data back to the initiating device at the same speed. This communication mechanism is compatible with contactless smart cards based on is014443a, Mifare and felica. Therefore, in the active mode, the NFC initiating module can use the same connection and initialization process to detect and establish contact with the contactless smart card or NFC target module.
When the NFC module is operating in active mode. As in the operation of RFID reader, this chip is completely controlled by MCU. MCU activates the chip and writes the mode selection into the ISO control register. MCU uses RF conflict avoidance command. So it doesn't have to undertake any real-time tasks. When each NFC module wants to send data to another NFC module, it must generate its own RF field. As shown in Figure 3, both the initiating module and the target module should generate their own RF field for communication. This is the standard mode of peer-to-peer network communication, which can obtain very fast connection settings.
Figure 3 NFC active communication mode
Figure 4 NFC passive communication mode
As shown in Figure 4, when the NFC module works in passive mode, the module is usually in power off or standby mode. If en2 = h, this module keeps the power system on. If en2 = L and en = L, the module is completely powered off. In order to operate in passive mode or tag emulator state, MCU must load a non-0 value into the module detection level register (bo-b2). This operation will turn on the RF metering system (powered by vext), so the system can still operate during complete power failure and its current consumption is only 3.5 μ A）。 RF metering always monitors the RF signal on the antenna input. When the RF level on the antenna input exceeds the value defined in the target detection level register, the NFC module is automatically activated (EN is the internal forced high level).
The mobile module mainly operates in passive mode. It can greatly reduce power consumption and prolong battery life. During an application session, the NFC module can switch its role between the initiating module and the target module. Take advantage of this feature. Devices with low battery power can require passive mode to act as target devices rather than initiating devices.
1.3 trf7970a communication interface design
The communication interface from the chip trf7970a to the reader can be configured in two ways: 8-wire parallel interface (do: D7) plus data_ CLK, or 3-wire or 4-wire serial peripheral interface (SPI). SPI interface uses traditional master device output / slave device input (MoSi), master device input / slave device output (MISO), IRQ and data_ CLK line. SPI can operate with or without slave device selection. This paper describes it in the form of parallel interface.
Communication is initiated by a start condition, which is usually followed by an address / command word (ADR / CMD). ADR / CMD is 8 bits long, and its format is shown in Table 2.
Table 2 address / command bit allocation
MSB (bit 7) determines whether the word is used as a command or an address. The last two columns in Table 2 show the function of independent bits, that is, whether an address or a command is written. Once the address word is sent, it enters the data reception waiting state. In continuous address mode (continuous mode = 1), the first set of data after the address is written (or read) to (from) the specified address. For each additional data, the address increment is 1. The persistent mode can be used to write a block of control register in a single data stream without changing the address; In discontinuous address mode (simple addressing mode), only one data word is waiting after the address.
The address mode is used to write or read configuration registers or FIFOs. When more than 12b data is written to FIFO, the continuous address mode shall be set to 1. Command mode is used to enter a command that causes the reader to act.
1.3.1 receiving of wireless interface data
When the receive operation is started (SOF is successfully detected), B6 of the IRQ status register is set. If the received number if{is less than or equal to 8 B, an interrupt request will be sent to MCU at the end of the receive operation. The MCU receives the interrupt request, and then it reads the IRQ. The status register (oxoc) is checked to determine the cause of the interrupt, and then the MCU reads data from the FIFO.
If the received packet is greater than 96 B, when the 96th byte is loaded into the FIFO (75% of the full capacity), an interrupt is issued before the receiving operation is terminated. MCU should re read the contents of IRQ status register to determine the cause of interrupt request. If the FIFO has reached 75% of the full capacity (marked by the flag B5 of the IRQ status register and reads the FIFO status register), the MCU makes space for the newly received data by reading the data from the FIFO. When the receive operation is completed, the interrupt is issued, and before completing the read operation, the MCU must check how many words are still in fif0.
If the reader detects a reception error, the corresponding error flag (framing error, CRC error) in the IRQ status register is set to indicate that the MCU reception is not completed correctly.
1.3.2 data transmission to MCU
FIFO should always be cleared by a reset command (0x0f) before starting data transmission. Data transfer is initiated by a selected command. Then, the MCU commands the reader to execute a continuous write command (ox3d) starting from register 0xld. The data written to register oxld is TX. Length byte L (upper and middle half bytes), followed by byte TX in register 0xle and length byte 2 (lower half byte and incomplete byte length). The TX byte length determines when the reader sends end of frame (EOF) bytes. After TX length bytes are written, FIFO data is loaded into register 0xlf. Its byte storage units are located between 0 and 127. After the first byte is written to FIFO, data transfer starts automatically. Since the addresses are arranged in order, TX length bytes and FIFO can be loaded by a continuous write command.
At the beginning of the transfer, the flag B7 (IRQ TX) of the IRQ status register is set, and an interrupt is sent at the end of the transfer operation to notify the MCU that the task has been completed.
2 software design
After the system is powered on, MCU first configures trf7970a chip, and first configures the working mode of the chip by two input pins (EN and en2) and several bits in the chip status control register (0x00). MCU transmits the configuration data to trf7970a chip through IZL. In Rx mode, the reception operation ends, and the external MCU is notified of the end of reception by setting pin 13 (IRQ) to high level; In TX mode, an interrupt request (IRQ) indicating that TX has been completed notifies the external MCU.
There is a framing logic part in trf7970a chip, in which the serial bit stream data is formatted into byte format. Special signals, such as frame start (SOF), frame end (EOF), communication start, communication end, are automatically removed; Parity bits and CRC bytes are also checked and removed. This "clean" data is then sent to the FIFO register of 128 B, which can be read by the external microcontroller. This means reducing the memory requirements of MCU, that is, reducing the cost of MCU. At the same time, shorten the software development time. The transmission mode of trf7970a is shown in Figure 5, and the reception mode of trf7970a is shown in Figure 6.
Figure 5 trf7970a launch mode Figure 6 trf7970a receiving mode
2.1 launch mode
(1) Select the power mode through pins en and en2 and several bits in the chip status controller (0x00) to make the module work in the active mode;
trf7970atb
NFC chip“
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