6.13.1 Target

When used as the NFC target, the chip is typically in a power down or standby mode. If EN2 = H, the chip keeps the supply system on. If EN2 = L and EN = L the chip is in complete power down. To operate as NFC target or card emulator, the MCU must load a value different from zero (0) in Target Detection Level register (b0-b2) which enables the RF measurement system (supplied by VEXT, so it can operate also during complete power down and consumes only 3.5 µA). The RF measurement constantly monitors the RF signal on the antenna input. When the RF level on the antenna input exceeds the level defined in the in Target Detection Level register, the chip is automatically activated (EN is internal forced high). The typical RF value that causes power-up for each value of B0 to B2 and the function of Target Detection Level register is listed in Table 6-15.

NFC Target Detection Level Register (0x18) – defines level for RF level for wake-up and gives information of NFCID size. This register is directly supplied by VEXT to ensure data retention during complete power down.

Default: reset to 00 at POR on VEXT (not on POR based on V_{DD_X}), not reset at EN = 0

When the voltage supply system and the oscillator are started and is stable, the osc_ok goes high (B6 of RSSI Level and Oscillator Status register) and IRQ is sent with bit B2 = 1 of IRQ register (field change). Bit B7 NFC Target Protocol in register directly displays the status of RF level detection (running constantly also during normal operation). This informs the MCU that the chip should start operation as an NFC TARGET device.

When the first command from the INITIATOR is received another IRQ sent with B6 (RX start) set in IRQ register. The MCU must set EN = H (confirm the power-up) in the time between the two IRQs as the internal power-up ends after the second IRQ. The type and coding of the first initiator (or reader in the case of a card emulator) command define the communication protocol type which the target must use. So the communication protocol type is available in the NFC Target Protocol register immediately after receiving the first command. The coding of the NFC Target Protocol register is described next.

NFC Target Protocol Register (0x19) – displays the bit rate and protocol type (active or passive) transmitted by initiator in the first command. It also displays the comparator outputs of both RF level detectors.

Default: reset to 00 at POR and EN = L. B0 to B4 are automatically reset after MCU read operation. B6 and B7 continuously display the RF level comparator outputs.

Based on the first command from INITIATOR following actions are taken:

• If the first command is SENS_REQ or ALL_REQ, the TARGET must enter the SDD protocol for 106 kbps passive communication. If bit B5 in NFC Target Detection Level register is not set, the MCU handles the SDD and the command received is send to FIFO. If the RF field is turned off (B7 in the NFC Target Protocol register goes low) at any time, the system sends an IRQ to the MCU with bit B2 (RF field change) in the IRQ register set high. This informs the MCU that the procedure was aborted and the system must be reset. The clock extractor is automatically activated in this mode.
• If the command is SENS_REQ or ALL_REQ and the card emulation bit in the ISO Control register is set, the system emulates an ISO/IEC 14443 A card. The procedure does not differ from the one previously described for a passive target at 106 kbps. The clock extractor is automatically activated in this mode.
• If the first command is a SENSF_REQ, the system becomes a TARGET in passive communication using 212 kbps or 424 kbps. The SDD is relatively simple and is handled by the MCU directly.
• If the first command is ATR_REQ, the system operates as an active TARGET using the same communication speed and bit coding as used by the INITIATOR. Again, all of the replies are handled by the MCU. The MCU should check for collision avoidance by checking the external RSSI as described in Section 6.5.1.2. When no RF field is detected from the external RSSI result, then the MCU can begin the process to send the reply.
• If the first command is coded as ISO/IEC 14443 B and the card emulation bit is set in the ISO Control register, the system enters ISO/IEC 14443 B emulator mode. The anticollision must be handled by the MCU, and the chip provides all physical level coding, decoding, and framing for this protocol.

Table 6-18 shows the function of the IRQ Status register in NFC and card emulation. This register is preset to 0 at POR = H or EN = L and at each write to ISO Control. It is also automatically reset at the end of read phase. The reset also removes the IRQ flag.