Texas Instruments TRF7970A Manual page 15

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Based on the first command from the INITIATOR, the 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 to begin; afterward, the baud rate can be changed to 212 kbps or 424
kbps, which is determined by the NFC initiator device. If bit B5 in the NFC Target Detection Level
register is not set, the MCU handles the SDD and the command received is send to FIFO. For
interoperability purposes, TI recommends allowing the MCU to handle the SDD process rather than
use the TRF7970A Auto-SDD feature to ensure interoperability with other NFC devices. If the RF field
is turned off (B7 in NFC Target Protocol register is 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 ISO Control register is set,
the system emulates an ISO/IEC 14443 A or ISO/IEC 14443 B tag. The procedure does not differ from
the one previously described for the case of 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 the 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 handle the timing requirements for collision avoidance. This is done by
using external RSSI to detect external RF fields before enabling RF on the TRF7970A.
• If the first command is a SENSB_REQ request and the card emulation bit is set in the ISO Control
register, the system enters ISO/IEC 14443 B emulation mode. The anticollision must be handled by the
MCU, and the chip provides all physical level coding, decoding, and framing for this protocol.
6.1.3.1 Active Target
If the first command received by the RF interface defines the system as an active target, then the receiver
selects the appropriate data decoders (ISO/IEC 14443 A or ISO/IEC 14443 B reader or peer-to-peer) and
framing option. Only the raw (decoded) data is forwarded to the MCU through the FIFO. SOF, EOF,
preamble, sync bytes, CRC, and parity bytes are checked by the framer and discarded.
The transmission of a response must occur after RF collision avoidance has been processed. The
recommended method for RF collision avoidance is to use external RSSI to detect any external RF field. If
successful, the NFC initiator can send the data that the MCU has loaded in the FIFO register. The coding
of this data is done by hardware coders either in ISO/IEC 14443 A format (106-kbps system) or in peer-to-
peer format for (212-kbps and 424-kbps systems). The coders also provide CRC and parity bits (if
required) and automatically add preambles, SOF, EOF, and synchronization bytes as defined by selected
protocol.
6.1.3.2 Passive Target
If the first command received by the RF interface defines the system as a passive target, then the receiver
selects the appropriate data decoders (ISO/IEC 14443 A or ISO/IEC 14443 B reader or peer-to-peer) and
framing option. Again, only the raw (decoded) data is forwarded to the MCU through the FIFO; SOF, EOF,
preamble, sync bytes, CRC, and parity bytes are checked by the framer and discarded. The receiver
works same as in the case of an active target.
The transmit system in passive target mode differs from active target and operates similar to the standard
tag. There is no RF collision avoidance sequence, and encoders are used to code the data for
ISO/IEC 14443 A or ISO/IEC 14443 B tag (at 106 kbps, to start) or peer-to-peer (at 212 kbps, to start)
format. The coding system adds all of the SOF, EOF, CRC, parity bits, and synchronization bytes that are
required by protocol. The response is transmitted over-the-air with a method known as load modulation.
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SLOS743L – AUGUST 2011 – REVISED MARCH 2017
Detailed Description
TRF7970A
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