Texas Instruments TRF7970A Manual page 14

TRF7970A
SLOS743L – AUGUST 2011 – REVISED MARCH 2017
The transmitter supports OOK and ASK modulation with selectable modulation depth. The TRF7970A also
includes a data transmission engine that comprises low-level encoding for ISO/IEC 15693, ISO/IEC 14443
A and B, and FeliCa. Included with the transmit data coding is the automatic generation of Start Of Frame
(SOF), End Of Frame (EOF), Cyclic Redundancy Check (CRC), and parity bits.
Several integrated voltage regulators ensure a proper power-supply noise rejection for the complete
reader system. The built-in programmable auxiliary voltage regulator V
20 mA to supply a microcontroller and additional external circuits within the reader system.
6.1.2 NFC Device Operation – Initiator
The transmitting system includes an RF level detector (programmable level) which is used for initial (or
response) RF collision avoidance. The RF collision avoidance sequence is started by sending a direct
command. If successful, the NFC initiator can send the data or commands, the MCU has loaded in the
FIFO register. The coding of this data is done by hardware coders in ISO/IEC 14443 A and B, or FeliCa
format. The coders also provide CRC and parity bits (if required) and automatically add preambles, SOF,
EOF, and synchronization bytes as defined by selected protocol.
The receiver system offers the same analog features (AGC, AM or PM, bandwidth selection, and so on)
as described previously in RFID and NFC reader and writer description. The system comprises integrated
decoders for passive targets (ISO/IEC 14443 A or ISO/IEC 14443 B tag or FeliCa) or active targets
(ISO/IEC 14443 A or ISO/IEC 14443 B reader or FeliCa). For all this options, the system also supports
framing including CRC and parity check and removal of SOF, EOF, and synchronization bytes as
specified by the selected protocol.
6.1.3 NFC Device Operation – Target
The activation of NFC target is done when a sufficient RF field level is detected on the antenna. The level
needed for wake-up is selectable and is stored in a nonvolatile register.
When the activation occurs, the system performs automatic power-up and waits for the first command to
be received. Based on this command, the system knows if it should operate as passive or active target
and at what bit rate. After activation, the receiver system offers the same analog features (for example,
AGC, AM/PM, and bandwidth selection) as in the case of an RFID reader.
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 the Target Detection
Level register (B0-B2) to enable the RF measurement system (supplied by VEXT, so it can also operate
during complete power down and consume 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 internally forced high).
When the voltage supply system and the oscillator are started and are stable, 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 NFC TARGET
device. When the first command from the INITIATOR is received, another IRQ sent with B6 (RX start) set
in the IRQ register. The MCU must set EN = H (confirm the power up) in the time between the two IRQs,
because 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 defines the communication protocol type that the target
must use. Therefore, the communication protocol type is available in the NFC Target Protocol register
immediately after receiving the first command.
14 Detailed Description
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(pin 32), is able to deliver up to
DD_X
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