6.10 Transmitter - Digital Section

The digital part of the transmitter is a mirror of the receiver. The settings controlled the ISO Control register (0x01) are applied to the transmitter just like the receiver. In the TRF7960A default mode (ISO Mode), the TRF7960A automatically adds all the special signals like start of communication, end of communication, SOF, EOF, parity bits and CRC bytes.

The data is then coded to modulation pulse levels and sent to the RF output stage modulation control unit. Just like with the receiver, this means that the external system MCU only must load the FIFO with data and all the microcoding is done automatically, again saving the firmware developer code space and time. Additionally, all the registers used for transmit parameter control are automatically preset to optimum values when a new selection is entered into the ISO Control register (0x01).

There are two ways to start the transmit operation:

• Send the transmit command and the number of bytes to be transmitted first, and then start to send the data to the FIFO. The transmission starts when first data byte is written into the FIFO.
• Load the number of bytes to be sent into registers 0x1D and 0x1E and load the data to be sent into the FIFO (address 0x1F), followed by sending a transmit command (see Direct Commands section). The transmission then starts when the transmit command is received.

At the end of a transmit operation, the external system MCU is notified by interrupt request (IRQ) with a flag in the IRQ register (0x0C) indicating TX is complete (example value = 0x80).

The TX Length registers also support incomplete byte transmission. The high two nibbles in register 0x1D and the nibble composed of bits B4 to B7 in register 0x1E store the number of complete bytes to be transmitted. Bit B0 in register 0x1E is a flag indicating that there are also additional bits to be transmitted which do not form a complete byte. The number of bits is stored in bits B1 to B3 of the same register (0x1E).

Some protocols have options so there are two sublevel configuration registers to select the TX protocol options.

• ISO14443B TX Options register (0x02). It controls the SOF and EOF selection and EGT selection for the ISO/IEC 14443 B protocol.
• ISO14443A High-Bit-Rate and Parity Options register (0x03). This register enables the use of different bit rates for RX and TX operations in ISO/IEC 14443 high-bit-rate protocol. Besides that, it also selects the parity method in case of ISO/IEC 14443 A high bit rate.

The digital section also has a timer. The timer can be used to start the transmit operation at a precise time in accordance with a selected event. This is necessary if the tag expects a replay in exact time window following the tag response. This is normally not the case with existing protocols but is needed in protocols when using 'fixed slot' command.

The TX timer uses two registers (register addresses 0x04 and 0x05). Register 0x04 uses 2 bits (B7 and B6) to define the trigger conditions. The remaining 6 bits of register 0x04 are the upper bits, and the 8 bits in register address 0x05 are the lower bits that preset the counter. The range of this counter is from 590 ns to 9.7 ms, in 590-ns increments.