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TRF7970A
www.ti.com
SLOS743K – AUGUST 2011 – REVISED APRIL 2014
Table 6-9. Minimum Crystal Requirements
Parameter
Frequency
Mode of Operation
Type of Resonance
Frequency Tolerance
Aging
Operation Temperature Range
Equivalent Series Resistance
Specification
13.56 MHz or 27.12 MHz
Fundamental
Parallel
±20 ppm
< 5 ppm/year
-40°C to 85°C
50 Ω
As an alternative, an external clock oscillator source can be connected to Pin 31 to provide the system
clock; pin 30 can be left open.
6.7
6.8
Transmitter – Analog Section
The 13.56-MHz oscillator generates the RF signal for the PA stage. The power amplifier consists of a
driver with selectable output resistance of nominal 4 Ω or 8 Ω . The transmit power level is set by bit B4 in
the Chip Status Control register (0x00). The transmit power levels are selectable between 100 mW (half
power) or 200 mW (full power) when configured for 5-V automatic operation. The transmit power levels
are selectable between 33 mW (half power) or 70 mW (full power) when configured for 3-V automatic
operation.
The ASK modulation depth is controlled by bits B0, B1, and B2 in the Modulator and SYS_CLK Control
register (0x09). The ASK modulation depth range can be adjusted between 7% to 30% or 100% (OOK).
External control of the transmit modulation depth is possible by setting the ISO Control register (0x01) to
direct mode. While operating the TRF7970A in direct mode, the transmit modulation is made possible by
selecting the modulation type ASK or OOK at pin 12. External control of the modulation type is made
possible only if enabled by setting B6 in the Modulator and SYS_CLK Control register (0x09) to 1.
In normal operation mode, the length of the modulation pulse is defined by the protocol selected in the
ISO Control register (0x01). With a high-Q antenna, the modulation pulse is typically prolonged, and the
tag detects a longer pulse than intended. For such cases, the modulation pulse length needs to be
corrected by using the TX Pulse Length Control register (0x06).
If the register contains all zeros, then the pulse length is governed by the protocol selection. If the register
contains a value other than 0x00, the pulse length is equal to the value of the register multiplied by
73.7 ns; therefore, the pulse length can be adjusted between 73.7 ns and 18.8 μs in 73.7-ns increments.
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 TRF7970A default mode the
TRF7970A automatically adds these special signals: 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.
Similar to working with the receiver, this means that the external system MCU only has to load the FIFO
with data and all the microcoding is done automatically, again saving the firmware developer code space
and time. Additionally, all of 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).
Note: FIFO must be reset before starting any transmission with Direct Command 0x0F.
There are two ways to start the transmit operation:
?
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.
Copyright ? 2011–2014, Texas Instruments Incorporated
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