Texas Instruments TRF7960 Manual page 19

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The value of the two external shunt capacitors on the crystal oscillator is calculated based on
the specified load capacitance of the crystal. The external capacitors (connected to the OSC
pins 30 and 31), are calculated as two capacitors in series plus C
capacitance of the oscillator gate plus PCB stray capacitance). The stray capacitance (C
can be estimated at 5 ±2 pF (typical).
As an example, given a crystal with a required load capacitance (C
C = ((C
L
18 pF = ((27 pF × 27 pF) / (27 pF + 27 pF)) + 4.5 pF
From this example, 18-pF capacitors would be placed on pins 30 and 31 to ensure proper
crystal oscillator operation.
The transmit power level is selectable as either half power of 100 mW (20 dBm) or full power of 200 mW
(23 dBm) when configured for 5-V automatic operation. The transmit output impedance is 8 Ω when
configured for half power and 4 Ω when configured for full power. Selection of the transmit power level is
set by bit B4 (rf_pwr) in the Chip Status Control register (see
automatic operation, the transmit power level is typically selectable as either 33 mW (15 dBm) in half-
power mode or 70 mW (18 dBm) in full-power mode (VDD_RF at 3.3 V). Lower operating voltages result
in reduced transmit power levels.
In typical operation, the transmit modulation is configured by the selected ISO Control register (address
0x01). External control of the transmit modulation is possible by setting the ISO Control register (address
0x01) to direct mode. While in direct mode, the transmit modulation is set by the ASK/OOK pin (pin 12).
External control of the modulation type is enabled by setting B6 = 1 (en_ook_p) in the Modulator and
SYS_CLK Control register (address 0x09). ASK modulation depth is controlled by bits B0, B1, and B2 in
the Modulator and SYS_CLK Control register (address 0x09). The range of the ASK modulation is 7% to
30%, or 100% (OOK).
Table 6-21 describes the coding of the Modulator and SYS_CLK Control register.
The length of the modulation pulse is defined by the protocol selected in the ISO Control register. 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 can be corrected by using the TX Pulse Length
register. If the register contains all zeros, then the pulse length is governed by the protocol selection. If the
register contains a value other than 00h, the pulse length is equal to the value of the register in 73.7-ns
increments. This means the range of adjustment is 73.7 ns to 18.8 µs.
Copyright © 2006–2017, Texas Instruments Incorporated
Table 6-9. Crystal Recommendations
PARAMETER
Frequency
Mode of operation
Type of resonance
Frequency tolerance
Aging
Operation temperature range
Equivalent series resistance
× C ) / (C + C )) + C
1 2 1 2 S
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SLOU186G – AUGUST 2006 – REVISED MAY 2017
SPECIFICATION
13.560000 MHz
Fundamental
Parallel
±20 ppm
<5 ppm/year
–40°C to 85°C
50 Ω, minimum
NOTE
) of 18 pF,
L
Table 6-11). When configured for 3-V
TRF7960 TRF7961
TRF7960, TRF7961
(the internal I/O
S
)
S
Detailed Description 19