Texas Instruments TRF7960 Manual page 35

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the RF and receivers are switched ON. When this command is received, the reader observes the digitized
receiver output. If more than two edges are observed in 100 μs, the window comparator voltage is
increased. The procedure is repeated until the number of edges (changes of logical state) of the digitized
reception signal is less than 2 (in 100 μs). The command can reduce the input sensitivity in 5-dB
increments up to 15 dB. This command ensures better operation in a noisy environment.
The gain setting is reset to maximum gain at EN = 0, POR = 1.
5.4.9 Test External RF (RSSI at RX input with TX OFF)
This command can be used in active mode when the RF receiver is switched ON, and the RF output is
switched OFF (bit B1=1 in the chip status register, rec-on. See
received on the antenna is measured and displayed in the RSSI levels register. The relation between the
3-bit code and the external RF field strength [A/m] must be determined by calculation or by experiments
for each antenna design. The antenna Q and connection to the RF input influence the result. The nominal
relation between the RF peak-to-peak voltage at the receiver inputs and its corresponding RSSI level is
presented as follows.
Receiver Input [mV ]
PP
RSSI level
If the direct command test RF internal or test RF external is used immediately after activation, it should be
preceded with a command enable RX to activate the RX section. For proper execution of the test RF
commands, the RX section must be enabled. This happens automatically when a data exchange between
the reader and the tag is done, or by sending a direct command enable RX.
5.4.10 Test Internal RF (RSSI at RX input with TX ON)
This command measures the level of the RF carrier at the receive inputs. Its operating range is between
300 mVp and 2.1 Vp with a step size of 300 mV. The two values are displayed in the RSSI levels register.
The command is intended for diagnostic purposes to set the correct RX_IN levels. The optimum RX_IN
input level is approximately 1.6 Vp, or an RSSI level of 5 or 6. The nominal relationship between the input
RF peak level and the RSSI code is presented as follows.
Receiver Input [mV ]
Pp
RSSI Level
5.4.11 Block Receiver
The block receiver command puts the digital part of receiver (bit decoder and framer) in reset mode. This
is useful in an extremely noisy environment, where the noise level could otherwise cause a constant
switching of the sub-carrier input of the digital part of the receiver. The receiver (if not in reset) would try to
catch a SOF signal, and if the noise pattern matched the SOF pattern, an interrupt would be generated,
falsely signaling the start of an RX operation. A constant flow of interrupt requests can be a problem for
the external system (MCU), so the external system can stop this by putting the receive decoders in reset
mode. The reset mode can be terminated in two ways. The external system can send the enable receiver
command. The reset mode is also automatically terminated at the end of a TX operation. The receiver can
stay in reset after end of TX if the RX wait time register (address 08) is set. In this case, the receiver is
enabled at the end of the wait time following the transmit operation.
5.4.12 Enable Receiver
This command clears the reset mode in the digital part of the receiver if the reset mode was entered by
the block receiver command.
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Table 5-9). The level of the RF signal
100 140
3 4 5
1200 1500
3 4 5
TRF7960
TRF7961
180 300
6 7
1800 2100
6 7
System Description 35