0
1
2
3
4
5
6
7
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
3.25
3.5
3.75
4
4.25
Input RF Carrier Level (V )
PP
RSSI Levels and Oscillator Status Register V
alue (0x0F)
27
SLOS743L – AUGUST 2011 – REVISED MARCH 2017
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Detailed Description
Copyright © 2011–2017, Texas Instruments Incorporated
6.5.1
Received Signal Strength Indicator (RSSI)
The TRF7970A incorporates in total three independent RSSI building blocks: Internal Main RSSI, Internal
Auxiliary RSSI, and External RSSI. The internal RSSI blocks measure the amplitude of the subcarrier
signal, and the external RSSI block measures the amplitude of the RF carrier signal at the receiver input.
6.5.1.1
Internal RSSI – Main and Auxiliary Receivers
Each receiver path has its own RSSI block to measure the envelope of the demodulated RF signal
(subcarrier). Internal Main RSSI and Internal Auxiliary RSSI are identical however connected to different
RF input pins. The Internal RSSI is intended for diagnostic purposes to set the correct RX path conditions.
The internal RSSI values can be used to adjust the RX gain settings or determine which RX path (main or
auxiliary) provides the greater amplitude and, hence, to determine if the MUX may need to be
reprogrammed to swap the RX input signal. The measuring system latches the peak value, so the RSSI
level can be read after the end of each receive packet. The RSSI register values are reset with every
transmission (TX) by the reader. This ensures an updated RSSI measurement for each new tag response.
The Internal RSSI has 7 steps (3 bit) with a typical increment of approximately 4 dB. The operating range
is between 600 mV
PP
and 4.2 V
PP
with a typical step size of approximately 600 mV. Both Internal Main
and Internal Auxiliary RSSI values are stored in the RSSI Levels and Oscillator Status register (0x0F). The
nominal relationship between the input RF peak level and the RSSI value is shown in
.
Figure 6-5. Digital Internal RSSI (Main and Auxiliary) Value vs RF Input Level in V
PP
(V)
This RSSI measurement is done during the communication to the Tag; this means the TX must be on. Bit
1 in the Chip Status Control register (0x00) defines if Internal RSSI or the External RSSI value is stored in
the RSSI Levels and Oscillator Status register (0x0F). Direct command 0x18 is used to trigger an Internal
RSSI measurement.
6.5.1.2
External RSSI
The external RSSI is mainly used to check for any external 13.56-MHz signals at the receiver RX_IN1
input. The external RSSI measurement should be used before turning on the transmitter to prevent RF
field collisions. This is especially important for active mode, when both devices emit their own RF field.
The level of the RF signal received at the antenna is measured and stored in the RSSI Levels and
Oscillator Status register (0x0F).
shows the relationship between the voltage at the RX_IN1
input and the 3-bit code.
