I have several modules that consist of the TRF7970A and MSP430F2370 running TI EVM firmware. Most of the modules work just fine reading tags. Two of the modules do not read tags and instead just return an RSSI value of 7F. I have found that if I change the power supply voltage from 5V to 3.3V these two modules read the tags just fine:
Inventory request response at 5V power supply:
010B000304142401000000 ISO 15693 Inventory request. [,7F]
Inventory request response at 3.3V power supply:
010B000304142401000000 ISO 15693 Inventory request. [1D8D2CE336C407E0,7F]
For all read attempts, the TRF7970A is set for a 5V power supply in both the Chip Status Control register (0x00) and Regulator Control register (0x0B).
Any idea why providing a lower power supply voltage makes the reader work better?