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TRF7970A: Cold Temp Operation

Lawrence Wong
Genius 15940 points
Part Number: TRF7970A
Other Parts Discussed in Thread: DLP-7970ABP

Customer implementing  TRF7970A rfid reader on one of their boards.

They  have a problem to turn on the rf field at low temperatures. When running the chip at room temperature it works fine, but when going to lower temperatures (48F in worst case, some of the boards 0F) it is not possible to enable the RF field.  If they keep the board enabled (EN high and EN2 low) all the time, they can enable the rf field at low temperatures. But if they disable the chip with EN= 0 and EN2= 0 the RF field will not start at these low temperatures. SPI communication works in both cases. They run the chip at 3.3V and have tested logic levels at both 1.8V and 3.3V.

The implementation is very similar to the TRF7970A eval board(NFC trans BoosterPack DevKit DLP-7970ABP)

They at first suspected the crystal, but the problem is still there when we use a signal generator as oscillator. What is the max recommended R0 for a 27.12MHz crystal/13.56MHz crystal, it is no specified in the datasheet.

  • 0
    TI__Mastermind 24130 points

    Hello Lawrence,

    I guess with R0 you mean the ESR. Unfortunately this spec parameter is not given. In the TRF7970A reference design an ITTI I30 series crystal with an ESR of 80 Ohm is used. I think the proper value of the shunt capacitors is more critical than the ESR.

    It would be possible to observe the clock at the SYS_CLK pin if enabled in register 0x09. Also the osc_ok bit in register 0x0F could be checked for stable crystal clock.

    It is strange that the issues persist if a signal generator is used as a clock source. Is it possible that other effects, such as condensing water or ice on the PCB could cause the issue?

    Best regards,

    Andreas.