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TRF7960A: Collision error when using external clock

Ales Kurecka
Prodigy 10 points

Part Number: TRF7960A

Hello,
we are using TRF7960A with an external 13.56 MHz clock fed to OSC_IN from MCU (OSC_OUT is left open). The MCU uses a 13.56 MHz 10ppm crystal and output to the TRF7960A is generated directly by MCU's oscillator. With this configuration, we always get collision error IRQ (0x42) almost immediately after RX enable. There is no difference between whether the ISO14443A tag is in a field or not.

When we set gain reduction for 15 dB (0x2C in 0x0A register), collision error occurs in about a third of the cases - sometimes we even manage to read the tag ATQA.

If we connect crystal directly to TRF7960A on the same hardware, everything is perfectly OK - no collision error is generated and the tag is successfully read.

What are the requirements for an external oscillator? Can the TRF7960A be used in this configuration? Is there anything documented anywhere that says otherwise?

Thank you in advance,
Ales

  • +1
    TI__Mastermind 30450 points

    Hello Ales,

    I would suspect some noise or jitter issue of the external clock, which might disturb the sub-carrier detection of the TRF7960A. Also the fact that gain reduction can improve the collision error, is indicating that the TRF7960A is misinterpreting the noise as a received signal.

    I would suggest to replace the external clock from the MCU by a "clean" and stable clock signal from a function generator, in order to check this possible root-cause.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    Prodigy 10 points

    Thank you for the suggestion.

    So we finally tested TRF7960A with a function generator and it doesn't seem to have a problem with the frequency or the stability of the external clock signal. The TRF7960A reads correctly with a clock frequency range of 13.30 to 13.82 MHz (with band-pass filter 450 kHz to 1.5 MHz), but it is very sensitive to the duty cycle. With a duty cycle lower than 49% or higher than 54% TRF7960A falls to systematic collision error.

    It seems to me that the TRF7960A uses one edge for timing and the other for sampling.

    The stability or duty cycle of MCU generated clock signal doesn't seem to be an issue. But the clock edges are pretty slow so the duty cycle in TRF7960A will probably easily fall out of range.

    It looks like the safest way is to connect the crystal directly to the TRF7960A and clock the MCU from SYS_CLK.