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LMX2594: LMK And LMX SPI Read Issue

Ajmal Rashid
Prodigy 60 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMK04828,

In our design, we are using the LMK04828B (1Nos) and LMX2594RHAR (2Nos) Clock Synthesizer IC's to provide the clock to RFSoC ADC/DAC reference clock and connected through SPI interface.

If the LMK and two LMX IC's are connected from  SoC through SPI interface as described in below figure. we are able to do SPI write properly and SPI read is not proper.

When connecting only one device in the SPI line, SPI read back is proper.

While analyzing, few observations are found capture in below.

* SPI MOSI, and CS select are proper and Voh level is 3.16V.
* SPI MISO signal Voh is 3.04V if we are only connecting one device to MISO line
* SPI MISO signal Voh is 1.44V if we are connecting two LMX to MISO line.
* SPI MISO signal Voh is 1.44V if we are connecting one LMX and one LMK to MISO line.

Please let us know, why this voltage misbehaving in the SPI MISO line if we are connecting more than one device in the MISO line. And what is the solution for the same.

  • 0
    TI__Guru* 86030 points

    Hi Ajmal,

    Your picture did not get through. I assume you have LMK04828 in 4-wire SPI mode and have the read back output pin shared with LMX2594's MUXOUT pin and SoC's MISO pin.

    Read back output pin of LMK device does not support tri-state, this pin cannot share the MISO with other devices. 

    MUXOUT pin of LMX device can be configured to tri-state. Put Register R1[3] = 0 = tri-state. Normal operation when R1[3]=1.

  • 0 in reply to Noel Fung
    Prodigy 60 points

    We are using the STATUS_LD1 in the LMK04828 as MISO pin. As per LMK04828  datasheet, this pin can be set as open drain. So what will the state of the pin 'STATUS_LD1' if we are configuring this  pin as SPI read back with open drain configuration and the chip select is high.

  • 0 in reply to Ajmal Rashid
    TI__Guru* 86030 points

    Hi Ajmal,

    Open drain is an option useful for analog lock detect output. For register readback, output from an open drain output is still either 0 or 1, no tri-state.