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LMX2615-SP: LMK04832 and LMX2615, 3V3 and 1V8 logic

Part Number: LMX2615-SP
Other Parts Discussed in Thread: LMK04832-SP, LMK04832

Currently I have five spare 3V3 and about twelve 1V8 logic pins on the FPGA: for the LMK04832-SP and the LMX2615-SP, could both CSb inputs be permanently tied low?

Datasheet for LMK04832 has omitted a key piece of information: can you confirm that any single-ended signal with an * is active low and any differential pin with an *

is the negative signal of the pair?

thank you,

Rajan.

  • Hi Rajan,

    Can CSb inputs be permanently tied low: Not permanently.

    When the CSb input stays low, the LMX2615 will auto-decrement the address after 16 data bits have been clocked in, so that data can continuously be clocked in until all registers in sequence have been programmed. The CSb input is used to reset the starting address so that individual registers may be programmed. If the final register in the programming sequence (R0) is programmed, and CSb is not asserted after programming, this is undefined behavior.

    Meanwhile, when the CSb input stays low, the LMK04832 will continue cycling inputs through the shift buffer on writes, and may stall or produce arbitrary output on reads after the valid data is shifted out (I have not checked this behavior). CSb toggle is required to latch in new data to the correct address.

    Please note that the LMK04832 is compatible with 1.8V, 2.5V, or 3.3V signaling. The LMX2615 is compatible only with 2.5V or 3.3V signaling.

    LMK04832 * signal polarity: I can confirm that any single-ended signal with * is active low. The only such signal is CS*.

    For differential pins with *, in almost all cases * is the negative signal of the pair. However, there is one exception: the OSCout buffer may be programmed to several different CMOS modes as described in Table 27 of the datasheet under the OSCout_FMT setting. As an example, "LVCMOS (Norm / Inv)" means OSCout polarity is positive, while OSCout* polarity is negative.

    Regards,

  • Dear Derek,

    Thank you for your reply.

    For the LMK04832, VIH (min) = 1.2 V which means I can drive its CSb input using 1V8 logic.

    For the LMX2615, page 5 of the datasheet suggests its CSb input can also be driven from 1V8 logic, however, VIH (max) is specified as 1.6 V which

    doesn't sound right for a 3V3 part.

    please advise.

    thank you and kind regards,

    Rajan.

  • Hi Rajan,

    Don't worry, it was a mistake. The true is VIH(min)=1.6V while VIL(max)=0.4V. We will fix the datasheet shortly.

  • Hi Noel,

    Would you agree I can then connect both CSb inputs to 1V8 logic and the remaining SPI signals to 3V3 logic.

    My uncle used to own a shop in Noel Street by the way

    Kind Regards,

    Rajan.

  • Hi Rajan,

    That will work. 

    I can't remember what is in Noel Street, I was there 14 years ago.