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LMX2615-SP: SPI interface alternate?

salman haque
Prodigy 10 points
Part Number: LMX2615-SP
Other Parts Discussed in Thread: LMX2694-EP

Can this part be programmed without the SPI interface?  Or is the only way to set the registers and thus frequency control through the SPI interface only?  Sometimes, we don't need frequency agility - a single frequency is good enough and SPI is not always available.  Setting the registers (and frequency control) with external resistors would be great.  This question applies not only to "LMX2615-SP" but also to "LMX2694-EP."   Does TI have any parts that can do what I described?  Thanks in advance for answering this question!

  • 0
    TI__Mastermind 33860 points

    Salman,

    Section 7.3.16 of the datasheet has a table of pin mode configurations that can be selected for LMX2615-SP - these options are pre-programmed and cannot be altered, so if the configuration you want is not present, SPI programming is the only method available. An equivalent function does not exist for LMX2694-EP. As far as I know we have no -EP rated devices which support pin-programmable frequency.

    While the idea of resistor-programmable frequency sounds nice, in practice the 32-bit fractional divider renders this a considerable challenge - there are 2^64 configurations to support in the fraction alone, setting aside the 19-bit N-divider, 7-bit R-divider, 8-bit pre-R-divider, and 5-bit channel divider. We would prefer some form of one-time programming or EEPROM, but both of these have significant drawbacks in a space-grade device with respect to radiation hardening requirements or process qualification. At some point, it becomes easier to point to a space-grade FPGA as a programming solution (since most space-grade applications also include ADCs or DACs and need some high-speed interfacing, DSP, etc). A resistor-programmable frequency knob would be novel, but I suspect making such a scheme usable for the >100 programmable bits needed would render such an interface too challenging to use.

    Regards,

    Derek Payne