How to let HMC765LPCE to identify which register to write from SPI command?

I would like to program the SPI (LE, Data and CLK) for the HMC765LPCE by using FPGA. I would like to achieve the output frequency of the PLL at 7912.5 MHz, is it correct to let R=1, Nint=158, and the Nfrac=2^22? How does HMC765 identify if I would like to program register 03H? This is not explained either in the datasheet or the operating guide. For ADF4108, it can identify which register to write given the control bits of C2,C1. Can you elaborate more on this for HMC765? 

  • 0
    •  Analog Employees 
    on Jan 18, 2018 3:24 AM

    Hi there,

    It looks like you might be using a PFD frequency of 50MHz as 50MHz * 158 = 7900MHz. 

    The remaining 12.5MHz is 25% of your PFD frequency and there are 24 fractional bits so Nfrac is  determined by using 12.5MHz / 50MHz = 0.25 * 2^24 = 4194304 (or 2^22).

    The user guide which can be found on the 'Full Product Page' for the HMC765 should help you with programming your FPGA.

    Best Regards,


  • Thank you Marty. I have 1 more question regarding the data format in programming the SPI.

    In Serial Port Write Operation of the the microwave PLL operating guide, ppA-13, it says:" A typical WRITE cycle is shown in Fig 7" with W/R bit plus register address (6 bits), and plus data (24 bits), finally plus a 1-bit redundancy. 
    I understand this is for 24 bits register Write operation. But if for register's length other than 24 bits, how shall I implement this procedure? For example Reg 03h, it has 16 bits. Do we need to patch '0's before the data or after the data to let it be in 24 bits format?
    Thank you and best regards,
    Liang Bin