HMC7044 SPI in open-drain mode?


I want to communicate with HMC7044 using SPI in open-drain mode. I want to use it with 1.8V instead of 3.3V, but I was confused about what resistors should be used for pull-up/pull-down. I tried 10k pull-up resistor from SDATA to 1.8V, and wrote to register 0x0054 for enabling open-drain mode (though i'm not sure if i could write correctly, but oscilloscope showed that my SCLK and SDATA were correct). With this configuration, when I tried an SPI read operation, hmc7044 SDATA was outputting an unstable signal around 0.8-1V. How should I connect resistors for open-drain mode to work properly? Thanks in advance.

  • Hi,

    Can you check the low and high voltage levels on SCLK,SDATA and SLEN when you are writing to the part? SDATA low level is 0.24 V as given in the datasheet Table 3 and it would be good to check whether this is satisfied. Another option is to write GPIO control registers and toggle it between force high and force low to check SPI write is okay. 

    After verifying SPI write is okay, you can check read operation. 


  • Hi Kudret,

    Below is the oscilloscope view from HMC7044's SDATA pin when I try to read from a register. In this setup I used 4.7k pullup resistor on SDATA pin. My low and high voltages fluctutate about 1V, so I guess I'm not able to write? When HMC7044 is not connected to my board (I'm using ZC702), high and low voltage levels are smooth and strictly 1.8 and 0V, respectively. What could be the reason of the fluctuation? Thank you in advance.


  • Hi Aysenur,

    Ripple is so high. It is possible that the part is getting falls rising/falling edges. You can try to put series resistors on SPI lines. Also, what is the SPI speed you are using? 


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