AD7770 SPI MISO Always Gives 0x20

We are using an STM32 board to communicate with AD7770 using SPI. As shown in the attached picture, the MCU is providing the correct clock, MOSI, and CS. However, the ADC MISO is always 0x20, regardless of any changes. This is shown in the attached oscilloscope capture. 

We tried introducing a delay between CS and the clock/MOSI becomes always 0x81. 

Mode 3 pin (Alert_SPI) is always high, even if the MCU is not connected. 

VIO is set to 3.3V

and +12V is connected to a 12V power supply.

The SCK(shown in green) is 500.9kHZ

We have not made any modifications to the board, and left it in the Default SPI mode. 

MISO is Blue

MOSI is Purple

CS is Yellow 

Clk is Green

  • 0
    •  Analog Employees 
    on Mar 30, 2018 7:27 AM

    Hi zlassy,

       What is the value of the voltage measured in the Mode3/Alert pin? you mentioned that you are using the SPI control mode. What are the status of the Format 0 and Format 1 pins? The conversion result of the sigma delta is clock out via the Doutx pins unless the SPI_SLAVE_MODE_EN bit is enabled. Would you be able to measure on the Doutx pins, are there signals coming out from these pins?

       From the signal you show, what is the data coming from the microcontroller (MOSI-purple) to the AD7770 , what is that data? My assumption is that microcontroller is writing to the AD7770 and calling a register and data from the register is 0x20. That is why I am asking more information on your set up and figure out the status of the AD7770 to have a picture and idea what possibly is happening.



  • Hi zlassy, I am having same problem about reciving only 0x20 in SPI. Could manage to get registers content but shifted one bit left if I set CS always LOW, but still can not write to registers. Have you find any solution to this issue? Can SPI mode be config in mode3 or mode0, and how?

    Thanks for your update.

  • 0
    •  Analog Employees 
    on Aug 24, 2018 12:50 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin