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AMC7891: SPI interface and register operation not working properly

Part Number: AMC7891
Other Parts Discussed in Thread: LMP92018,


Need help from TI expert.

We are now using AMC7891 on one product. Related circuit is designed with referring to LMP92018 EVB that has same pin function as AMC7891. The LMP92018 EVB works well with my CPU and code, but when testing the AMC7891 by writing command to AMC7891 via SPI, like configuring the reference voltage to internal mode, or write GPIO out reg to toggle some pins, the AMC7891 has no response and nothing changed on GPIO.

When we monitoring the SPI waveform with oscilloscope, it shows the same bit sequence as the datasheet requires. And when writing command on SDI, the SDO pin also replies waveform of last command word, which indicates the SPI interface seems working, at least it clocks in the command bits into the SPI shift register. But the chip wise, seems does not execute the command.

we have the 5V AVDD applied later after 3.3V GPIOVdd and SPIVdd is applied. GPIOs is designed to be all output mode, and some GPIOs is externally pull up to 3.3V with a 10K resistor to enable the lines set to high level as default.

Actually the AMC7891 should be very simple to configure and operate, it is not a complex IC. But I am very confused about this issue.

Is there any specified requirement on the external circuit design to make sure it works correctly? Or any specified requirement on SPI to enable chip execute command correctly ?

Can any expert help?