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ADS1299: SPI Communication Problem

Part Number: ADS1299

I tried to use a STM32 F411 Nucleo board to communicate with OpenBCI V3 daisy module, which is basically an ADS1299 module.
For HW connection:
V3              Nucleo Board
DVDD <---> 3.3V
GND   <---> GND
MISO <--->  PC2/SPI2 MISO
MOSI <--->  PC3/SPI2 MOSI
SCK   <--->  PB10/SPI2 SCK
CS2   <--->  PB4 /GPIO Output -- firmware toggle

On V3 daisy module, CLKSEL is connected to DVDD, so ADS1299 uses internal clock.


For the first step, I don't touch the analog part, just want to make sure digital com works.
I tried SPI to read 1 byte from register address 0. SPI send 3 bytes: 0x20, 0x00, 0x00, but the reception buffer is always 0. I tried all 4 SPI modes with a rather slow baud rate 1Mbps.


Can anyone enlighten me what I did wrong or what else I need to do, to make it work?