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Getting ad7991 into high speed

Hi, I can use perfectlly ad7991 in standar mode, but I can't make it work in fast mode neither high speed mode.  Actually, in the datasheet I don't see a procedure to make it work in fast mode, so I asume I have only to increase the speed from 100khz to 400khz, but it only works in 100khz.  According to page 25, it seems like to configure high speed mode, you should be in fast mode but there is nothing about how to set it into fast mode.  I followed the procedure for high speed mode by giving first the high speed master code (I put 0Fh) followed a repeated start with the device address and a read bit function and a 2us clock stretch.  Then I begin to read but there is no data at all.  I only get '0'.  Can anyone help me with this? Is there something I hava overlooked?