ADS1255 Reading Dealy timing Setting

HI， 

I  am reading the register via SPI in TMS320F28062.  The ADS1255 shows that it requires t6>50*master clock period delay, the master clock period is 0.1us(10MHz), so I think t6=10us is reasonable, But it does not work till I set it to 60us. So I am not sure there is something wrong, my code is below, Could you help me check that, Thank you!

// Set Register
SpiaRegs.SPITXBUF = 0x1300;
while (SpiaRegs.SPISTS.bit.INT_FLAG != 1) { } 
SpiaRegs.SPIRXBUF = SpiaRegs.SPIRXBUF; 
DELAY_US(60);                                                   //60us Delay,    function run in RAM ( I have checked the time using oscilloscope)
SpiaRegs.SPITXBUF = 0x0000;
while (SpiaRegs.SPISTS.bit.INT_FLAG != 1) { } 
SpiaRegs.SPIRXBUF = SpiaRegs.SPIRXBUF; 
DELAY_US(60);
SpiaRegs.SPITXBUF = 0x0000;
while (SpiaRegs.SPISTS.bit.INT_FLAG != 1) { } 
SampleRate = SpiaRegs.SPIRXBUF;
DELAY_US(60);