High clock rate of LPC1788-SPI performs error

Discussion created by lpcware Employee on Jun 15, 2016
Content originally posted in LPCWare by qq on Wed Jul 22 00:20:04 MST 2015
I have configured an LPC1788 as SPI master on SSP1 (CPOL = 0; CPHA = 0) to communicate with a Nor Flash chip at 20 MHz.The Nor Flash chip needs the chip select pulled down (I do that with a GPIO), then a byte of command data need to be transmitted and some bytes of data need to be read and then the chip select needs to be pulled up again.
So I use Chip_SSP_WriteFrames_Blocking() function to send any data and the data does get transmitted and
Chip_SSP_ReadFrames_Blocking () function to read any data from the Nor Flash accordingly.
So everything's fine.However, As the frequency of the SSP for more than 20 MHz, the SSP can't work properly.
My question is: Why the SSP can't work properly more than 20 MHz.