Please find the answer for your queries below.
1. Does any input SCK speed not greater than 133MHz for SDK work? For example set up 90MHz for SCK.
Answer) The maximum clock frequency supported by our device S25FL128L is 133MHz. You can operate this device at any frequency less than 133MHz. So, 90MHz is OK.
2. Shall the value read from the erased memory be 0xFF?
Answer) Yes, data in the erased memory will be 0xFF.
Thanks Pradipta! I find this very helpful, too.
When you say any frequency, does that mean I can go as low as 1MHz?
Yes you can go as low as 1MHz.