As mentioned in the datasheet, the device S29GL128S is available in three speed grades 100ns, 110ns and 120ns. You cannot access (read/write) the flash device at a speed more than this maximum spec.
Example: If you are using S29GL128S device with speed grade 100ns, then you cannot read a byte/word from this flash with less than 100ns access time.
tCE spec is the time required for the flash device to drive data out when OE# is LOW and address lines are already stable. This is same as the maximum operating speed (minimum access time) of the flash device. As you can see in the datasheet, the minimum access time and the value of tCE spec are same. So, we recommend you to use tCE = 100ns/110ns/ 120ns (depending of the device you use) while reading data from flash.
Please let me know, if you need any clarifications.
Thanks and Regards,
Thank you for your reply.
We use the VCC=VIO type, so S29GL128S90TFI010 has a 90ns access time.
tACC is designed with enough time (> 90ns) for spec.
We want to know the margin of tCE.
Could you tell us the typical value of tCE.