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SDRAM Timing Spec
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The refresh command time t_rfc is not t_ref (marked with nr.1).
t_ref is the refresh cycle which is 64
ms and you correctly used it to calculate refresh command period.
t_rfc is not reported in datasheet but you can assume it is equal to t_rc, then 70ns.
Please note that those you used are minimal values. If you want to be conservative, you can rise those values and exclude problems due to any abnormally delayed signal. (clearly you pay the price of slower access to sdram...)
The only parameter that work the other way is refresh period: this should be reduced to ensure correct operation.
Honored Contributor
