Auto-Refresh is command. In the datasheet, you will see the m@x refresh cycle time is around 16ms to 64ms. And you will see in refresh cycle time, you must preform how many time autorefresh. Like 16ms will be 1k refresh ( k=1024 ),32ms is 2k refresh and 64ms is 4k refresh. You must execute precharge all first and then do auto-refresh. Precharge must spend 1~3 cycles.each time, you do auto-refresh, 1~3 cycles is panic. But you can use burst refresh,then you can save nk X prechage time. The penality is system must wait for burst refresh cycle end. I think you will choose distribute refresh, 15.6us = 64ms/4096 will have one refresh. I hope this will answer your first question.