What different in Fine Granularity Refresh mode?

This function provide in BIOS ver. 0082, you can find it in the bottom of DRAM Timing Control page.

if FGR mode auto or enabled, BIOS tRFC will unavailable and only can available set tRFC2 value, when it disabled, BIOS tRFC2 will unavailable to set, only tRFC available.

there is some different between FGR mode enabled and disabled, you can choose what mode you want to use.

in general, FGR mode = enabled, more flexible and compatible to use, when FGR mode = disabled it might cause system unstable.

Refresh Operation
The Refresh command (REF) is used during normal operation of the DDR5 SDRAMs. This command is non persistent, so it must be issued each time a refresh is required. The DDR5 SDRAM requires Refresh
cycles at an average periodic interval of tREFI.

There are three types of refresh operations supported by DDR5 SDRAMs.
– Normal Refresh: By issuing All Bank Refresh (REFab) command in Normal Refresh mode
– Fine Granularity Refresh: By issuing All Bank Refresh (REFab) command in Fine Granularity Refresh mode
– Same Bank Refresh: By issuing Same Bank Refresh (REFsb) command in Fine Granularity Refresh mode

For Normal Refresh and Fine Granularity Refresh operations, all banks of the SDRAM must be precharged and idle for a minimum of the precharge time tRP(min) before the All Bank Refresh command (REFab) can be issued. The refresh addressing is generated by the internal refresh controller during the refresh cycle. The external address bus is only required to be in a valid state once this cycle has started. When the refresh cycle has completed, all banks of the SDRAM will be in the precharged (idle) state. A delay between the Refresh command and the next valid command, except DES, PDE and non-Target ODT commands, must be greater than or equal to the minimum Refresh cycle time tRFC(min) as shown in Figure 62 and Figure 63. Note that the tRFC timing parameter depends on memory density and the refresh mode setting, which can be set to Normal Refresh mode or Fine Granularity Refresh (FGR) mode.

The DDR5 SDRAM has two different Refresh modes with two different refresh cycle time (tRFC) settings. There is a Normal Refresh mode setting and a Fine Granularity Refresh (FGR) mode setting. The FGR mode provides a shorter refresh cycle time (tRFC2) but also requires All Bank Refresh commands (REFab) to be provided twice as often (tREFI is divided by two, i.e., tREFI2 = tREFI1/2).

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