Texas Instruments TMS320DM643 manual Refresh Scheduling, Self-Refresh Mode, Refresh Urgency Levels

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Peripheral Architecture

2.9Refresh Scheduling

The DDR2 memory controller issues autorefresh (REFR) commands to DDR2 SDRAM devices at a rate defined in the refresh rate (RR) bit field in the SDRAM refresh control register (SDRCR). A refresh interval counter is loaded with the value of the RR bit field and decrements by 1 each cycle until it reaches zero. Once the interval counter reaches zero, it reloads with the value of the RR bit. Each time the interval counter expires, a refresh backlog counter increments by 1. Conversely, each time the DDR2 memory controller performs a REFR command, the backlog counter decrements by 1. This means the refresh backlog counter records the number of REFR commands the DDR2 memory controller currently has outstanding.

The DDR2 memory controller issues REFR commands based on the level of urgency. The level of urgency is defined in Table 12. Whenever the refresh level of urgency is reached, the DDR2 memory controller issues a REFR command before servicing any new memory access requests. Following a REFR command, the DDR2 memory controller waits T_RFC cycles, defined in the SDRAM timing register (SDTIMR), before rechecking the refresh urgency level.

In addition to the refresh counter previously mentioned, a separate backlog counter ensures the interval between two REFR commands does not exceed 8× the refresh rate. This backlog counter increments by 1 each time the interval counter expires and resets to zero when the DDR2 memory controller issues a REFR command. When this backlog counter is greater than 7, the DDR2 memory controller issues four REFR commands before servicing any new memory requests.

The refresh counters do not operate when the DDR2 memory is in self-refresh mode.

	Table 12. Refresh Urgency Levels
Urgency Level	Description
Refresh May	Backlog count is greater than 0. Indicates there is a backlog of REFR commands, when the DDR2 memory
	controller is not busy it will issue the REFR command.
Refresh Release	Backlog count is greater than 3. Indicates the level at which enough REFR commands have been performed
	and the DDR2 memory controller may service new memory access requests.
Refresh Need	Backlog count is greater than 7. Indicates the DDR2 memory controller should raise the priority level of a
	REFR command above servicing a new memory access.
Refresh Must	Backlog count is greater than 11. Indicates the level at which the DDR2 memory controller should perform a
	REFR command before servicing new memory access requests.

2.10 Self-Refresh Mode

Setting the self refresh (SR) bit in the SDRAM refresh control register (SDRCR) to 1 forces the DDR2 memory controller to place the external DDR2 SDRAM in a low-power mode (self refresh), in which the DDR2 SDRAM maintains valid data while consuming a minimal amount of power. When the SR bit is asserted, the DDR2 memory controller continues normal operation until all outstanding memory access requests have been serviced and the refresh backlog has been cleared. At this point, all open pages of DDR2 SDRAM are closed and a self-refresh (SLFRFR) command (an autorefresh command with DDR_CKE low) is issued.

The DDR2 memory controller exits the self-refresh state when a memory access is received or when the SR bit in SDRCR is cleared to 0. While in the self-refresh state, if a request for a memory access is received, the DDR2 memory controller services the memory access request, returning to the self-refresh state upon completion. The DDR2 memory controller will not wake up from the self-refresh state (whether from a memory access request or from clearing the SR bit) until T_CKE + 1 cycles have expired since the self-refresh command was issued. The value of T_CKE is defined in the SDRAM timing 2 register (SDTIMR2).

After exiting from the self-refresh state, the DDR2 memory controller will not immediately start executing commands. Instead, it will wait T_SXNR + 1 clock cycles before issuing non-read commands and T_SXRD + 1 clock cycles before issuing read commands. The SDRAM timing 2 register (SDTIM2) programs the values of T_SXNR and T_SXRD.

SPRU986B–November 2007 DDR2 Memory Controller 29

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