EMIF Module Architecture
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SPNU563A–March 2018
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External Memory Interface (EMIF)
21.2.5.8 Power Down Mode
To support low-power modes, the EMIF can be requested to issue a POWER DOWN command to the
SDRAM by setting the PD bit in the SDRAM configuration register (SDCR). When this bit is set, the EMIF
will continue normal operation until all outstanding memory access requests have been serviced and the
SDRAM refresh backlog (if there is one) has been cleared. At this point the EMIF will enter the power-
down state. Upon entering this state, the EMIF will issue a POWER DOWN command (same as a NOP
command but driving EMIF_CKE low on the same cycle). The EMIF then maintains EMIF_CKE low until it
exits the power-down state.
Since the EMIF services the refresh backlog before it enters the power-down state, all internal banks of
the SDRAM are closed (precharged) prior to issuing the POWER DOWN command. Therefore, the EMIF
only supports Precharge Power Down. The EMIF does not support Active Power Down, where internal
banks of the SDRAM are open (active) before the POWER DOWN command is issued.
During the power-down state, the EMIF services the SDRAM, asynchronous memory, and register
accesses as normal, returning to the power-down state upon completion.
The PDWR bit in SDCR indicates whether the EMIF should perform refreshes in power-down state. If the
PDWR bit is set, the EMIF exits the power-down state every time the Refresh Must level is set, performs
AUTO REFRESH commands to the SDRAM, and returns back to the power-down state. This evenly
distributes the refreshes to the SDRAM in power-down state. If the PDWR bit is not set, the EMIF does
not perform any refreshes to the SDRAM. Therefore, the data integrity of the SDRAM is not assured upon
power down exit if the PDWR bit is not set.
If the PD bit is cleared while in the power-down state, the EMIF will come out of the power-down state.
The EMIF:
• Drives EMIF_CKE high.
• Enters its idle state.
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