RM0390 Rev 4 313/1328
RM0390 Flexible memory controller (FMC)
324
If the next access is sequential and the current access crosses a bank boundary, the
SDRAM controller activates the first row in the next bank and initiates a new read/write
command. Two cases are possible:
• If the current bank is not the last one, the active row in the new bank must be
precharged.At a bank boundary, the automatic activation of the next row is supported
for all rows/columns and data bus width configuration.
• If the current bank is the last one and the selected SDRAM device is connected to
Bank 1, the automatic activation of the next row in device connected to SDRAM Bank2
is not supported. A PALL software command must be issused on Bank1 before any any
access on Bank2.
SDRAM controller refresh cycle
The Auto-refresh command is used to refresh the SDRAM device content. The SDRAM
controller periodically issues auto-refresh commands. An internal counter is loaded with the
COUNT value in the register FMC_SDRTR. This value defines the number of memory clock
cycles between the refresh cycles (refresh rate). When this counter reaches zero, an
internal pulse is generated.
If a memory access is ongoing, the auto-refresh request is delayed. However, if the memory
access and the auto-refresh requests are generated simultaneously, the auto-refresh
request takes precedence.
If the memory access occurs during an auto-refresh operation, the request is buffered and
processed when the auto-refresh is complete.
If a new auto-refresh request occurs while the previous one was not served, the RE
(Refresh Error) bit is set in the Status register. An Interrupt is generated if it has been
enabled (REIE = ‘1’).
If SDRAM lines are not in idle state (not all row are closed), the SDRAM controller generates
a PALL (Precharge ALL) command before the auto-refresh.
If the Auto-refresh command is generated by the FMC_SDCMR Command Mode register
(Mode bits = ‘011’), a PALL command (Mode bits =’ 010’) must be issued first.
11.7.4 Low-power modes
Two low-power modes are available:
• Self-refresh mode
The auto-refresh cycles are performed by the SDRAM device itself to retain data
without external clocking.
• Power-down mode
The auto-refresh cycles are performed by the SDRAM controller.
Self-refresh mode
This mode is selected by setting the MODE bits to ‘101’ and by configuring the Target Bank
bits (CTB1 and/or CTB2) in the FMC_SDCMR register.
The SDRAM clock stops running after a TRAS delay and the internal refresh timer stops
counting only if one of the following conditions is met:
• A Self-refresh command is issued to both devices
• One of the devices is not activated (SDRAM bank is not initialized).
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