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Datasheet, Volume 1 57
Power Management
4.3.2 DRAM Power Management and Initialization
The processor implements extensive support for power management on the SDRAM
interface. There are four SDRAM operations associated with the Clock Enable (CKE)
signals that the SDRAM controller supports. The processor drives four CKE pins to
perform these operations.
The CKE is one means of power saving. When CKE is off, the internal DDR clock is
disabled and the DDR power is reduced. The power-saving differs according to the
selected mode and the DDR type used. For more information, refer to the IDD table in
the DDR specification.
The DDR defines 3 levels of power down that differ in power saving and in wakeup
time:
1. Active power down (APD): This mode is entered if there are open pages when de-
asserting CKE. In this mode the open pages are retained. Power-saving in this
mode is the lowest. Power consumption of DDR is defined by IDD3P. Exiting this
mode is defined by tXP – small number of cycles.
2. Precharged power down (PPD): This mode is entered if all banks in DDR are
precharged when de-asserting CKE. Power-saving in this mode is intermediate –
better than APD, but less than DLL-off. Power consumption is defined by IDD2P1.
Exiting this mode is defined by tXP. The difference relative to APD mode is that
when waking-up in PPD mode, all page-buffers are empty.
3. DLL-off: In this mode the data-in DLLs on DDR are off. Power-saving in this mode is
the best among all power modes. Power consumption is defined by IDD2P1. Exiting
this mode is defined by tXP and tXPDLL (10–20 according to the DDR type) until
first data transfer is allowed.
The processor supports 6 different types of power down. The different modes are the
power down modes supported by DDR3 and combinations of these. The type of CKE
power down is defined by configuration. The options are as follows:
1. No power down
2. APD: The rank enters power down as soon as the idle-timer expires, independent of
the bank status
3. PPD: When idle timer expires, the MC sends PRE-all to rank and then enters power
down
4. DLL-off: Same as option 2 but DDR is configured to DLL-off
5. APD, change to PPD (APD-PPD): Begins as option 1, and when all page-close timers
of the rank are expired, it wakes the rank, issues PRE-all, and returns to PPD.
6. APD, change to DLL-off (APD_DLLoff): Begins as option 1, and when all page-close
timers of the rank are expired, it wakes the rank, issues PRE-all, and returns to
DLL-off power down.
The CKE is determined per rank, when it is inactive. Each rank has an idle counter. The
idle counter starts counting as soon as the rank has no accesses, and if it expires, the
rank may enter power down while no new transactions to the rank arrive to queues.
The idle counter begins counting at the last incoming transaction arrival.