DRAM Power Management and Initialization

The processor implements extensive support for power management on the memory interface. Each channel drives 4 CKE pins, one per rank.

The CKE is one of the power-saving means. 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 processor supports four different types of power-down modes in package C0 state. The different power-down modes can be enabled through configuring PM PDWN config register. The type of CKE power-down can be configured through PDWN_​mode (bits 15:12) and the idle timer can be configured through PDWN_​idle_​counter (bits 11:0).

The different power-down modes supported are:

• No power-down (CKE disable)
• 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 fined by tXP – small number of cycles. For this mode, DRAM DLL should be on.
• PPD/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 IDD2P. Exiting this mode is defined by tXP, but also tXPDLL (10–20 according to DDR type) cycles until first data transfer is allowed. For this mode, DRAM DLL should be off.
• Precharged power-down (PPD): This mode is entered if all banks in DDR are pre-charged when de-asserting CKE. Power-saving in this mode is intermediate –better than APD, but less than DLL-off. Power consumption is defined by IDD2P. Exiting this mode is defined by tXP. The difference from APD mode is that when waking-up, all page-buffers are empty.) The LPDDR does not have a DLL. As a result, the power savings are as good as PPD/DDL-off but will have lower exit latency and higher performance.

The CKE is determined per rank, whenever 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 arrives to queues. The idle-counter begins counting at the last incoming transaction arrival.

It is important to understand that since the power-down decision is per rank, the IMC can find many opportunities to power down ranks, even while running memory intensive applications; the savings are significant (may be few Watts, according to DDR specification). This is significant when each channel is populated with more ranks.

Selection of power modes should be according to power-performance or thermal trade off of a given system:

• When trying to achieve maximum performance and power or thermal consideration is not an issue: use no power-down
• In a system which tries to minimize power-consumption, try using the deepest power-down mode possible – PPD/DLL-off with a low idle timer value
• In high-performance systems with dense packaging (that is, tricky thermal design) the power-down mode should be considered in order to reduce the heating and avoid DDR throttling caused by the heating.

The default value that BIOS configures in PM PDWN config register is 6080 – that is, PPD/DLL-off mode with idle timer of 0x80 (128 DCLKs). This is a balanced setting with deep power-down mode and moderate idle timer value.

The idle timer expiration count defines the # of DCLKs that a rank is idle that causes entry to the selected power mode. As this timer is set to a shorter time the IMC will have more opportunities to put the DDR in power-down.