RZ-G/RZG hardware

← RZ-G

DDR

DDR (auto) refresh and self-refresh

There is often a confusion between DDR auto refresh and self refresh, this small note is meant to shed some light on these two terms.

DDR refresh or auto refresh is simply the command that is sent from the DDR controller (DDRC) to the DDR chip periodically (in the range of milliseconds) to refresh the DDR content. This happens while normal operation, so all the signals including the clock are active.

DDR self-refresh instead is a low power mode that is entered by issuing the (auto) refresh command and keeping the CKE low. In this case the DDR chip uses an internal counter to trigger the refresh. In self refresh mode all the DDRC signals but CKE are "don't care" and they may not even be actively driven to save power.

In some cases, especially for DDR used in temperature sensitive applications, there is a feature called Auto Self-Refresh (ASR) that is basically an self-refresh with a variable refresh time automatically computed based on the temperature. If ASR is not supported normally the refresh time should be configured before the self-refresh mode is entered.

In RZ/G2 there is a low power mode where all the power supplies but the DDRC I/Os (mainly to drive CKE) are off, to better support the so called suspend-to-RAM, that allows to keep the DDR content and save power. When the power supplies are back the MPU will reboot quicker by restoring its state from the one saved in the DDR before entering the low power mode. Obviously, in order for the suspend-to-RAM to be supported and all the power supplies off, there should be a PMIC that supports it and some software routines that handles the handshake, as well as the shutdown / wake up.

Timer

TBD