74.10.2 Backup with SDRAM in Self-Refresh (BSR) Mode

Backup with SDRAM in Self-Refresh (BSR) mode is an extension of Backup mode with the application context saved in the external DDR memory operating in Self-refresh mode, in the perspective to restart the application faster. In this mode, VBAT and VDDIODDR power inputs must be maintained, as well as the DDR component power inputs. It is good practice to use a combination of LPM and SHDN outputs from the Shutdown Controller (SHDWC) to signal the power management circuit to enter this special powering mode. See BSR Mode Entry and Wake-up for detailed waveforms.

The device exits BSR mode when an active wake-up event is triggered by the SHDWC. Upon this wake-up event, the SHDWC automatically toggles its SHDN output back to VBAT and this signal typically helps to restart all the power supply channels at board level. The LPM pin is not automatically reset by the SHDWC. It must be software-reset in the early steps of the wake-up process.

The system power consumption in BSR mode is mainly that of the DDR, therefore the choice of the DDR type is of prime importance. In a similar way, the choice of the regulator(s) that maintain(s) DDR supplies in BSR mode should optimize efficiency at low current.