[riot-notifications] [RIOT-OS/RIOT] cpu/stm32: make backup SRAM available (#16870)

fabian18 notifications at github.com
Fri Sep 24 20:50:45 CEST 2021

@fabian18 commented on this pull request.

> +void backup_ram_init(void);
+ * @brief   Check whether the backup RAM regulator is on and backup RAM
+ *          content is retained while asleep
+ */
+bool backup_ram_is_retained(void);
+ * @brief   Enable the backup RAM regulator to retain it´s content
+ *          during standby mode
+ */
+void backup_ram_sleep(void);
+ * @brief   Disable the backup RAM regulator

So if I am not mistaken, to measure the current consumption on the `nucleo` boards,
it is as simple as removing a jumper and connect a multimeter to the `IDD` pins. For the `nucleo-f767zi` it is Jumper 5. (And do `ETH->MACPMTCSR |= 1` to power down Ethernet. )
[datasheet section 6.7](https://www.st.com/resource/en/data_brief/nucleo-f767zi.pdf)
When the regulator is switched on explicitly I get at most `34.30 mA` in normal operation mode
And when the regulator is not explicitly enabled I get at most `34.19 mA` in normal operation mode.

So, if at all, the difference is about `110 µA`.
How do we conclude? Keep the regulator enabled by default all the time?
If you think it is worth to disable it, then I must get a coin cell and remove a solder bridge to know what is going on, when the board is powered from `VBat`, because it could be that the regulator must be turned on all the time in that case anyways.

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