cpu/stm32: add backup battery monitoring (VBAT)

[fabian18]

Contribution description

This PR adds a driver to monitor the backup battery status for many stm32 based boards with ADC.
To check which boards support battery monitoring, I did:
grep -I -r '_VBAT' | cut -d' ' -f1 | sort | uniq in cmsis-header-stm32.
As a reference of how to calculate the voltage value from the ADC sample I found this.

Testing procedure

I tested tests/driver_vbat with nucleo-f767zi.

2021-10-14 17:36:51,385 # main(): This is RIOT! (Version: 2021.10-devel-960-g0ec3a-cpu_stm32_add_vbat)
2021-10-14 17:36:51,385 # 
2021-10-14 17:36:51,388 # RIOT backup battery monitoring test
2021-10-14 17:36:51,388 # 
2021-10-14 17:36:51,393 # This test will sample the backup battery once a second
2021-10-14 17:36:51,393 # 
2021-10-14 17:36:51,394 # 
2021-10-14 17:36:51,395 # VBAT: 2984[mV]
2021-10-14 17:36:52,397 # VBAT: 3233[mV]
2021-10-14 17:36:53,399 # VBAT: 3246[mV]
2021-10-14 17:36:54,400 # VBAT: 3210[mV]
2021-10-14 17:36:55,402 # VBAT: 3178[mV]
2021-10-14 17:36:56,404 # VBAT: 3216[mV]
2021-10-14 17:36:57,406 # VBAT: 3207[mV]
2021-10-14 17:36:58,408 # VBAT: 3213[mV]

Issues/PRs references

This PR includes #16870 and was created to have battery monitoring independent.

[benpicco]

This needs a rebase 😉

[benpicco]

Sorry for losing track of this again!
Looks good in general, I like a dedicated, simple API for something simple as opposed to a big generic API that makes the simple things hard.

It would be a laudable task to unify the stm32 ADC implementations, maybe adc_ng will get us there.

A bit of bikeshedding ahead 😉

[benpicco]

since you started that bikeshedding last time 😉

Why not periph_vbat?
Battery monitoring seems to be more a peripheral thing than battery RAM.

[fabian18]

The name periph_vbat is surely comprehensible. But I personally would not call it like that because probably no datasheet would list it as an extra peripheral because it is a feature of the ADC. I saw that there is cpu/efm32/drivers/coretemp/coretemp.c which is also samples an ADC channel. So I decided to implement VBAT similar to this. So you think it should rather be periph_vbat?

[benpicco]

Although that is efm32_coretemp (and I think a generic version should be periph_coretemp).

Generally I understand the periph_ prefix as something that needs a per-CPU implementation as opposed to a generic module. (I see that this makes the case for periph_backup_ram…)

[fabian18]

Yes I think it should just be coretemp 😅. Because if my use case or application requires the core temperature, I´d rather do USEMODULE += coretemp and not care if my CPU is efm32. Or better, coretem could pull in efm32_coretemp for efm32 but that´s not the case right now.

[fabian18]

Ah yes or periph_coretemp like you said.

[fabian18]

Anyway, I have no problem with periph_, if you think it´s better.

[benpicco]

I think this would better belong to cpu/stm32/include/periph/<line>/periph_cpu.h but if you prefer it here I won't argue

[fabian18]

Since nothing else uses it, I´d keep it there.

[benpicco]

Murdock found some issues

[fabian18]

When doing TEST_KCONFIG=1 BOARD=nucleo-f767zi make -C tests/periph_vbat menuconfig, VBAT_ADC_VREF_MV does not appear anywhere. I have not figured out why, yet.

[benpicco]

Not sure, maybe @leandrolanzieri has an idea.
Murdock appears to be happy though - please squash.

[leandrolanzieri]

The reason it does not show up when TEST_KCONFIG=1 is that, when that flag is active, Makefile dependency resolution does not run -> USEMODULE_PERIPH_VBAT is not defined. Although it implies some duplication, I'd go with the option of defining it again, in the first menuconfig. It could look something like:

menuconfig MODULE_PERIPH_VBAT
    bool "Backup Battery monitoring driver"
    depends on HAS_PERIPH_VBAT
    depends on HAS_PERIPH_ADC
    depends on TEST_KCONFIG
    select MODULE_PERIPH_ADC

if MODULE_PERIPH_VBAT

config MODULE_PERIPH_INIT_VBAT
    bool "Auto initialize VBAT ADC line"
    default y if MODULE_PERIPH_INIT

config VBAT_ADC_VREF_MV
    int "ADC reference voltage in mV"
    default 3300

endif # MODULE_PERIPH_VBAT

menuconfig KCONFIG_USEMODULE_PERIPH_VBAT
    bool "Configure backup battery monitoring peripheral driver"
    depends on USEMODULE_PERIPH_VBAT
    help
        Configure backup battery monitoring peripheral driver using Kconfig.

config VBAT_ADC_VREF_MV
    int "ADC reference voltage in mV"
    default 3300
    depends on KCONFIG_USEMODULE_PERIPH_VBAT
    help
        This is the reference voltage (VREF) of the ADC.
        Often VREF is connected with VDDA, which is equal to VDD.

[fabian18]

Thank you. I am going to squash that in right away.

[fabian18]

Thank you.