[PATCH 0/3] rtc: m41t80: set xtal load capacitance from DT

[Dennis Lambe Jr]

The m41t82 and m41t83 have an adjustable internal capacitance that 
defaults to 25 pF per xtal pin. This patch series adds the ability to 
configure it via the devicetree.

Patch 1 just switches the CONFIG_OF-dependent block in m41t80_probe() 
from an ifdef guard to an if(IS_ENABLED(...)) guard, so that I don't 
need to use __maybe_used on my new functions and variables.

Patch 2 is the DeviceTree YAML changes.

Patch 3 is the actual added functionality.

The desired capacitance comes from quartz-load-capacitance property, 
following the example of two other RTC ICs that have adjustable internal 
load capacitance, the NXP pcf85063 and pcf8523.

The m41t82 supports much finer-grained control over the capacitance than 
those chips and calls the feature "analog calibration", but it looks to 
me like it's essentially the same kind of thing.

My use case for this is:

ST specifies not to add any additional external load capacitance[1], but 
the MikroElektronika RTC 9 Click board[2] has a 22 pF cap on each xtal 
pin[3]. The resulting combined capacitance appears to be outside of the 
operating range of the xtal, because when power is removed from the 
boards I'm testing with, the RTC reports an Oscillator-Fail flag on the 
next power on.

I found I could work around the problem by reducing the internal load 
capacitance as low as it will go.

I have tested on the VersaLogic Zebra, an NXP i.MX6 ARM. I made sure it 
compiles cleanly on amd64 with CONFIG_OF=no.

References:
[1] https://www.st.com/resource/en/application_note/an3060-applications-guide-for-serial-realtime-clocks-rtcs-stmicroelectronics.pdf
[2] https://www.mikroe.com/rtc-9-click
[3] https://download.mikroe.com/documents/add-on-boards/click/rtc-9/rtc-9-click-schematic-v100.pdf

Dennis Lambe Jr (3):
  rtc: m41t80: probe: use IS_ENABLED for CONFIG_OF
  dt-bindings: m41t80: add xtal load capacitance
  rtc: m41t80: set xtal load capacitance from DT

 .../devicetree/bindings/rtc/st,m41t80.yaml    | 18 ++++
 drivers/rtc/rtc-m41t80.c                      | 84 +++++++++++++++++--
 2 files changed, 94 insertions(+), 8 deletions(-)

-- 
2.25.1