Hi Sam,
On 09/07/2018 12:18 AM, Sam Ravnborg wrote:
> Hi all.
>
> The NXP RTC support that two different capacitors are connected.
> The default (what the RTC is programmed to after reset) is 7 pF.
> But one may also connect 12.5 pF to save power.
> To use the 12.5 pF the RTC needs to be properly configured.
>
> The RTC can only support either 7 pF or 12.5 pF.
>
> A wrongly configured RTC will loose several hours in a week.
>
>
> We came up with the following binding:
> ==========================================
> * NXP PCF8523 Real Time Clock
>
> NXP PCF8523 Real Time Clock
>
> Required properties:
> - compatible: Should contain "nxp,pcf8523".
> - reg: I2C address for chip.
>
> Optional property:
> - nxp,quartz_load_12.5pF: The capacitive load on the quartz is 12.5pf,
> which differ from the default value of 7pf
>
> Example:
>
> pcf8523: pcf8523@68 {
> compatible = "nxp,pcf85063";
> reg = <0x68>;
> nxp,quartz_load_12.5pF;
> };
> =======================================
>
> The Q:
> The above scheme will SILENTLY break out-of-tree Linux users because
> we will change so if nothing is specified the RTC is configured to 7 pF.
That's the expected behaviour.
> And you need to specify if 12.5 pF is used.
>
> But all users of the kernel driver will assume 12.5 pF, because this
> is the driver default. But not the default of the RTC.
I've opened the driver rtc-pcf85063 and PCF85063A.pdf datasheet, and I don't
see why 12.5 pF internal oscillator capacitor selection is the default one,
it is not.
The only touched configuration of PCF85063_REG_CTRL1 register is STOP bit,
CAP_SEL bit value is unmodified by the driver.
So I would assume it should be fine just to add a new property for 12.5pF
load capacitance.
>
> We will change all in-kernel device trees when we add this, but...
>
> Should we try do do this in a way so we respect the current
> kernel driver implmentation, or should we respect the reset default
> value of the RTC?
>
> Sam
>
--
Best wishes,
Vladimir
