linux-kernel - RE: [PATCH 2/2] rtc: pcf2127: add battery-low INTn generation

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Date:   Wed, 3 Apr 2019 15:49:03 +0000
From:   Flavio Suligoi <f.suligoi@...m.it>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
CC:     Alessandro Zummo <a.zummo@...ertech.it>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-rtc@...r.kernel.org" <linux-rtc@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/2] rtc: pcf2127: add battery-low INTn generation

> > > On 03/04/2019 16:52:45+0200, Flavio Suligoi wrote:
> > > > The pcf2127 has an automatic battery-low detection function.
> > > >
> > > > In case of battery-low event, an interrupt generation through
> > > > the pin INTn (active low) can be enabled, setting the flag BLIE
> > > > in the register Control_3.
> > > >
> > > > This function is activated by the "battery-low-hw-alarm" DT
> property.
> > > >
> > > > Example of use for an NXP i.MX7D board:
> > > >
> > > > &i2c3 {
> > > > 	clock-frequency = <100000>;
> > > > 	pinctrl-names = "default";
> > > > 	pinctrl-0 = <&pinctrl_i2c3>;
> > > > 	status = "okay";
> > > >
> > > > 	pcf2127@51 {
> > > > 		compatible = "nxp,pcf2127";
> > > > 		reg = <0x51>;
> > > > 		battery-low-hw-alarm;
> > > > 		status = "okay";
> > > > 	};
> > > > };
> > > >
> > >
> > > So I'm curious, how do you then use that signal? I have a (not yet
> sent)
> > > series adding alarm support for the pcf2127. The issue having BLIE is
> > > that then this will prevent the alarm to work properly.
> > >
> > > So my guess is that you have nINT connected to an LED or something
> that
> > > the user can see?
> > >
> >
> > I'm working on our custom embedded board with an NXP i.MX7D,
> > developed here, in Asem (www.asem.it).
> > The nINT pin is connected to a GPIO of the MX7 and then
> > used by an applicative sw, to generate an alarm for the user.
> >
> 
> Then, you should probably not enable BLIE because this will cause issues
> with the alarm functionnality.. It is certainly enough to use
> RTC_VL_READ periodically.

We use the nINT signaling solution because of this pin, in addition
to be used by the CPU, can be also connected to an external connector,
available for the final user.
Anyway, even if the BLIE is set, the sw low voltage alarm works,
with the message (displayed about every 12 minutes):

rtc-pcf2127-i2c 2-0051: low voltage detected, check/replace RTC battery.

What kind of issues did you find with BLIE enabled?

Flavio