A question regarding RTC - MCP7940N

jaikumar · Joined: 15 Dec 2006 Posts: 109

Hi all,
I am using the MCP7940N RTC.

http://www.farnell.com/datasheets/1754398.pdf

I was wondering if there is any way of finding out if the
battery was removed and put back during power off.

Can i set a internal RAM location to some value say 0xAA and then check later on when power is re applied.

OSCON bit does not seem to help here.

Thanks,
Jay.

Ttelmah · Joined: 11 Mar 2010 Posts: 19546

You don't need to fiddle.

Just read the Power Fail Time Stamp registers (18 to 1B).

If these contain a time, then the battery was OK all the time the chip was off. If they are empty. then the battery has been off, since the last main power failure.

These store the time when the main supply goes off, but their contents are lost if the battery goes off as well.

jaikumar · Joined: 15 Dec 2006 Posts: 109

Thanks Ttelmah.

Regards

Ttelmah · Joined: 11 Mar 2010 Posts: 19546

I happened to want to ensure the chip always had a valid time, and 'force' a reinitialization when the battery had been off. Suddenly 'twigged' I could use these registers. Smile

jaikumar · Joined: 15 Dec 2006 Posts: 109

Want to know if i can apply power to RTC using the port pin of 16f887 instead of from regulator.
I know it sounds ridiculous but i have one reason
I read some where in the datasheet or application notes that the applied voltage should drop suddenly and should not droop for the RTC to work properly.
Since i am driving a motor i have a big cap. The controller is protected by a
supervisor. So i want to know if i can drive the RTC from a port pin.

Regards,
Jay.

Ttelmah · Joined: 11 Mar 2010 Posts: 19546

Probably yes.

However where did you read this?.

If you look at the data sheet, there is a _minimum_ rise time specified, but no 'maximum'. The_PIC_ however has a maximum rise time, to reset properly, which with a large supply capacitor may well cause problems (you need an external MCLR circuit, if the supply takes significant time to rise...).

jaikumar · Joined: 15 Dec 2006 Posts: 109

Ttelmah,

Please see below app note:
http://ww1.microchip.com/downloads/en/AppNotes/01365C.pdf

Although this app note is for the MCP794XX, as far as the RTC goes
they function the same.

Page-2 - Power-UP

it also specifies about using the MFC pin. But if i don't want to use it.
Can i just leave the pin open. Its not clear on that.

Regards,
Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19546

The bit you are looking at there, is only if the supply droops below the minimum operating Vdd voltage, and then recovers. Seriously if your supply to the PIC does this, you have much bigger problems.....

If you are running the motor from the same supply as the PIC, 'think again'. Separate the rails. Have a PIC supply, with it's own capacitor. This can even be fed from the motor rail, but separated (perhaps an inductor, or resistor), from the actual motor rail. You have to understand that motors, can do some nasty things like acting as a generator, when switched off. This can result in the rail going above it's normal voltage. The energy both from this and the inductive nature of the drive coils, has to go _somewhere_, and you need to design your supply to handle this.