Save eeprom before WDT restart

ntc · Joined: 18 Mar 2015 Posts: 8

If I want to save a data to EEPROM before WDT restart, can I have do ?
I think have an wdt interrupt to process this work ?
Thanks

Ttelmah · Joined: 11 Mar 2010 Posts: 19539

You need to always start by telling us the chip. The 'PIC' as a generic term covers about seven actual chip families with different capabilities and restrictions.

However, generally, may not be possible. In standard operation, the watchdog restarts the chip, unless it is asleep, in which case in many cases it can wake on the watchdog, which could then write. If used this way, why not write before going to sleep, which is safer.

But, I then have to ask 'why bother'. When the chip restarts with a watchdog, the registers are saved (not lost), so you can test the 'restart_cause', and then write the EEPROM.

Generally though, beware of anything that writes to the EEPROM on a regular basis. It will always surprise you just how quickly you can kill a chip with repeated writes. Even using the slowest watchdog, which on many chips is something like 30 seconds, this will kill the EEPROM in about 70 days.....
Anything that writes more frequently than once every few _hours_ should be treated with care. The EEPROM in general is designed for things that change _infrequently_.

ELCouz · Joined: 18 Jul 2007 Posts: 427 Location: Montreal,Quebec

temtronic · Posted: Thu Mar 19, 2015 5:21 pm

How much 'data' ? If less than 56 bytes, an RTC chip like the DS1307 is a great 'poor mans' FRAM WITH a real time clock for 'free'...

just another option..

Jay

ntc · Joined: 18 Mar 2015 Posts: 8

Thanks all.
In my program, i'm using WDT to reset the chip when it hanging. So, I think can't using an WDT ISR to save data to EEPPROM.
I'm using PIC18F4680, and wanted to save about <20 byte. In my circuit, I'm using RTC DS1307 chip, can you tell me clear?
Thanks.

ntc · Joined: 18 Mar 2015 Posts: 8

Ttelmah · Joined: 11 Mar 2010 Posts: 19539

I'd be working out why the chip is hanging.....

Seriously it either implies a coding problem (something like an array overflow overwriting part of memory), or a hardware problem (electrical noise etc..). I have some PIC's dating back to the first they made (actually earlier than Microchip claim for their first chips - I pointed this out to them a few years ago....), that in some cases have run in instruments _continuously_ since being started (some now for about 39 years)!. Had a few fail recently, they started to lose their ROM's, but the watchdog to bodge round a hang is very dangerous. About the only genuine reason for a watchdog recovery being needed, is a strike by a subatomic particle corrupting memory.
The key thing with using the watchdog, is to get a 99.999% reliability first.

temtronic · Posted: Fri Mar 20, 2015 5:01 am

I have to agree with MrT that you need to find out why the WDT is timing out, resettting the PIC. I've got about 25 years of using them and have never needed to implement the WDT feature.
Also since you are using the DS1307,save your data to the CMOS RAM in it.The driver here has the 'reads/write' functions already in it !

cheers
Jay

asmboy · Joined: 20 Nov 2007 Posts: 2128 Location: albany ny