#FUSES question [SOLVED]

[benoitstjean]

Compiler: 5.026
Device: PIC24EP512GP806

Quick question on fuses.... Can you use #FUSES anywhere in the code to modify the behaviour of the MCU _at that point in the code_ ? Or is it the last #FUSES encountered used for the life of the program?

Like for instance, if I use #FUSES at the begining of my code with a bunch of fuses configured and somewhere along the way, inside a function, I call #FUSES again with different fuses.... What should I expect?

The reason I am asking is because I want to disable the watchdog timer when the MCU falls in sleep mode after I call a function I wrote.

In that function, I disable a bunch of interrupts and I do a bunch of other things but I also want to make sure that before I call < sleep( SLEEP_FULL ); >, I need to make sure that the WDT gets disabled because I don't want the MCU to revive itself upon expiry of the WDT.

The MCU can only wake-up upon external interrupts from the serial port or two input interrupt pins.

Many thanks,

Benoit

[newguy]

A fuse is something that, as far as I know, can only be "burned" by an external programmer. Unless devices with the ability to write program memory can modify the fuse locations? In which case, if that's even possible, modifying them on the fly would be rather involved. You'd need to write the new value to that location and you might also need to program a whole page?

[benoitstjean]

Ok, that would make sense.

So these are my fuses:

#fuses HS, PR, WDT, WINDIS, WPOSTS16, NOJTAG, NODEBUG, NOIESO

In my main C file, rght at the start, I do this:

setup_wdt( WDT_16S );

Then at some point in time after a pre-determined timer expires, I go in shutdown mode and when I call the shutdown function:

- I disable and clear a bunch of interrupts
- Stop the timers;
- Call <setup_wdt( WDT_OFF );> ;
- Disable perripheral power with <#byte PMDx = getenv("byte:PMDx")> followed by <PMDx = 0xFF> and do this from 1-8
- Then I call <sleep full>;

So far, the thing goes to sleep but automatically reboots after a few seconds and I call the <restart_cause();> function and keeps returning WATCHDOG TIMER.

Benoit

[newguy]

I know that there are some processors with an either/or fuse/no fuse WDT setup. If you use the WDT fuse, you don't have software control of the thing, which explains what you're seeing. You'll have to dig into the data sheet for that processor (or family reference manual) to see if the WDT can be only software controlled. If it can, you'll have to change to NOWDT and then assume software control of it in code.

[Ttelmah]

You always need to read the data sheet.

Like a lot of other chips, if the watchdog is enabled in the fuses, it cannot be turned off.

Page 143 in the sheet. Note at the bottom:

"2: If the FWDTEN Configuration bit is ‘1’ (unprogrammed), the WDT is always enabled, regardless of the SWDTEN bit setting."

To be able to disable the watchdog when you sleep, you need to turn the watchdog _off_ in the fuses. Then at the start of your code, enable it in software. You can then disable it in software before you sleep.

[benoitstjean]

Hmm... So, I changed to this:

.h File:
#fuses HS, PR, NOWDT, WINDIS, WPOSTS16, NOJTAG, NODEBUG, NOIESO

.c file at the start:

[Ttelmah]

Yes.
The reason for the fuse, is that some code has the requirement that the watchdog cannot be turned off. The fuse ensures this. However means you can't use sleep, unless you accept that this will wake from the watchdog.

[benoitstjean]

Hi TTelmah,

So from your reply, the code I posted just above where I disable the WDT fuse and use the setup_wdt( WDT_ON ) and WDT_OFF is the way to go, correct? Anyhow, that's what's currently happening - it works.

Thanks,

Benoit

[Ttelmah]

Yes.

The setup_wdt function controls the SWDTEN (software watchdog enable) bit.
The fuse is the FWDTEN (fuse watchdog enable) bit.

With the fuse on, the software bit does not work. With it off, you have to enable and disable the watchdog when wanted.