Disabling MCLR temporarily "on the run"

Torello · Joined: 29 Sep 2006 Posts: 118

Hi,

The Mclr reset pin can be disabled by a configuration bit. For what I understand (chapter 23.1 18LF4620) it is possible to program the configuration bit by the pic self.

Has anyone done this? I use the Mclr, but at some (short) moments (writing to eeprom, modifying circular buffer pointers etc.), I really don't want the user to (async) reset the mcu. It will corrupt that data. Applying save copies and locking flags is quite a pain in the behind. So temporally disabling the Mclr would be great.

Any jump start sample code much appreciated

Regards,
Edwin

RLScott · Joined: 10 Jul 2007 Posts: 465

Torello · Joined: 29 Sep 2006 Posts: 118

Yep, you're quite right.
But at this moment it is easier to find a software workaround, than modify 200 pcb's hardware.... Wink

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Two remarks:

Configuration bits are self-programmable with some PIC families, e. g. 18F, but have a limited endurance as any flash cells. They are not intended for frequent reprogramming.

In a typical application utilizing self-programming, e.g. with a bootloader, ID locations would be write protected (same as the bootloader area) to achieve fail-safe behaviour.

Torello · Joined: 29 Sep 2006 Posts: 118

Hi Fvm,

You're absolutely right! After one nights sleep, I though of that also. Assuming it would be possible, the pic would program these bits very frequently and wear out the flash.

Conclusion: idea rejected.

Thanks all!

Ttelmah · Guest

In most cases, where you can reprogram these bits, the change _doesn't come into effect till the next restart_.
Why not just program the pin as a normal I/O.
Have a 'heartbeat' tick inside the machine, use this for everything that needs timings, and have it as one 'job', simply read the state of the pin. If the same heartbeat is also used to reset the watchdog, based on a counter being fed from the 'main' code, when the pin is seen, you can simply stop resetting the watchdog. Have this set to some interval like perhaps 10*the heartbeat (so with a 50Hz heartbeat, perhaps a 0.2 second watchdog), and when you stop resetting it, yu will get a hardware restart. A reset 1/5th second after the pin is pushed, is quite fast enough to seem 'immediate' to a user, and since it is software controlled, you can disable it when wanted.

Best Wishes

Torello · Joined: 29 Sep 2006 Posts: 118

Hi Ttelmah,

To give some insight in "though" decisions:
The application I'm working on is fully sealed and waterproof. The user can wake up the device by activating it with a little magnet. A normal switch would not survive (salt) water.
If I would not use the Mclr, I would never be able to get out of a hangup situation (tricky). If I would make two magnetic switches, a wrong "approach" of the user would trigger a reset anyway...
Maybe using an external interrupt as reset line could be an alternative. Athough this will not absolutely aways get you out of a hangup I think.

Thanks,
Edwin

RLScott · Joined: 10 Jul 2007 Posts: 465

Torello · Joined: 29 Sep 2006 Posts: 118

Hi RLScott,

That is exactly what I've implemented. Note, that if one invalid set is detected at startup and the valid one is read, you want to rewrite the eeprom. Be sure to take the invalid set first! (nice pitfall).

Not only eeprom stuff but also a circular buffer (ram) pointers I need to maintain save copies. Pitfall at this is that the backup write pointer, not always exactly points to the last location written. Because you don't know if the last (interrupted) write has been completed. A possible overwrite of this location can happen, creating garbled data. etc, etc

Thanks,
Edwin.