LVD required BROWNOUT fuse?

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

Hi all,
I am playing with the low voltage detect feature of the PIC18F452, it seems that to get the LVD to work I need the BROWNOUT fuse set. I can't see why this is as the data sheet doesn't mention it. I want to be as tight as possible with current, at the moment I have 50uA in sleep mode with the BROWNOUT fuse. If I set the NOBROWNOUT fuse the PIC won't enter sleep at all...any ideas?

Keep well,

Will

Ttelmah · Guest

You realise that the LVD interrupt will wake the processor from sleep?. Hence if you are sending it to sleep without disabling LVD, it'll wake up again.
With the brownout reset enabled, the processor will go into a shutdown mode if the voltage drops below the brownout voltage, not 'sleep'.
The point about LVD, is it allows you to execute code _before_ the procesor stops. You could (for instance), have an interrupt routine, which then turns off the voltage reference circuit (this is drawing quite a lot of your 50uA), disabled the LVD interrupt, and puts the processor to sleep. However remember you then have the responsibility to wake the chip up again, when the power is restored.
Have a look at document 70056b on the Microchip site, which describes how to use the LVD.

Best Wishes

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

I disable the interrupt and set LVDEN to 0 before issuing sleep(); what is strange is that if BROWNOUT fuse is set everything works perfectly, however if I use NOBROWNOUT the pic never goes to sleep (or maybe it does then as you say wakes up instantly). The only interrupts I have enabled during sleep are INT_RB for the buttons. I don't use the WDT at all. I'm off to read that document. Cheers,

Will

Ttelmah · Guest

Key thing. You need to clear the interrupt 'flag', as well as just disabling the interrupt (do this after disabling the module). Basically, if it has triggered in the past (even during power up), the flag will already be set. Wake up, does not require the interrupt to occur, just the flag to be set... :-)
LVD does work without brownout enabled on the chip (I use it).

Best Wishes