[Opendispense] Latest additions

Harry McNally harrymc@ucc.gu.uwa.edu.au
Mon Aug 4 02:28:11 2003

On Mon, 4 Aug 2003 01:05:10 +0800 dagobah@ucc.gu.uwa.edu.au (Bernard Blackham) wrote:

> On Sun, Aug 03, 2003 at 10:32:51PM +0800, Harry McNally wrote:
> > > There's also another circuit attached to PE3 which puts the pin high if
> > > the 24VUN (unregulated?) drops below 20ish volts. Still tracking down
> > > where they're used. I think I'm currently on a roll with the coin mech :)
> > 
> > I think this may sense that mains power has been lost. The switch
> > mode regulator that provides 5 volts will hold up for a while as the
> > 24VUN collapses so the micro may have time to finish any nonvolatile
> > RAM writes and maintain system sanity for when power returns. I
> > was chatting to Nick about using that for our own system preservation
> > on power fail.
> The only reference I can find to the PE3 pin with this circuitry on it
> is in the following snippet of assembly in the main loop, which it seems
> holds the reset line of the changer high until 24V power is restored.
> Perhaps it's a power-saving technique? Or stops the changer from
> dropping out coins just because the powers flaky?

I'd say the latter Bernard. If we have/can get the datasheet for
the coin mech it may specify holding off operation below a 
certain supply voltage.

I'm surprised that is the only reference. The Dallas battery
backed RAM will cease responding to control signals when
Vcc drops below say 4.5 volts (or so) but I would have thought
some protection would be needed from partial updates of
the sales and price data in the RAM. There you go.


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