Re: How to acheive glich free SSR outputs on the BasicX LCDX module at powerup?

Started by David June 11, 2011
Hack the board.
You can always use the hot-metal glue to put it back the way it was.
DAvid

--- In b..., "ron outous" wrote:
>
> Yes you are correct, in that the 2803 is driven by a 74HC595. I did some
> continuity tests that indicate the /MR is tied to +5vdc and the /OE is tied
> to GND. If only the pcb designer had used an RC network to allow the /OE
> some time to hold the 595 outputs low. Something like a .1uf and a 10k would
> have worked great, holding the 595 outputs inactive for about 700ms. I sure
> there is a more precise way to calculate this, but I just use .7RC. Close
> enough, I think. Anyway, without really hacking the board, I have just put
> Call RelayInitialize() as the first line in Main(). That seems to minimize
> the glitch effects on the 2803 outputs somewhat. What else do you think
> could be done?
>
> _____
>
> From: b... [mailto:b...] On Behalf Of
> David
> Sent: Thursday, June 09, 2011 6:36 AM
> To: b...
> Subject: [BasicX] Re: How to acheive glich free SSR outputs on the BasicX
> LCDX module at powerup?
>
>
> Isn't the 2803 connected through an SPI chip, rather than direct to the AVR.
> On my LCD-serial, the precursor to the LDC+, the 2803 is driven from a
> 74HC595 which has /OE and /MR, are they used?
> DAvid
>
> --- In b... , Tom Becker
> wrote:
> >
> > > ... Is there any way to configure the BX35 so the ULN2803A outputs do
> not sink
> > >on powerup?
> >
> > I think this is caused by a characteristic of AVRs, on which the BX35 is
> built.
> > AVR I/O pins float (high-impedance) during a processor reset, as occurs at
>
> > powerup. My guess is that the 2803 outputs might go low, perhaps weakly,
> while
> > the inputs float high. Had I designed the LCDX I might have put 10k
> pulldown
> > resistors from the 2803 inputs to ground tp prevent that - and that might
> be a
> > solution for you.
> >
> > That said, I switch mechanical relays with an LCDX and I can't say that I
> see
> > this effect on that project; it seems to reset fine without relay
> glitching.
> > Perhaps the 2803 doesn't sink enough current with floating inputs to pull
> the
> > relay coils closed in my case but, in your case, the SSR inputs are
> sufficiently
> > sensitive to glitch. Maybe you can drive the SSRs differently to avoid it,
> too.
> >
> > I/O states when in reset are a proper concern for many projects. Pressing
> Reset
> > in a panic shouldn't cause motors to suddenly run full-blast - or cause
> firework
> > ignitors to light - for instance. Outputs should be designed to adopt the
> safe
> > state when in the reset state.
> >
> >
> > Tom
> >
>
>