Okay, no more ASCII art from me, after seeing what Outlook did to
the tab stops.  Bloody Microsoft.  Where's my bourbon? :-)

Gustaf

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Gustaf J. Barkstrom
> Sent: Friday, February 27, 2004 8:54 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] Release of magic smoke
>
>
> > At 02:45 AM 2/27/2004 -0500, Spehro Pefhany wrote:
> >
> > It's called latch-up and results from current going through the on-chip
> > protection diodes (too much current) which triggers the GIANT PARASITIC
> > SCR which lives inside every PIC, right across the power supply.
>
> Ah, I see.  Thank you for the description.
>
> >
> > >By the way, the circuit worked very well until I turned up the
> > duty-cycle.
> >
> > It was probably marginal.
>
> Sounds like the rest of my life. :)
>
> > I suspect your problem is one of circuit layout. You must not allow
> > motor current to flow between the ground pin on the darlington array
> > and the Vss pin on the PIC.
>
> Ah. Here's what my power and grounding looks like:
>
> -------------------------------------------------------------
> | 5v bus        (big copper pad area)                                   |
> -------------*------------*-----------*---------*-----------*
>                  |                |               |     |               /
>                  |                |               | (zener) |
>       / motor
>            [74244]      [PIC]           [ULN]--|<---*           / coil
>                  |                |               | \
>               /
>                  |                |               |  \------------------*
> ------   ----*------------*-----------*         (to ULN outs)
> |GND *-|<-*      (big copper pad area)    |
> ------   ------------------------------
>
> It seems my circuit does EXACTLY what you prescribe against!
> Laying it out like I did above, I can "feel" the oscillations
> latching-up across the PIC.
>
> The diode to ground is an "anti-idiot" diode to protect
> against inproper polarity hookup to the power supply.  Then
> again, it probably raises ground to something like 0.1v??? That's
> not good if it does...
>
> I guess I should ask the group: are beginners of my sort welcome here?
>
> >IOW if you think of a "star" connection
> > at the ground of the power supply, one point goes off to the PIC
> > and one goes off to the ULNxxxx ground. You do NOT ground the PIC
> > to the supply and run the connection from there to the ULNxxx (although
> > the opposite would be okay). There should be a BFC (electrolytic)
> > paralleled with a 0.1 ~ 1uF ceramic close to the PIC and the motor
> > from ground to Vdd/5V. 1000uF 6.3V low impedance (like you
> > find on a PC motherboard) works fine.
>
> Okay, so drawing this again, I get the following???
>
> -------------------------------------------------------------
> | 5v bus        (big copper pad area)                                   |
> ---*---------*----*-----*-------------*---------*-----------*
>    |             |      |       |                 |     |               /
>    |             |      |     |           | (zener) |           / motor
> [74244]    [PIC]        -     -         [ULN]--|<---*           / coil
>     \____    |  -.1uF   - 1000uF          | \                           /
>            |   |        |     |           |  \------------------*
> ------  -*---*----*-----*-------------*         (to ULN outs)
> |GND *-|<-*      (big copper pad area)    |
> ------  -------------------------------
>
> And, now for a new, improved design (I think):
>
> ----------------------------------    -----------------------
> | Logic Vdd plane. (+5v)         |    | Motor V+ plane.     |
> ---*---------*----*-----*---------    *---------*-----------*
>    |             |      |       |                 |     |               /
>    |             |      |     |           | (zener) |           / motor
> [74244]    [PIC]        -     -         [ULN]--|<---*           / coil
>     \____    |  -.1uF   - 1000uF          | \______________     /
>            |   |        |     |           |                \----*
> ---------*---*----*-----*---------        *----------------------
> | Logic GND plane.               |    | Motor ground plane. |
> ----------------------------------    -----------------------
>
>
> > BTW, you might want to put a 1K on each PIC output. Yes, there's a
> > resistor inside the ULNxxx, but trust me. There's also an isolation
> > junction diode to the substrate...
>
> I didn't draw them, but I will put the 1k resistors in as well.
>
> <gripe> Why on earth do they draw the isolation diodes with dotted
> connection lines on the datasheet? </gripe>
>
> > The zener on the catch diodes is just fine, provided it doesn't
> > get too warm.
>
> Is a catch diode a different term for "free wheeling" diode?
> I saw the term "free wheeling diodes" on the datasheet, so I'm
> just wondering.
>
> I can't thank you enough!
>
> Gustaf
>
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