Using a small 8-bit micro (LPC900) as external reset / watchdog device!?

"Jim Granville" <no.spam@designtools.co.nz> wrote in message
news:4209542a$1@clear.net.nz...
>   Note that to replace a Vdd Monitor device, you need a DEFINED reset
> drive, from below 1V Vdd. - to get that in a uC, you need one that
> drives the Reset pin and has a data sheet defined state - eg the
> C8051F30x series state 0.6V is /RST force threshold, whilst AVRs do not
> drive the RST pin.
>   The ideal WDOG_uC would
> ** have a defined Reset Out for < 1V Vcc
> ** Have a Brown Out detector that defaulted ON
> ** Have an adjustment range on the BOD threshold
> ** Draw under 1uA, with BOD alive.
>
>   I have not seen one that meets all of the ideals....

I don't know the PICs well, but that was also my concern. The one project I
was involved in with PICs had brown-out troubles...

For something as important as reset (and hardware watchdoggery), I have
confidence in dumb, essentially analog/mixed devices whose job it is to
monitor that the rail is within specs (e.g. 5V +/-5%) and to hold the CPU in
reset if not. Nothing else. I don't mind (too much) paying a premium for
such a vital function. I don't currently have that same confidence in e.g.
PICs.

The MAX1232 is my old staple part; 50uA operating current (typ.) is fine for
my purposes.

YMMV.

Steve
http://www.fivetrees.com

Steve at fivetrees wrote:
> "Jim Granville" <no.spam@designtools.co.nz> wrote in message
> news:4209542a$1@clear.net.nz...
> >   Note that to replace a Vdd Monitor device, you need a DEFINED
reset
> > drive, from below 1V Vdd. - to get that in a uC, you need one that
> > drives the Reset pin and has a data sheet defined state - eg the
> > C8051F30x series state 0.6V is /RST force threshold, whilst AVRs do
not
> > drive the RST pin.
> >   The ideal WDOG_uC would
> > ** have a defined Reset Out for < 1V Vcc
> > ** Have a Brown Out detector that defaulted ON
> > ** Have an adjustment range on the BOD threshold
> > ** Draw under 1uA, with BOD alive.
> >
> >   I have not seen one that meets all of the ideals....
>
> I don't know the PICs well, but that was also my concern. The one
project I
> was involved in with PICs had brown-out troubles...
>
> For something as important as reset (and hardware watchdoggery), I
have
> confidence in dumb, essentially analog/mixed devices whose job it is
to
> monitor that the rail is within specs (e.g. 5V +/-5%) and to hold the
CPU in
> reset if not. Nothing else. I don't mind (too much) paying a premium
for
> such a vital function. I don't currently have that same confidence in
e.g.
> PICs.
>
> The MAX1232 is my old staple part; 50uA operating current (typ.) is
fine for
> my purposes.
>
> YMMV.
>
> Steve
> http://www.fivetrees.com


Steve and Jim,

I did not think about the brownout of the small micro because I was
planing to use a battery backup for that one. Basically the small micro
would switch to battery when power fails. To monitor the main power,
the comparator with its own reference can come in very handy. During
active power on, the small micro would act as watchdog / critical data
storage and during power off it would be buffered. Large CPUs in new
processes have power dowm currents that are enough to operated a small
micro while still being active.
The LPC900 series offers an EEPROM functionality with byte writes and
can be used when power gets turned off to save critical process data
from RAM to flash. Then it can go into power down itself, draining the
battery probably less than the self discharge.
During all your inputs, the ideas have evolved and I can see now that
more functionality on the micro really adds value while watchdog/reset
just by itself is probably better handled by a low end reset circuit.

Thanks everybody for the knowledge you are willing to share!  

An Schow

> > If not all functionas are needed, a smaller AVR can easily be used.
> > If a small LCD controller is needed an ATmega169 can be used etc.
> >
> > I think that conservatism is the answer in many cases.
>
>   Note that to replace a Vdd Monitor device, you need a DEFINED reset
> drive, from below 1V Vdd. - to get that in a uC, you need one that
> drives the Reset pin and has a data sheet defined state - eg the
> C8051F30x series state 0.6V is /RST force threshold, whilst AVRs do not
> drive the RST pin.

Nope, not neccessary,
Connect a pulldown resistor to any I/O pin.
When the AVR get brownout, then it is reset.
At reset, the I/O pin becomes an input, and the pulldown resistor will
activate the reset to the main CPU.
When the AVR thinks it is time to pull the main CPU out of reset, it defines
the I/O port as an output and drives it high
So far, none that I spoken to has complained about this.


>   The ideal WDOG_uC would                                        AVR.
> ** have a defined Reset Out for < 1V Vcc                  - Pull down
resistor fixes this...
> ** Have a Brown Out detector that defaulted ON       - Yep, fuse option
> ** Have an adjustment range on the BOD threshold    - The newer AVRs have
64 levels. only three visible
> ** Draw under 1uA, with BOD alive.                          - Would be
real nice. now it is more like 10 uA.
>
>   I have not seen one that meets all of the ideals....
>

-- 
Best Regards
Ulf at atmel dot com
These comments are intended to be my own opinion and they
may, or may not be shared by my employer, Atmel Sweden.