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

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Our design needs to have a timed reset (shorter!) to get up to speed
faster. We use a 32-bit CPU with external crystals and the required
time to keep the device in reset is min. 10 ms. If I use a
RC-combination to generate the reset pulse and include the factory
tolerance my typical reset low time will be much longer than 10 ms.

External supervisory circuits are more expensive and a lot less
versatile than low end micros. If the micro has an internal oscillator
and a defined startup time, it is very easy to control the reset
timing. It is also very easy use the device as watchdog with
programable actions. This watchdog could even try to establish
communication with the larger micro before it generates a reset.
The LPC903 or the LPC904 with ADC seem to be perfect.

With all these benefits and still so many reset devices on the market,
I must have missed some important disadvantages of such a
configuration.

For one the supervisor circuit uses less power than a micro but the
micro is still less than 100 uAs active.
What else am I missing?

Your feedback is appreciated, Schwob


Re: Using a small 8-bit micro (LPC900) as external reset / watchdog device!?
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We pay about $.40 for a 5-pin reset circuit.  We've
also done a watchdog/reset/power sequencer based on
a PIC.  The PIC will work fine if you need to do a
lot of specialized things, but the cost will be higher
after you include development, part cost, programming
and board space.

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


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You also have to go to the extra step of programming the micro.  The
reset chip is simply soldered into place and it works.

Dan



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

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right, but you can get from micro much more, like I2C slave RTC,ADC, battery
powered RAM, EEPROM , software protection key and Reset device for more
powerful (ARM,MIPS) system.

r.



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Re: Using a small 8-bit micro (LPC900) as external reset / watchdog device!?
wrote in comp.arch.embedded:

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I've used an approach like this for ten years, but I wouldn't use it
just for reset and watchdog.  You still need a power supply monitor,
and perhaps an external reset circuit anyway, in any safety critical
or high reliability.

It can be extremely useful to put a secondary microcontroller as an
intelligent slave on a board.  I used an 8051 deritave with a 486 a
long time ago, and our current architecture uses a 16-bit HS12 from
Motorola as a slave/IO coprocessor to an ARM 9.

But in addition to the programmable watchdog function, the coprocessor
handles relatively low speed tasks to spare the main processor
overhead, such as running an A/D converter, debouncing switch and
mechanical sensor inputs, controlling LED display outputs.

On our newest product, the hand controllers are plug-and-play over a
low speed CAN interface,  The coprocessor sends out a broadcast status
request four times a second, assembles all the replies from however
many are connected (there are multiple ports), and sends them to the
main processor in a single 115k baud packet using the serial protocol
we developed.

It is also extremely handy for the board assembly house to
troubleshoot boards, and locking the board -- and the whole system --
into a fail safe state if something goes wrong.

The typical 32-bit processor/controller requires a large number of
devices and board traces to be working to do anything at all.  The
secondary micro uses no external memory and really needs nothing but
power and its oscillator.

--
Jack Klein
Home: http://JK-Technology.Com
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Re: Using a small 8-bit micro (LPC900) as external reset / watchdog device!?
Hello Jack,

great ideas. Watchdog and Reset are probably not worth the while but
adding some sanity check functionality is really excellent.
In fact I have seen hot plug functionality with CAN in the past as well
but the really cheap micros don't offer this yet. The HC12 is a little
too expensive for what I am looking at, the ARM7 device is competing in
price with many of the HC12s. Looked at PIC but the 10-series is just a
toy, the PIC12F is somewhat similar in functionality to the LPC904 but
does not offer the same serial features or the same great internal
oscillator. The smallest AVRs seem more expensive.

In fact your approach could be best implemented with the LPC916,
multiple serial interfaces, 2k flash and an ADC and I got quotes
definitely below $1. Could offload the CPU quite a bit for serial
communication and some low end analog inputs.

Thank you for your great feedback!

An Schwob

Jack Klein wrote:
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speed
oscillator
market,
coprocessor
status


Re: Using a small 8-bit micro (LPC900) as external reset / watchdog device!?
That's funny, I just answered a similar question this morning on LPC2000
groups. Someone had mentioned they used a TPS3306-20-Q1 but when I
checked the TI 1K price it was over a $1. For years I have used little
PIC12C509s for reset and ISP control etc, what flash micros don't need
some easy kind of ISP control? The newer devices such as the LPC900s or
PIC12Fs are even better and cheaper. To save double-talk I'll just
repost my previous message.

REPOST from http://groups.yahoo.com/group/lpc2000 /

The problem with reset chips is that they are overly expensive (why?)
and customized. I find it cheaper to employ an 8-pin micro
(PIC12F,LPC900)complete with power-on timer/brown-out detect etc. I can
configure these for my reset threshold, reset time etc. These micros are
typically stand-alone, not requiring any external circuitry such as
crystal etc to operate.

PLUS!, there is always the bonus of having it monitor the coms line and
being able to reset or ISP the main processor as well as being able to
operate as an independant configurable watchdog.

Summary:
Configurable intelligent watchdog/reset/monitor *costs less* than a
fixed reset chip.

my2cents
*Peter*


-- An Schwob in USA wrote:

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Re: Using a small 8-bit micro (LPC900) as external reset / watchdog device!?
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I'd look closely at the Power On reset, or Vpor, on the uC, if you
want to use it as a WDOG/RST. The SiLabs C8051F305 etc family might
be a better solution, as they have a more robust and precise BOD and
/RST pin-drive handling.

-jg


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