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

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

Reply to
An Schwob in USA
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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.

Reply to
Jim Stewart

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

Reply to
Dan

On 7 Feb 2005 16:37:53 -0800, "An Schwob in USA" wrote in comp.arch.embedded:

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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Reply to
Jack Klein

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 Kle> On 7 Feb 2005 16:37:53 -0800, "An Schwob in USA"

speed

oscillator

market,

coprocessor

status

Reply to
An Schwob in USA

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

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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 > Our design needs to have a timed reset (shorter!) to get up to speed

Reply to
Peter Jakacki

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.

Reply to
Raivo Leini

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

Reply to
Jim Granville

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