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Circuit fails when power gradually drops

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 17 June, 2008 » Circuit fails when power gradually drops « Previous Next »

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Diode Speedzeitghost13/06/08  08:39 am
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amr_bekhit
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Username: amr_bekhit

Post Number: 367
Registered: 06-2005


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Posted on Friday, 06 June, 2008 - 04:50 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

I recently purchased an RFID security unit (RFID reader and keypad) that can be used as an electronic lock. While experimenting with the unit, I noticed that the unit seems to wipe/garble it's memory if the power supply voltage slowly increases to its nominal value. However, if I instantneously apply the supply voltage the unit continues to operate correctly. The unit is mounted in a lorry and so runs off the battery (through a 24-12V DC-DC converter).

I'm thinking of maybe using an uninterruptible power supply or a battery/charger combo to keep the unit running in the short term. But I was wondering if anyone knew why a circuit could fail like that?

Thanks

--Amr
Helm PCB - My personal site.
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amr_bekhit
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Username: amr_bekhit

Post Number: 368
Registered: 06-2005


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Posted on Friday, 06 June, 2008 - 04:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just after posting, I had an idea as to why it might fail: say the micro on the board runs at min 2V and the EEPROM runs at min 4V, then the micro starts working before the EEPROM and tries to access it and this causes the EEPROM to be garbled somehow??

--Amr
Helm PCB - My personal site.
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chippie
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Username: chippie

Post Number: 144
Registered: 11-2005


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Posted on Friday, 06 June, 2008 - 07:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If the micro is starting before the power has come up for the eeprom, is there a write cycle to the eeprom that's causing the corruption or is it that the data read from the epprom is being misinterpreted by the micro?
eccentric millionaire financed by 'er indoors
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mikehibbett
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Post Number: 798
Registered: 04-2005

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Posted on Friday, 06 June, 2008 - 07:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

That's what the reset input is for Amr. Just extend your reset time, to ensure the power supply is in range first.

Mike
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amr_bekhit
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Username: amr_bekhit

Post Number: 369
Registered: 06-2005


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Posted on Friday, 06 June, 2008 - 08:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the replies - I didn't design the unit, it was purchased but has problems working. I was wondering what you guys thought about it, since I've never had this problem in my own circuits.

I'm thinking of adding a small circuit before the RFID unit to monitor the power supply and switch off/on the unit depending on how stable the voltage is.

--Amr
Helm PCB - My personal site.
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zeitghost
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Username: zeitghost

Post Number: 1159
Registered: 01-2006

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Posted on Friday, 06 June, 2008 - 09:19 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You may find that a reset generator chip will come in handy... you can get these in a wide variety of voltage levels... Maxim do a shedload.

I worked on a radio telemetry modem that would quite happily refuse to reset with the rs232 connected because the max232 managed to work backwards and keep the Vcc above 2V or so...

A 4V reset chip fixed it.
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amr_bekhit
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Username: amr_bekhit

Post Number: 370
Registered: 06-2005


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Posted on Friday, 06 June, 2008 - 10:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hmm..I've never heard of them before, they sound interesting. In fact, it looks like these reset chips do exactly what I was going to build myself, so they look like a viable solution. Thanks for pointing them out, I'll have a look at them.

--Amr
Helm PCB - My personal site.
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sounded_simple
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Username: sounded_simple

Post Number: 330
Registered: 12-2005

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Posted on Monday, 09 June, 2008 - 10:05 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Yeah sounds like a standard brownout problem, older PICs were prone to it.

Check out microchips MCP100 range.
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grab
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Username: grab

Post Number: 750
Registered: 05-2005


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Posted on Monday, 09 June, 2008 - 10:07 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I don't usually bother with a special reset chip. I've usually got a spare gate somewhere on the circuit, so feed that with a resistor-capacitor (to put on a small delay after the power's come up) and you get a fine reset signal.

It's not so clever on power-down, because it keeps things going even when the power's vanishing. If you find you have problems on power-down, you can put a reverse-biased diode across the capacitor. That'll discharge the capacitor immediately when you lose power. At this point though you're starting to get a bit more technical, so a proper reset chip starts looking more promising. But usually it's only on power-up that you particularly want a synchronised reset of everything.

Graham.
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zeitghost
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Username: zeitghost

Post Number: 1161
Registered: 01-2006

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Posted on Monday, 09 June, 2008 - 11:46 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

http://www.maxim-ic.com/products/supervisors/

They're as simple or as complex as you like... :o)
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amr_bekhit
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Username: amr_bekhit

Post Number: 371
Registered: 06-2005


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Posted on Tuesday, 10 June, 2008 - 09:13 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the replies chaps,

I ended up going with the MCP101 - a small 3 pin TO-92 chip, very simple and does the job effectively. The security unit now works fine with the power supply.

Thanks for your advice!

--Amr
Helm PCB - My personal site.

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