Hi All, I enconuter a big problem, when I insert a SmartMedia memory card to my system, my system power will be shifted, the VCC will be shit down to ground abruptly and then reutrn to VCC, the ground is doing the same thing, from 0V to -5V.my system ( embedded system ) will crash after that, in my case, it will auto reset ( restart) when I insert SmartMedia card, but it will not happen every time, but it happens.I main chip is running 5V, other components is running at 3.3V.Could you please give me any hints?Thank you very much!Best regards,F.Y.

What are these voltages measured relative too? Where is the groundlead of your scope probe connected? If part of your ground net (withthe probe tip) is reading -5v relative to another part (with the groundclip) you have a serious problem! Are there even any negative powersupplies in the system?If it were only the VCC dropping, it would seem like an inrush currentissue. I'd try another smartmedia card if one were available. Alsowould considering putting together something to power up the card withno signals connected, with a small series resitor in the ground lead toit, and measure the voltage across that resitor, hence the startupcurrent. You should expect some startup current, which would besupplied by capacitors located close to the card socket, but itshouldn't be huge.But what you are seeing in the ground suggests something moresystematic is wrong with the circuit or your measurement setup.

SmartMedia Card will crash my system ?

Jun 25, 2005 23 Replies

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Boki 21 years ago

I distributed the decoupling capacitance in every chips' VCC and GND..., is that correct?

4 layer.

Yes, I have dedicated ground and power planes.

3.3 and 5 V I/O tolerant is good point, but... ... does it possible no tolerant between 3.3V and 5V? I remember we have to apply more then 10V or 15V to make MOSFETs breakdown, isn't it?

Where? close to CPU and DRAM. Yes, a large decoupling capactitor across the supply rails, but I think due to the complicated GND, we still have to add some L to critical components, right?

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Pooh Bear 21 years ago

In its current state I suspect it isn't actually !

I think you may have a basic problem understanding how power supplies need to be capable of high peak currents when powering digital circuits.

How much 'decoupling' capacitance do you have on your pcb ? How is it distributed ?

How many layers on the pcb btw ? Do you have any dedicated ground and power planes. What's the system clock frequency ?

If you have mixed 3.3 and 5 volts logic you *have* to use parts that are 'mixed voltage I/O tolerant' where appropriate..

Where the memory card plugs in, you do have a large decoupling capacitor across the supply rails don't you ?

Graham

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Pooh Bear 21 years ago

You have 1 per IC ? That's good. Multilayer ceramic presumably ?

Good.

It doesn't work like that Boki ! IC's have 'parasitic paths' that can cause breakdown problems if any pin exceeds ( or falls below ) substrate potential by about 0.5 V. It all depends on the precise situation. Indeed I just expereinced such a problem myself - although in very different circumstances and for a very unexpected and subtle reason. I had a chip going 'bang' on power-down. Took me a while to realise why. It was indeed a substrate reverse-bias problem caused by a parasitic current path through another IC !

What's complicated about it. That sounds like bad news ! This may be a clue. And you need a big cap right next to the memory card socket.

Very carefully and only ever in critical locations where you're *totally sure* of why you're doing it. And not in the ground path ever in this kind of application.

Graham

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<bokiteam 21 years ago

Vbs ( body , source ) ++, Vth -- Vth --, Id ++

Very good point, I will remember that!

Thanks a lot, Graham!

Best regards, Boki.

"Pooh Bear" ???????: snipped-for-privacy@hotmail.com...

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