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8051F020 series, 5V tolerant input schematic?

If you are dealing with 5V logic signals, isn't the threshold VCC/2, or around 2.5V?

How about clamping with a 3.9V zener to ground with a small series resistor to limit the transient current? As long as the desired signal gets above the threshold, do you care if it goes all the way to 5V?

For a BZX84C3V9, Fig. 2 on page 3 shows it doesn't start to conduct until you get above 3V, and 50mA doesn't even reach 5V:

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Don't know what all the tolerances are, but so far nothing else seems to have any chance of working in your application.

JK

Reply to
John K
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I still haven't seen described how a "rock solid" 5V supply produces such gross transients.

As for thresholds... a 3.3V logic block, with 5V tolerant inputs, will threshold roughly at VDD/2, or 3.3V/2, or 1.65V. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

The usual, through outside cabling, induced by heavy machinery being turned on et cetera. Unit is plastic (has to be), a decent ground structure is difficult to impossible.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

It doesn't. It's the signal lines that do.

[...] 
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

So "induced" transients? How much actual energy? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Hard to say how much arrives, all I know from reports is that it has frozen the uC at times. So we had to do something about it. We should be good with those clamps. I was just wondering how far things can go above abs max and for how long. Of course we won't be sending in 7V for hours. When I redesign stuff I like to make it not just good enoug to pass a test, but ideally almost bullet-proof.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

The 7V limit only applies to those pins which are configurable I/O. If it's an input-only pin, it can stand a whole lot more... some processes can be as high as 50V.

But you said uC... they're probably ALL I/O :-( ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

They state a 5.8V abs max, 7V is just what one would typically see if a monster pulse comes along and leans into the clamp diodes.

Yup, all I/O :-(

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

If you can get a handle on the energy involved (*), I can show you some simple clamps that are reliable and tight on the limits.

(*) Just patch in a diode to +5V and current probe the data line. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

I could design some clamps that are more precise but there is so little space that just adding the teeny clamp arrays was already a challenge. Also, it's thousands of miles away.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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