Clipper problem

Ron H

Joined Apr 14, 2005
7,012
Do let us know what is the outcome of this enquiry.

But a side question, the clamp(series R+ESD diodes) only works if there is load drawing some current on the VCC line. Now these uC can all go to SLEEP and consumes less a uA or less current drawn from the VCC bus.

Would forming a clamp with a high series resistor have the possibilities of raising the VCC potential over the allowable limit of the chip?
If there were no other load on the supply, you would be correct. Most positive regulators can't sink current. A battery should work OK.
 

Ron H

Joined Apr 14, 2005
7,012
I searched the Microchip Knowledge Base for AN521, and found this entry:

Source: TICKET 1-81455
Date Created: 6/12/2007
Relevance: 63%
Last Updated: 6/27/2007
Subject: Area: PICmicro Products, Product Group: PIC18F6XJXX, Product: PIC18F65J10

Description: I am using the PIC 18F24J10 in an application which requires zero cross detection of the AC line. I would like to apply your an521 to this application. I plan to use a comparator input (AN0, AN1) instead of a standard I/O pin. Can I still just use a single 5M resistor as the input to comparator input pin? Do you still recommend this method for detecting zero crossing?

Resolution: That method was really just for the PIC16C54, and actually operated the device outside of its specifications. It was possible to get away with it on older devices, and was designed into a number of earlier products (at the users risk) and worked well, which is why the appnote was created about 10 years ago. Please note that the existance of an application note does not guarantee suitability, particularly for other devices.

As a general rule, newer devices do not function well when exposed to such conditions. The electrical specification states you can not go below ground or above Vdd on an I/O line, spec Vih, Vil. That design will as the voltage exceeds Vdd or Vss by a schottky diode drop on either swing. The tiny induced currents will play havoc with the newer parts, particularly the A/D and other analog sections. Since the absolute maximum ratings are not violated, the device is not in danger of permanant damage, but the violation of the operating specifications voids the warranty and risks perfomance issues. I'd not recommend doing this.

There is nothing wrong with using the comparator, or any other pin, just avoid going above Vdd and below ground.
Hopefully, we can put this to bed now.:D
 

Thread Starter

phuzionz

Joined Dec 5, 2008
47
+5V
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Cathode
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Anode
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--series resistor--- signal to be clamped------>
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Cathode
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Anode
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GND
So to say it in short words, never use the internal diodes in the µC, always build a circuit by yourself to be sure.

Is the circuit above good enough to work fine, nothing have to be added?

Jef
 
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