I affirm again - >> ...... ***NEVER**** use direct RS232 input into a >> microprocessor pin via a resistor and assume that the >> processor protection diodes will clamp the voltage >> swing adequately. In such cases you must ALWAYS ensure >> that the voltage never swings outside the processor >> supply range (or, where specified, the maximum limits >> specified by the manufacturer for NORMAL (not absolute >> maximum) operation.). > Nonsense. The input clamp diodes ensure that the input voltage is > not exceeded, as long as the series resistor limits the current within > spec. Here we go again :-) This is a regular topic and one debated with religious fervour. Look in archive for why it is as I say above. I assure you that I am correct and that great sorrow shall fall upon the head of those who disregard this warning. Maybe something along these lines should also be in a FAQ ? I have attached a long explanation that I recently sent to another list. I will also give a short explanation here. Ignore or disbelieve this at your peril: - Microcontrollers have TWO sections in the spec sheets. "Absolute Maximum" ratings and "typical operating conditions". This also applies to essentially ALL other ICs you will ever see. - The absolute maximum specs are those beyond which the device may be destroyed or permanently damaged. Correct operation at these specs (or near them) is NOT GUARANTEED - all that is guaranteed is non-destruction if you don't exceed them. - The typical specs are always contained within the range of the absolute maximum specs. - The specifications which cover operation of the clamping diodes and how much current they can pass and the permissible voltage by which pins can exceed Vcc with the clamping diodes conducting are **ALWAYS** in the "absolute maximum" section of the spec sheet. Proper operation of the processor is NOT guaranteed when the clamping diodes are conducting. [[[It would in fact be possible for a manufacturer to provide input protection diodes that could conduct safely during normal operation but overwhelmingly this is not done. There may indeed be devices where this is done but I am not personally aware of any]]]. - Typical; operation conditions are set such that the clamping diodes NEVER conduct (except, to quibble, at essentially leakage current levels). - The reason that clamping diodes must NEVER be allowed to conduct during normal operation is that current which flows through them enters the IC at points where such currents are not usually expected and can (and does) bias on or off various parts of the IC in totally undefined ways. This can and does happen but in any given situation you MAY get away with it. Something which works on one occasion may misbehave on another. YMWV. Expect your processor to possibly do anything it is capable of doing under these conditions (and a few things it is incapable of as well). *** Competent designers always ensure that during normal operation their designs ALWAYS meet specifications in the typical operating conditions section of the spec sheet. A design in which the body / protection diodes conduct during normal operation is an incompetent design and bad engineering and will bring sorrow upon the heads of its perpetrator. Using protection diodes to clamp pin excursions under certain conditions can be acceptable as long as abnormal circuit operation of any possible nature is acceptable during these conditions. (eg a 1 megohm resistor of suitable ratings between an input and a pin may allow full 230 VAC mains to be temporarily applied to the input. Correct processor operation during such an incident would not be guaranteed. Now to spoil the warning: Experience and a good understanding of electronic principles MAY allow you to violate the above requirements in marginally minor ways with a reasonable expectation of success. Proper operation under such conditions is not guaranteed. (Example: split the input resistor into 2 series resistors and clamp the middle to Vcc with a small si gnal silicon diode such as 1N4148). This nominally violates many pin input typical operation specs but USUALLY works OK in practice. If you understand why it usually does work and why it mightn't then you can decide whether it's OK to do it in your application. That wasn't that short, was it :-) ? regards Russell McMahon "M., for what it's worth, E. (elec)" (The N years experience is worth rather more :-) ) PS: Starting a reply with "Nonsense." is probably inadvisable (unless you are Olin who is so competent that he can almost always get away with it :-) ) ___________ > I want to measure a 12 V pulse with the MSP430 direct. A simple way by > other Controllers like PIC's is to take only a resistor which limit > the current to the protection diodes inside the chip. But are the > MSP320 Inputports protected against over voltage ? What is the maximum > allowed current ? How I can find it in the datasheets ? A better solution is to supply the input voltage via a resistor and clamp the pin to ground with an a zener diode slightly less than Vdd (eg 4v7 when using a 5v Vdd supply). (cathode to pin, anode to ground) OR Again use a series resistor and clamp the pin to supply with a SCHOTTKY diode eg BAT85. (Anode to pin, Cathode to Vdd). This clamps the pin at a level below which the protection diode starts to conduct (because the Schottky diode conducts at a lower forward voltage than the internal silicon diode). *** You should NOT use the internal diodes for pin clamping *** during normal operation BECAUSE - Using the input protection diodes to limit pin voltage during normal operation is asking for major and ill defined problems. This is certainly true for PICs and will also be true for almost all other processors as well. A *careful* reading of the PIC datasheets will reveal that the spec for pin voltages outside supply rails is in the "absolute maximum ratings" section and NOT in the "normal operation" section. ie the processor will survive but is NOT guaranteed to operate properly. As noted in another reply, the relevant data for the MSP430 is also in the "absolute maximum ratings" section. MANY people use the protection diodes in this manner and have (or appear to have) no problems. But unpredictable operation can occur at any time and sometimes does. I have personally seen semi-random processor glitching when this is done with a PIC. The problems occur because, unless specifically designed for this not to happen, the current in the protection diodes flows in relatively arbitrary parts of the IC and may bias junctions on or off in abnormal ways. When this occurs, anything that can happen may happen and sometimes does. Whenever this topic comes up on other lists people almost invariably enthusiastically (attempt to) rebut the above FACTS and claim that it is OK to use protection diodes to limit pin voltages during normal operation. Before agreeing with them respondents should carefully check the data sheets for their favourite processor and see which section the "pin voltage outside supply voltage range" spec occurs. It is possible for manufacturers to provide a system whereby the clamp diodes did not reverse bias the intrinsic substrate diodes in the IC or cause similar problems but few if any do this as it could incur substantial extra cost and the requirement is better addressed by proper circuit design. Russell McMahon -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.