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uC detect input from 13VDC

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Here is what MicroChip says on one of their data sheets. Note that the implication is that the device will not latchUp if the current is limited to +-20mA, however, on another line it says that the pin must be constrained to +0.3V above and below Vdd and Vss, respectively. How can you obey both of these constraints simultaneously?

By the presence of a simple series resistor, when the CLAMPING diodes (NOT ESD diodes) will provide any required protection. MicroChip even stick direct mains in via a suitable series resistor, for zero crossing detection etc.

I would also suggest you take datasheets with a 'pinch of salt', as it's rare to get one that isn't full of errors.
 
Could always do something like this with single shottky. Provided your input can sink a little current.
You mean to enable the internal pull-up. The forward drop of a Schottky will add about 0.3V to the Vcesat of the external driver, which is likely ok to be detected as a zero with some noise margin. If Vcesat is >0.5V and if the diode is Si with a forward drop of 0.65V, the combined Vlow doesn't leave much noise margin...
 
Nigel, did you even read the two Microchip App notes linked to by Pete?
https://ww1.microchip.com/downloads/en/AppNotes/93013a.pdf
and
https://ww1.microchip.com/downloads/en/AppNotes/93009A.pdf

They effectively say: We told you it was ok, but now we are telling you not to do it.

The protection diodes serve for both ESD protection and clamping the voltage if current is injected into a CMOS input. The ESD test is done with the chip unpowered, so LatchUp is not an issue.

**broken link removed**

The simulated "whole body ESD 5000zap" just cant blow any aluminum or blow any gates.
1167Fig07.gif


There is a separate go-no acceptance test to see if the chip latches while powered and some amount of current is injected. IC makers usually do not spec that number on their data sheets. Back when I was testing ICs, we used +-5mA as a go/nogo test.
 
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I guess this all boils down to:

1 If you want a simple design, using minimum parts count, for a device which is to be used in a non-critical application, then just use the series resistor and the internal ESD protection diodes.

2 If you need something for a critical application, where reliability trumps component cost then use a resistive divider and maybe a zener diode from the input pin to 0v.
Or, maybe use an opto-isolator.

This debate is the sort of thing which would take place in a "design review meeting" where a product was being designed.
The advantages and dis-advantages of each approach would be considered and compared with the requirements specification of the end product.
It is called "engineering".

JimB
 
You mean to enable the internal pull-up. The forward drop of a Schottky will add about 0.3V to the Vcesat of the external driver, which is likely ok to be detected as a zero with some noise margin. If Vcesat is >0.5V and if the diode is Si with a forward drop of 0.65V, the combined Vlow doesn't leave much noise margin...
Yeah, it will be close, just a suggestion that perhaps can be considered.

Obviously worse if the output is open collector, but is that the case?
 
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