Protecting PIC Inputs

srnet

Senior Member
Well, I have seen that before.

Whilst the point about the clamp doides is well made, and may well be true, by implication that only appiles to pins with analogue capabilty, which is obviously not the case for all PIC pins.

Yet the datasheets apply the 0.3V limit to all pins. So if the application note is correct in its details, then the datasheets are missleading at best.
 

Armp

Senior Member
As a circuit designer for many years I just have to chuckle when customers think they know better than the manufacturer about critical things like overvoltage specs! They probably don't ever change the oil in their cars either.

It is SO EASY to meet the specs, why not do it? Must be a 'cultural' thing I guess....
 

mrburnette

Senior Member
<...>
Yet the datasheets apply the 0.3V limit to all pins. So if the application note is correct in its details, then the datasheets are missleading at best.
The way I read the sheet, Microchip acknowledges that the 0.3V only applies to Analog pins, but states they are using this figure for everything rather than separate 0.3 and 0.7 because silicon evolution is making more use of analog inputs. Pretty hard to be misleading when they specifically state they are misleading.

- Ray
 

srnet

Senior Member
I'm not sure how you read that but maybe you should contact M'chip with your observations.
"Choose Your Pins Wisely
If the design simply does not allow the extra
components, then choosing pins with no analog
features is the best choice for overvoltage. Purely
digital pins do not have pass-gates, therefore they are
not subject to the same constraints. "
 

srnet

Senior Member
From what I can see, of the '2' PICAXEs, only the 18M2 uses an analogue capable pin for serial in, all the others are digital only pins.

A PICAXe programmed from a standard serial lead or a AXE027 with a lower volts powered PICAXE (3V for instance) appears to rely on the clamping diodes for the standard download circuit to work, is this so ?

Was it therefore by design from Rev Ed, putting serial in (mostly) on digital only pins so as to avoind the problem described in the Microchip application note ?
 
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