8051F020 series, 5V tolerant input schematic?

John, I don't know the particulars, I was just thrown into the mix to run an LTspice simulation... otherwise I know not much. All I know is that the problem flummoxed the FAE, and the factory has gone silent on us. I can find out more next week when I meet again with the principals. ...Jim Thompson

How do you end up in that position? Is there no way to increase the size of the board when it gets over 90% or so? It is very difficult to route a tightly-packed pcb, so it costs more.

Is the enclosure the limiting factor? If so, is it possible to split the board in two and double the available space? This would increase the chance of coming up with and new idea that could vastly increase the value of the product. But when your mind knows a pcb is 99% full, you automatically discard any new ideas because there is no room.

I am amazed at the number of times you and JL mention you cannot use some solution to a problem because there is not enough room.

surge or

up

rail

surges

it

diodes.

zener

boards.

Perhaps you need to tell them to give you the space you need or got h***.

?-)

Usually through the telephone, "Yo, George, can you help us get this through cert?".

Absolutamente no :-(

No chance, it's an existing and very tight unit. Sometimes they have to be that way, earlier this year I designed something similar but way different market. Even when dropping to 0402 it barely fit.

Comes with the nature of our turf. I design a lot of sensor stuff and that has to live in crammed quarters. Same for aerospace, there you typically get told "We have this much volume, here are the dimensions, and we need something that is xx percent better than what the competition has".

Think about it like a retrofit market. For example, if you designed a new fancy timer/dimmer/whatever, it would still have to fit in the standard North American NEMA box. If it didn't you could not sell it. Now multiply that by 0.1 :-)

or

Not sure what the last word should mean but no, this is a normal situation. I see things like that several times a year.

Very good answer. Thanks!

I've moved away from diodes to the rail. Many regulators can't sink current and there is always chance on injecting noise into the supply lines.

I've used these low leakage zener diodes to improve a design recently:

or

You need a shunt that kicks in if the rail wants to ride up. Only an issue if the total load on the rail has phases where it is very light. Noise isn't an issue because there is usually lots of capacitance on a rail and the diodes are only for major ESD events and such.

Zeners are nice but for many digital circuits the 5% tolerance is too much. Like the 5V6 in the datasheet which at 5mA can be anywhere between

or

we use PESD3V3L4UG quad esd zeners in a sot353 and resistor arrays maybe something like NUF6010MU ?

-Lasse

How about this: series resistor, shunt diode to 3.3V , port pin

There seems to be considerable confusion about what a "5V-tolerant" input looks like, schematic-wise.

Would everyone please post what they think it looks like? ...Jim Thompson

That doesn't protect very well because the parasitic substrate diode will take a substantial portion of the hit. The only thing that works is:

R1 -> shunt diodes -> R2 -> port pin.

R2 can be small because there will only be 1-2V max across it and it only has to make sure the substrate diode gets no more than a few mA.

But ... too many parts in this case.

Oops, sorry, your are right. At a 5V-tolerant pin it would work. Problem is that we only have 5V in the area where the protectors are.

If you only have 5V around driving a 5V-tolerant pin... what are you fretting over? ...Jim Thompson

The concern are surge events that make these lines carry nasty transients, which in turn can lock up or damage the uC.

So you have a "dirty" 5V supply? ...Jim Thompson

No, rock-stable. But clamping against it will lead to up to 7V spike amplitude if it's a huge transient. Those small clamper arrays can easily reach 2V Vf or more for a few usec.

_Where_ are these "huge transients" coming from? ...Jim Thompson

Absolute max "rating" is 7V, but "simple" inputs (no drain or source connected, just gates) can probably take 20V. ...Jim Thompson