Hysteresis on dsPIC ports?

Obviously somewhere between zero and 0.6*Vdd. ;-)

They've historically maintained a minimum of 0.15*Vdd on the earlier 16C chips. So likely typical is somewhere well in between (0.3*Vdd?). Really, there are no guarantees, since it's not in the datasheet, and your best bet is to measure a few if you want a good idea of the current typical.

Or use the comparator if it's important that it be close to some specific value. CMOS ST chips are almost as bad- hysteresis has a huge range before it hits the data sheet limits.

Best regards, Spehro

Measure it! Then publish and take *all* of the credit.

Hand a board a copy of the email results to Dave the EE blog guy, Sit back and watch the fireworks.

Steve

Well, that will give use the value for 'that' particular part. What would be typical, and what would be guaranteed may be two VASTLY different things. Just look at the spec sheet for a 74C14 and see. If you design for the typical, you will be bit on the ass when you get that run of non-typical parts.

Oh yeah, he'd tear them apart in mid-air :-)

"Sir, why are this quarter's results so dismal?" ... "Well, we have been daved."

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

I don't have one here and the client hasn't built it yet. Plus we'd need guaranteed limits, not just from one wafer. MicroChip called it "unpublished electrical characteristics". Pathetic. Taught me a lesson about these products.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

My suspicion is that PICs are so popular these days that their "support" department is the same as you get with most software companies -- the 1st tier are essentially just there to read the data sheet back to you, since the answers to a very large percentage of support requests are found right there.

Of course, they should have automatically escalated your request to the next tier when it became clear they didn't know the answer to your specific question. Someone's making a judgement call that that spec isn't very important, I'd guess, which is poor.

---Joel

On a sunny day (Tue, 26 Jan 2010 08:59:56 -0800) it happened Joerg wrote in :

Bull, there is a large difference between 'Schmitt characteristics' and 'is a Schmitt trigger input'. It means in fact that you *cannot* use it as a Schmitt trigger input, but that you *should expect* a Schmitt trigger like behaviour when driving it with some ramp. I am not saying Microchip is all in the clear here, but *unless* it is specified as a Schmitt trigger input, you cannot assume so. It is simply nice that they bothered to point out that gate's behaviour, they did not have to. LOL

Blowing off a client with a remark along the lines of "the sales guys will dig into that if your sales volume is deemed worthy" is, ahem, not a very polite way to deal with their most important part of biz, the sales channel.

Yep. So far I haven't used PICs in any of my "from scratch" designs. And I guess after this episode I won't. I need parts that are characterized a bit more thorough than those.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

with some ramp.

specified

Nope. Quote from the dsPIC datasheet: "All I/O input ports feature Schmitt Trigger inputs ..."

did

Sorry, for my line of work that is not good enough. How come Texas Instruments is able to diligently specify the guaranteed min-max values? See page 24:

_That's_ how a good datasheet is written.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

On a sunny day (Tue, 26 Jan 2010 10:05:14 -0800) it happened Joerg wrote in :

did

I remember reading a datasheet of the new PICs I have, it clearly shows min and max levels for the logical inputs. What more do you want? ftp://panteltje.com/pub/pic_io.jpg

Note that this is marked 'preliminary', and is about the 18F14K22 that I just got. I am pretty sure most of these PICs use the same silicon solution. There must exist some datasheet for your type of PIC, This is simply under DC characteristics.

they did

This is what I want and what other mfgs provide: A min-max guaranteed _spec_ for the hysteresis.

got.

It isn't, see page 263 (attention readers on slow connections, >4MB file):

All they tell you is in which range your high and low levels must be, no hysteresis data. When I asked about it they didn't know. In mission critical apps noise immunity is sort of important and without guaranteed specs you can't guarantee your design. No dice with me there, that's a clear red flag for me.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

On a sunny day (Tue, 26 Jan 2010 10:28:05 -0800) it happened Joerg wrote in :

No, that datasheet says: if the level is above .8 * Vdd then it will be a one, and if below .2 * Vdd it will be a zero. So you have < .2 * Vdd noise margin from ground and .8 * Vdd noise margin from Vdd.

From that follows the max noise you can have on the supply line and ground. I do not see your problem with this spec.

Vdd.

Unfortunately that is not what such a spec says. It essentially says that they don't guarantee anything if you are between 0.2*VDD and

0.8*VDD. That has nothing to do with hysteresis. And most definitely not with noise margin.

Well, the guy at Microchip saw it but could not help.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

from Vdd.

Maybe do as others here have done, write an E-mail directly to Steve Sanghi ?:-)

It's been a couple of years since I did some oscillator design work for them. Some of those people (I believe Don Gerber, for sure) are still there. Tell me what part and what pin spec is your problem? I'll ask privately. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

On a sunny day (Tue, 26 Jan 2010 11:17:17 -0800) it happened Joerg wrote in :

from Vdd.

No, you do not interpret this right. It says the input line noise can peak, for a 5V supply, when input is high, to Vdd .2 x 5 = 1 V below Vdd, so to >4V, and the noise on the input can peak to < +1 V when at zero. When noise stays below or above those values, the input will not flip, and the hysteresis will not happen.

Because there is nothing top help, you do not understand the spec.

one,

from Vdd.

It is this controller from their dsPIC series:

DSPIC33FJ256GP710

Main thing is they should write the hysteresis min-max values into the datasheet like other manufacturers do. And most of all train their support staff a wee bit :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

one,

from Vdd.

Again, this does _not_ guarantee the input won't switch to low at, say,

1.3V or high at 3.5V. It only says that they don't guarantee it will switch there but it could (and probably does). It has nothing to do with hysteresis and noise margin. I bet Jim could elucidate.

Then why did they not write back that the hysteresis is 0.2*VDD to

0.8*VDD? One of the reasons is that this would still not specify a min-max hysteresis. Which you need in order to calculate noise margins properly.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

one,

from Vdd.

A quick surf of DSPIC33FJ256GP710, for the data sheet, shows NO MENTION of hysteresis.

What makes you think there IS hysteresis?

A definition of inputs with ZERO = 0.7*VDD is industry standard. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.