mmihai wrote:
> On Aug 22, 1:11 pm, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
> 
>> Any slave is allowed to clamp the SCK line,
>> so a 'special case bus' is quite useless, or
>> at least it is not I2C anmyore. Call it then
>> something else.
> 
> It's a sub set of I2C.
> Being allowed does not mean it must to. Doc pointer: http://www.i2c-bus.org/singlemaster/
> You'll find plenty of other [software] implementations which don't
> check if SCL is high after it is released.
> 
> One can figure out from the datasheet of the devices if handshake
> (i.e. clock stretching) is needed or not.
> --
> mmihai
> 
> 
Very True.  I get call from them every so often saying Our Battery is 
defective. A very common mistake does not make it acceptable.

If the manufacture changes the chips it may stop working.
One brand of I2C bus monitor holds the clock on occasion.

On 23 aug, 14:13, rickman <gnu...@gmail.com> wrote:
> On Aug 23, 12:59 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
> wrote:
>
> > Terry Given wrote:
> > > absolutely. This is a pretty good example of a *really* terrible idea.
> > > It saves $0.002 worth of resistors, brutally violates the I2C spec, and
> > > doubtless has some interesting dynamic implications.
>
> > I couldn't agree more.
>
> > Why would anyone want to do this ?
>
> If you have a really low power design, it eliminates the current
> through the resistors when the bus is low.  Maybe this is not a huge
> current, but it can be significant in very low power designs.

Right, It all depends.
It depends on what _is_ on that i2c bus.
For example if you only drive a PCF8574 IO expander, those are only
slaves,
and have no clock pull down transistor.
In such a case it is OK, and even better, to drive the clock from a
normal
CMOS output, and leave out the clock line pull up resistor.
Gives a better clock waveform, and less power consumption.
Same for PCF8591 etc..
I have done so, in the early days the i2c spec did not even have the
clock pull down stuff,
was only single master, and slave acknowledged by pulling sda low.

On Aug 23, 12:59 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
> Terry Given wrote:
> > absolutely. This is a pretty good example of a *really* terrible idea.
> > It saves $0.002 worth of resistors, brutally violates the I2C spec, and
> > doubtless has some interesting dynamic implications.
>
> I couldn't agree more.
>
> Why would anyone want to do this ?

If you have a really low power design, it eliminates the current
through the resistors when the bus is low.  Maybe this is not a huge
current, but it can be significant in very low power designs.

However, like DRAM, it will require periodic refreshes just by having
some bus activity if your operational activity is not frequent
enough.

On Aug 23, 1:17 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
> DJ Delorie wrote:
> > Eeyore <rabbitsfriendsandrelati...@hotmail.com> writes:
> > > Why would anyone want to do this ?
>
> > Well, in the original article, he admitted that he just forgot to
> > design them in
>
> That's a bit like forgetting to tie your shoelaces !

Actually, he didn't forget them, he thought the MCU had them built
in.  A mistake, don't we all make mistakes?

"mmihai" <iiahim@yahoo.com> schreef in bericht 
news:1187808526.744654.296830@r23g2000prd.googlegroups.com...
> On Aug 22, 11:08 am, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
>
>> It is allowed that a station in the I2C bus clamps
>> a signal to ground. If you're attempting to precharge
>> a clamped signal, you'll create a straight short - not
>> a too recommendable practice.
>>
>> The clamping can be done on both the SCK and the SDA
>> line.
>
> Yes, I know.
>
> Read the "simple bus" description form the article.
>
> I never claimed this is better than I2C from the original spec.
> It is just a way to run I2C in some /special/ cases as stated in the
> text.
>
> --
> mmihai
>
>

Once had to add a device to such a "special". Nothing documented but "I2C". 
So, the bitbanging master did not look after ACKs and crashed when the clock 
pulse was stretched. Also bus resistance was too high as was bus 
capacitance. Could only survive by by adding too low pullup resistors so 
violating the specs a little bit more. Then added a 82B715 to isolate my own 
part of the bus. Which was the easy part. Programming around the other 
violations took some more time. So I can't but repeate it's bad engineering 
practice. Stick to the specs as long as you can. Preferable a little bit 
longer.

petrus bitbyter 

On Aug 23, 2:53 pm, Terry Given <my_n...@ieee.org> wrote:
> Robert Adsett wrote:
> > In article <1187829284.987009@ftpsrv1>, Terry Given says...
>
> >>Robert Adsett wrote:
>
> >>>In article <1187808526.744654.296...@r23g2000prd.googlegroups.com>,
> >>>mmihai says...
>
> >>>>On Aug 22, 11:08 am, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
>
> >>>>>It is allowed that a station in the I2C bus clamps
> >>>>>a signal to ground. If you're attempting to precharge
> >>>>>a clamped signal, you'll create a straight short - not
> >>>>>a too recommendable practice.
>
> >>>>>The clamping can be done on both the SCK and the SDA
> >>>>>line.
>
> >>>>Yes, I know.
>
> >>>>Read the "simple bus" description form the article.
>
> >>>>I never claimed this is better than I2C from the original spec.
> >>>>It is just a way to run I2C in some /special/ cases as stated in the
> >>>>text.
>
> >>>I think the problem is people are having trouble envisioning any case in
> >>>which it makes sense to leave the resistors out.
>
> >>If you made, say, 1 billion somethings, you could save about $1,000,000
>
> > Of course you won't need very many support calls or warranty claims to
> > eat that right back up and more.
>
> > Robert
>
> absolutely. This is a pretty good example of a *really* terrible idea.
> It saves $0.002 worth of resistors, brutally violates the I2C spec, and
> doubtless has some interesting dynamic implications. Why stop there -
> why not power the I2C peripheral via SCL, SDA using its internal ESD
> diodes, and ensuring that there is enough bus activity to keep the
> supply pumped up.....

Works a treat with CMOS logic, just keep at least one input high at
all times, don't draw too much current, keep your speeds low, and
don't rely on meeting the specs :->

Ick!

Dave.

DJ Delorie wrote:

> Eeyore <rabbitsfriendsandrelations@hotmail.com> writes:
> > Why would anyone want to do this ?
>
> Well, in the original article, he admitted that he just forgot to
> design them in

That's a bit like forgetting to tie your shoelaces !

Graham

Eeyore <rabbitsfriendsandrelations@hotmail.com> writes:
> Why would anyone want to do this ?

Well, in the original article, he admitted that he just forgot to
design them in, and didn't want to uglify the board with a couple of
afterthought resistors.

I agree it's a bad idea, but he did go through all the math and
engineering to see if it would work reliably enough for his purposes.

Me, I'm following a friend's advice and putting a grid of solder pads
on the back of my boards to add 0603 or SOIC parts later if needed.
At least they're less visible that way.

Terry Given wrote:

> absolutely. This is a pretty good example of a *really* terrible idea.
> It saves $0.002 worth of resistors, brutally violates the I2C spec, and
> doubtless has some interesting dynamic implications.

I couldn't agree more.

Why would anyone want to do this ?

Graham

Robert Adsett wrote:
> In article <1187829284.987009@ftpsrv1>, Terry Given says...
> 
>>Robert Adsett wrote:
>>
>>>In article <1187808526.744654.296830@r23g2000prd.googlegroups.com>, 
>>>mmihai says...
>>>
>>>
>>>>On Aug 22, 11:08 am, Tauno Voipio <tauno.voi...@INVALIDiki.fi> wrote:
>>>>
>>>>
>>>>
>>>>>It is allowed that a station in the I2C bus clamps
>>>>>a signal to ground. If you're attempting to precharge
>>>>>a clamped signal, you'll create a straight short - not
>>>>>a too recommendable practice.
>>>>>
>>>>>The clamping can be done on both the SCK and the SDA
>>>>>line.
>>>>
>>>>Yes, I know.
>>>>
>>>>Read the "simple bus" description form the article.
>>>>
>>>>I never claimed this is better than I2C from the original spec.
>>>>It is just a way to run I2C in some /special/ cases as stated in the
>>>>text.
>>>
>>>
>>>I think the problem is people are having trouble envisioning any case in 
>>>which it makes sense to leave the resistors out.
>>>
>>
>>If you made, say, 1 billion somethings, you could save about $1,000,000
> 
> 
> Of course you won't need very many support calls or warranty claims to 
> eat that right back up and more.  
> 
> Robert

absolutely. This is a pretty good example of a *really* terrible idea. 
It saves $0.002 worth of resistors, brutally violates the I2C spec, and 
doubtless has some interesting dynamic implications. Why stop there - 
why not power the I2C peripheral via SCL, SDA using its internal ESD 
diodes, and ensuring that there is enough bus activity to keep the 
supply pumped up.....

Cheers
Terry