I2C without resistors

Here you'll find a writing about a resistor-less I2C implementation:

        http://www.delajii.net/~mmihai/Articles/I2C_precharged.html

As the name of the link is saying, it's a 'precharged' I2C bus.

Enjoy!
--
mmihai

On Aug 22, 2:24 pm, mmihai <iia...@yahoo.com> wrote:
> Here you'll find a writing about a resistor-less I2C implementation:
>
>        http://www.delajii.net/~mmihai/Articles/I2C_precharged.html
>
> As the name of the link is saying, it's a 'precharged' I2C bus.
>
> Enjoy!
> --
> mmihai

Cute but nasty!

Assuming your SDA line does have enough capacitance to do this,
anything else that switches nearby is going to play havoc. I don't
think I'd be able to sleep at night if I implemented this.
I'd simply add the resistor and gets a good nights sleep!

Dave.

On Aug 21, 10:37 pm, "David L. Jones" <altz...@gmail.com> wrote:

> Cute but nasty!
>
> Assuming your SDA line does have enough capacitance to do this,
> anything else that switches nearby is going to play havoc. I don't
> think I'd be able to sleep at night if I implemented this.
> I'd simply add the resistor and gets a good nights sleep!

As I wrote, /use on your own risk/.
The noise margin will be worse than a standard I2C (no secret about
that) but it's up to you to decide if it's appropriate for your
application.

Anyway, should you decide to use it, do some calculations for
yourself:
 - an I/O pin capacitance in is pF range (2-20pF, depends on the
package)
 - input current for a I/O is less than 1u (F123 has 10nA per data
sheet)
 - the level on SDA while in HiZ has to hold long enough for you clock
cycle (that's depends on your MCU speed, faster the better).

Study the spec of your devices and do the math.
--
mmihai

On Tue, 21 Aug 2007 23:06:43 -0700, mmihai <iiahim@yahoo.com> wrote:

>On Aug 21, 10:37 pm, "David L. Jones" <altz...@gmail.com> wrote:
>
>> Cute but nasty!
>>
>> Assuming your SDA line does have enough capacitance to do this,
>> anything else that switches nearby is going to play havoc. I don't
>> think I'd be able to sleep at night if I implemented this.
>> I'd simply add the resistor and gets a good nights sleep!
>
>As I wrote, /use on your own risk/.
>The noise margin will be worse than a standard I2C (no secret about
>that) but it's up to you to decide if it's appropriate for your
>application.
>
>Anyway, should you decide to use it, do some calculations for
>yourself:
> - an I/O pin capacitance in is pF range (2-20pF, depends on the
>package)
> - input current for a I/O is less than 1u (F123 has 10nA per data
>sheet)
> - the level on SDA while in HiZ has to hold long enough for you clock
>cycle (that's depends on your MCU speed, faster the better).
>
>Study the spec of your devices and do the math.

resistors are cheap.

"mmihai" <iiahim@yahoo.com> schreef in bericht 
news:1187762803.647774.215680@l22g2000prc.googlegroups.com...
> On Aug 21, 10:37 pm, "David L. Jones" <altz...@gmail.com> wrote:
>
>> Cute but nasty!
>>
>> Assuming your SDA line does have enough capacitance to do this,
>> anything else that switches nearby is going to play havoc. I don't
>> think I'd be able to sleep at night if I implemented this.
>> I'd simply add the resistor and gets a good nights sleep!
>
> As I wrote, /use on your own risk/.
> The noise margin will be worse than a standard I2C (no secret about
> that) but it's up to you to decide if it's appropriate for your
> application.
>
> Anyway, should you decide to use it, do some calculations for
> yourself:
> - an I/O pin capacitance in is pF range (2-20pF, depends on the
> package)
> - input current for a I/O is less than 1u (F123 has 10nA per data
> sheet)
> - the level on SDA while in HiZ has to hold long enough for you clock
> cycle (that's depends on your MCU speed, faster the better).
>
> Study the spec of your devices and do the math.
> --
> mmihai
>

I can't but consider it bad engineering practice. The I2C is well specified 
and you'd stick to it as long as possible. This type of "solution" may work 
in a one of a kind or small series. But even then, the price of the extra 
resistors outweight the risks by far. Large series will give more problems 
as you will get unrecognised, intermittend problems in only some of the 
devices. The costs will easily grow way beyond the cost of correcting the 
design.

petrus bitbyter 

mmihai wrote:
> Here you'll find a writing about a resistor-less I2C implementation:
> 
>         http://www.delajii.net/~mmihai/Articles/I2C_precharged.html
> 
> As the name of the link is saying, it's a 'precharged' I2C bus.
> 
> Enjoy!
> --
> mmihai
> 

Interesting, but why?

The two resistors necessary in I2C are cheap and small and (bonus) let 
the link work according to spec.

I would never do this even on a hobby project. Interesting that it can 
be done, although it's probably been done inadvertently anyway ;)

Cheers

PeteS

mmihai wrote:
> Here you'll find a writing about a resistor-less I2C implementation:
> 
>         http://www.delajii.net/~mmihai/Articles/I2C_precharged.html
> 
> As the name of the link is saying, it's a 'precharged' I2C bus.
> 
> Enjoy!
> --
> mmihai


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.

Read the spec for details.

-- 

Tauno Voipio
tauno voipio (at) iki fi

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

mmihai wrote:
> 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


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.

-- 

Tauno Voipio
tauno voipio (at) iki fi

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