Regarding driving of SCL and SDA pins of I2C

Hi all,

This is a basic qustion regarding SDA and SCL pins.
Since both these pins are bidirectional these should pins need to be
tristated , so that the slave can acknowledge on SDA.

But i have seen in some docs that a '1' need to be converted to a 'Z'
while driving on SDA and SCL, what is the reason behind this????

Thanks in advance,
Praveen

<praveen.kantharajapura@gmail.com> schrieb im Newsbeitrag
news:47cf10b7.0504130430.9a34497@posting.google.com...
> Hi all,
>
> This is a basic qustion regarding SDA and SCL pins.
> Since both these pins are bidirectional these should pins need to be
> tristated , so that the slave can acknowledge on SDA.
>
> But i have seen in some docs that a '1' need to be converted to a 'Z'
> while driving on SDA and SCL, what is the reason behind this????
>
> Thanks in advance,
> Praveen

well in order to drive '1' (EXERNAL RESISTIVE PULLUP) you need to Z the wire
eg tristate it.
0 is driven as 0
1 is driven (or relased) as Z, ext pullup will pull the wire high

Antti

<praveen.kantharajapura@gmail.com> wrote in message 
news:47cf10b7.0504130430.9a34497@posting.google.com...
> Hi all,
>
> This is a basic qustion regarding SDA and SCL pins.
> Since both these pins are bidirectional these should pins need to be
> tristated , so that the slave can acknowledge on SDA.
>
> But i have seen in some docs that a '1' need to be converted to a 'Z'
> while driving on SDA and SCL, what is the reason behind this????
>
> Thanks in advance,
> Praveen

They're not tri-stated as such, merely a pull down or open collector output, 
with an external resistive pull-up.  If using a micro or FPGA then set the 
output to a "0" and use the tri-state enable to turn the pin on or off. 

praveen.kantharajapura@gmail.com wrote:
> Hi all,
> 
> This is a basic qustion regarding SDA and SCL pins.
> Since both these pins are bidirectional these should pins need to be
> tristated , so that the slave can acknowledge on SDA.


 No, both pins are not bidirectional. Only the master device drives the SCK
line, and all slaves must leave their SCK's as input.


> But i have seen in some docs that a '1' need to be converted to a 'Z'
> while driving on SDA and SCL, what is the reason behind this????
> 
> Thanks in advance,
> Praveen


 As others have said, usually SCK and SDA have a 1-10k pullup resistor to Vdd.
This makes the signal a 1 while no device is pulling a pin low. So set the
output pin to a zero, and toggle it as being an input versus an output, to
generate your digital signal.

 Since SCK is never "tristated", you can just drive it as 0/1 output using the
master device and omit the pullup resistor. Of course, the master device must be
able to source and sink a few mA. The PIC line of microcontrollers have no
problem doing this; the Atmels probably work the same.

 Furthermore, if the Atmels have a pin which is "open collector output only",
then that would work for SDA without needing to tristate it. Just set it as an
output and "0" will pull SDA low, and "1" will release it (alowing the pullup
resistor to make SDA "1".)

 For power-hungry applications, you can increase the pullup resistors at the
expense of speed and noise rejection. 100k works well for shielded,
battery-powered applications.

 Cheers,
 MCJ

"Mark Jones" <abuse@127.0.0.1> wrote in message 
news:upWdnTeAPOxSqcDfRVn-sA@buckeye-express.com...

> No, both pins are not bidirectional. Only the master device drives the SCK
> line, and all slaves must leave their SCK's as input.

Yes, they are.

Slaves can pull down SCL to 'wait state' traffic to match their own speed.

That's why it has to be open-collector.

> Since SCK is never "tristated", you can just drive it as 0/1 output using 
> the
> master device and omit the pullup resistor.

No, this is seriously against the spec!

You should never drive it active high.

Otherwise slaves may be damaged when they try to wait-state the bus master.


I've heard this bad advice before - people say they get away with it, but 
they cannot say it obeys the I2C spec.

In article <upWdnTeAPOxSqcDfRVn-sA@buckeye-express.com>, abuse@127.0.0.1 
says...
> praveen.kantharajapura@gmail.com wrote:
> > Hi all,
> > 
> > This is a basic qustion regarding SDA and SCL pins.
> > Since both these pins are bidirectional these should pins need to be
> > tristated , so that the slave can acknowledge on SDA.
> 
> 
>  No, both pins are not bidirectional. Only the master device drives the SCK
> line, and all slaves must leave their SCK's as input.

Not true, a slave device can extend a cycle through clock stretching and 
the only way to do that is for the slave device to be able to hold the 
clock line low.

http://www.i2c-bus.org/clockstretching/

> 
> 
> > But i have seen in some docs that a '1' need to be converted to a 'Z'
> > while driving on SDA and SCL, what is the reason behind this????
> > 
> > Thanks in advance,
> > Praveen
> 
> 
>  As others have said, usually SCK and SDA have a 1-10k pullup resistor to Vdd.
> This makes the signal a 1 while no device is pulling a pin low. So set the
> output pin to a zero, and toggle it as being an input versus an output, to
> generate your digital signal.
> 
>  Since SCK is never "tristated", you can just drive it as 0/1 output using the
> master device and omit the pullup resistor. Of course, the master device must be
> able to source and sink a few mA. The PIC line of microcontrollers have no
> problem doing this; the Atmels probably work the same.
> 
>  Furthermore, if the Atmels have a pin which is "open collector output only",
> then that would work for SDA without needing to tristate it. Just set it as an
> output and "0" will pull SDA low, and "1" will release it (alowing the pullup
> resistor to make SDA "1".)
> 
>  For power-hungry applications, you can increase the pullup resistors at the
> expense of speed and noise rejection. 100k works well for shielded,
> battery-powered applications.
> 
>  Cheers,
>  MCJ
> 

James Beck wrote:
> In article <upWdnTeAPOxSqcDfRVn-sA@buckeye-express.com>, abuse@127.0.0.1 
> says...
> 
>>praveen.kantharajapura@gmail.com wrote:
>>
>>>Hi all,
>>>
>>>This is a basic qustion regarding SDA and SCL pins.
>>>Since both these pins are bidirectional these should pins need to be
>>>tristated , so that the slave can acknowledge on SDA.
>>
>>
>> No, both pins are not bidirectional. Only the master device drives the SCK
>>line, and all slaves must leave their SCK's as input.
> 
> 
> Not true, a slave device can extend a cycle through clock stretching and 
> the only way to do that is for the slave device to be able to hold the 
> clock line low.
> 
> http://www.i2c-bus.org/clockstretching/
> 

 Explain that to a Noob.

 Please.

Mark Jones wrote:
> praveen.kantharajapura@gmail.com wrote:
> > Hi all,
> >
> > This is a basic qustion regarding SDA and SCL pins.
> > Since both these pins are bidirectional these should pins need to
be
> > tristated , so that the slave can acknowledge on SDA.
>
>
>  No, both pins are not bidirectional. Only the master device drives
the SCK
> line, and all slaves must leave their SCK's as input.

I do not agree with u mark

If a slave can't
receive or transmit another complete byte of data until it
has performed some other function, for example servicing
an internal interrupt, it can hold the clock line SCL LOW to
force the master into a wait state.




>
>
> > But i have seen in some docs that a '1' need to be converted to a
'Z'
> > while driving on SDA and SCL, what is the reason behind this????
> >
> > Thanks in advance,
> > Praveen
>
>
>  As others have said, usually SCK and SDA have a 1-10k pullup
resistor to Vdd.
> This makes the signal a 1 while no device is pulling a pin low. So
set the
> output pin to a zero, and toggle it as being an input versus an
output, to
> generate your digital signal.
>
>  Since SCK is never "tristated", you can just drive it as 0/1 output
using the
> master device and omit the pullup resistor. Of course, the master
device must be
> able to source and sink a few mA. The PIC line of microcontrollers
have no
> problem doing this; the Atmels probably work the same.
>
>  Furthermore, if the Atmels have a pin which is "open collector
output only",
> then that would work for SDA without needing to tristate it. Just set
it as an
> output and "0" will pull SDA low, and "1" will release it (alowing
the pullup
> resistor to make SDA "1".)
>
>  For power-hungry applications, you can increase the pullup resistors
at the
> expense of speed and noise rejection. 100k works well for shielded,
> battery-powered applications.
> 
>  Cheers,
>  MCJ

Antti Lukats wrote:
> <praveen.kantharajapura@gmail.com> schrieb im Newsbeitrag
> news:47cf10b7.0504130430.9a34497@posting.google.com...
> > Hi all,
> >
> > This is a basic qustion regarding SDA and SCL pins.
> > Since both these pins are bidirectional these should pins need to
be
> > tristated , so that the slave can acknowledge on SDA.
> >
> > But i have seen in some docs that a '1' need to be converted to a
'Z'
> > while driving on SDA and SCL, what is the reason behind this????
> >
> > Thanks in advance,
> > Praveen
>
> well in order to drive '1' (EXERNAL RESISTIVE PULLUP) you need to Z
the wire
> eg tristate it.
> 0 is driven as 0
> 1 is driven (or relased) as Z, ext pullup will pull the wire high

In order to drive a '1' , i will not tristate it to 'Z' i will drive a
'1' only.
Any issues(Hardware malfunction) if i drive a'1' instead of 'Z'

> 
> Antti

You can read a summary here:

http://www.i2c-bus.org/Termination.277.0.html

But I recommend visiting the Philips site and downloading the
specification:

http://www.semiconductors.philips.com/markets/mms/protocols/i2c/

You can probably safely ignore the multimaster and high-speed modes,
but you should be aware of their existance. I personally don't know of
anybody using high-speed mode, but it does make the I/O requirements
more interesting.

-Keith