[TriEmbed] Basic I2C Question

Thu Sep 11 11:04:17 CDT 2014

The device specs state the amount of capacitance they exert on the bus and
specs for your master should state the maximum capacitance it will support.

I don't think you can hook up a capacitance meter as the capacitance can't
be measured with the device off, and most capacitance meters do not work
well in powered circuits.

On Thu, Sep 11, 2014 at 11:46 AM, Jeff Highsmith <jeff at jeffhighsmith.com>
wrote:

> Rodney,
>
> Can bus capacitance be easily measured? I've got five I2C sensors hooked
> up to the project I'm working on, though all within a square foot. Can I
> just put my multimeter in capacitance mode and measure across SDA and SCL
> when the circuit is off?
>
> Thanks!
> Jeff :)
>
>
> On Sep 11, 2014, at 11:39 , Rodney Radford wrote:
>
> As long as the I2C devices are close to the microprocessor, you do not
> need additional pull up resistors.
>
> However, as you increase the distance (say to an extra sensor board), the
> long wires add additional capacitance to the I2C lines, turning those sharp
> edges into something that looks more like a sine wave.  The same occurs as
> you add more devices as each device has it's own internal capacitance. The
> additional pull up resistors help correct this, allowing you to put your
> sensors further away (or add more sensors).
>
> So if you are going off-board with your sensors, yes, i would definitely
> recommend adding pull up resistors. The lower the value of the resistors,
> the more pull up you get and the sharper your edges become - at the
> increase of additional current required for your device, so it is a trade
> off.
>
> I worked on a setup years ago where I used I2C as a communication bus for
> a large aquarium monitoring system where I had I2C temperature sensors in
> each aquarium, I2C port expanders to drive small 2x16 line LCD displays,
> and port expanders to control turning on heaters to each aquarium.  The
> aquariums were in an external storage shed that was not insulated, so by
> measuring the external temperature we could start increasing the water
> temperature as the external temperature dropped, to prevent the water from
> getting too cold before the heaters cut in.  In hind site, it may have been
> easier to insulate the shed, but the system worked well for years.  I was
> running I2C lines over CAT-5 30' and it worked like a charm.  I did
> optoisolate the I2C lines back to the computer (which was a Windows PC that
> was bit-banging the I2C out the PC's parallel port) to prevent lightning of
> other high static charges from damaging the port.
>
> Of course now I would replace all of that with a few small microprocessors
> and a Raspberry Pi, but this was back in the early/mid 90s...
>
> On Thu, Sep 11, 2014 at 11:28 AM, Charles McClelland <chip at mcclellands.org
> > wrote:
>
>> Adam,
>>
>> Thank you for the response your and Jon’s note raises another question:
>>
>> Is there an advantage to putting resistors on the circuit board versus
>> simply using the Arduino’s internal pull-ups?  Power savings or - as Jon
>> mentioned - the ability to tune for performance?
>>
>> On the RTC - here is my reader’s digest version of the bewildering
>> variety that Maxim offers:
>> - DS1337 - Great but no battery backup to keep clock set when I change
>> the batteries
>> - DS1338 - Adds battery backup but drops alarms
>> - DS1339 - Battery backup, alarms and an alarm pin - and it is cheaper
>> than the DS1338.
>>
>> I am hoping the library / commands for all three will be similar and
>> similar.  Thoughts?
>>
>> thanks,
>>
>> Chip
>>
>>
>> On Sep 11, 2014, at 8:50 AM, Adam Haile <email at adamhaile.net> wrote:
>>
>> There's no need for them at all.
>> I2C only needs pull up resistors on the bus as a whole, NOT for each
>> device.
>> Also, the Wire library in Arduino automatically enables the internal
>> pullup resistors when you call begin(). You can also do the same by setting
>> a pin as an input and then writing HIGH to it, but Wire takes care of it
>> for you.
>>
>> Sure, having extra won't help but you certainly don't need more than one
>> set.
>>
>> For the RTC, check out the DS1337, it is the DS1307's big brother and
>> includes alarms. However, there are no alarm pins, you just have to poll
>> for if the alarm is "going off". But there is lots of code out there
>> already for the DS1337. If you need alarm interrupts, I'm not sure what to
>> use... nothing I'm familiar with that has separate alarm pins.
>>
>> On Thu, Sep 11, 2014 at 8:28 AM, Charles McClelland <chip at mcclellands.org
>> > wrote:
>>
>>> First, I wanted to thank Paul for the excellent presentation on
>>> interrupts.  I now have a Watch Dog Timer routine which is saving me from
>>> having to put a clock on one project.
>>>
>>> On my other project though, I still need a clock.  And since I am
>>> already using an I2C bus, I thought I could simply add one to that bus.
>>> That way I will have two I2C sensors connected to an Arduino (ATMEGA328).
>>> My question is how to best implement the pull-up the resistors.  Should I
>>> put resistors on each (perhaps 10k since they will effectively be in
>>> parallel) or should I put one set of pull up resistors on the main board
>>> for both?  It seems like the latter is the better approach but I did not
>>> know if there was some reason to do one over the other.
>>>
>>> On a related note, I need a 3.3V I2C Real Time Clock with Alarms - I am
>>> looking at the DS1339 and hoping I can use some of the DS1307 library since
>>> they are both from Maxim.  Any other suggestions?
>>>
>>> Thanks,
>>>
>>> Chip
>>>
>>>
>>>
>>> _______________________________________________
>>> Triangle, NC Embedded Computing mailing list
>>> TriEmbed at triembed.org
>>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>> TriEmbed web site: http://TriEmbed.org <http://triembed.org/>
>>>
>>
>>
>>
>> _______________________________________________
>> Triangle, NC Embedded Computing mailing list
>> TriEmbed at triembed.org
>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>> TriEmbed web site: http://TriEmbed.org
>>
>>
> _______________________________________________
> Triangle, NC Embedded Computing mailing list
> TriEmbed at triembed.org
> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> TriEmbed web site: http://TriEmbed.org
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20140911/ba4afde6/attachment.htm>