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'[PIC]: I2C Questions'
2002\02\27@180203 by Sean Alcorn - Avion Sydney

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Hi everybody,

I am a relative newbie, and am designing a series of products based on the
PIC16F87X and possibly the PIC16F628 that will all need to communicate with
each other (multi Master) over distances of up to say 50m.

Is I2C a suitable choice? Can I run the PICs as slow as 4MHz, or would I
need to go higher than this? I plan to send simple commands and
acknowledgements only.

Also what is a "Controller Area Network"

Sorry if these are silly questions.

Regards,

Sean

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2002\02\28@020757 by Simon-Thijs=20de=20Feber?=

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I do not know if 50m is achievable by I2C.
Check the datasheet of an 82B715.
This is an I2C driver which has low Z outputs and can
drive quite some current.

grtz

Simon




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2002\02\28@023059 by Martin Buehler

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depends on the speed of the connection.
i use i2c single master on a wireload of about 50..100meters per segment,
and i run it at about 1khz. i have a star topology with serial resistors
(about600..800 ohms) in the scl and sda wires at each node.
that works fine!
tino





Spehro Pefhany <.....speffKILLspamspam@spam@INTERLOG.COM>
Gesendet von: pic microcontroller discussion list <PICLISTspamKILLspamMITVMA.MIT.EDU>
27.02.02 16:53
Bitte antworten an pic microcontroller discussion list


       An:     .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
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       Thema:  Re: [PIC}: I2C Questions


At 02:17 AM 2/28/02 +1100, you wrote:
>Hi all,
>
>I am a relative newbie, and am designing a series of products based on
the
>PIC16F87X and possibly the PIC16F628 that will all need to communicate
with
>each other (multi Master) over distances of up to say 50m.
>
>Is I2C a suitable choice?

No. It is intended for very short distances. You want, at a minimum,
RS-485,
for which you can use the on-board UART in those chips. I prefer isolation
(optical or transformer) for noisy environments.

>Can I run the PICs as slow as 4MHz, or would I
>need to go higher than this? I plan to send simple commands and
>acknowledgements only.

4MHz should be fine as far as comms are concerned.

>Also what is a "Controller Area Network"

CAN is a relatively simple network used in automobiles and similar
situations.
It's also part of the industrial standard Devicenet. Do a web search and
you can find lots of information on it.

Here's a presentation that should get you up to speed:
http://www.ece.cmu.edu/~ece540/lecture/09_can.pdf

>Sorry if these are silly questions.

The only silly question is one that should have been asked but, wasn't.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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2002\02\28@080027 by Olin Lathrop

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> Check the datasheet of an 82B715.
> This is an I2C driver which has low Z outputs and can
> drive quite some current.

If you're saying it drives actively high instead of just low, then that is
non-standard.  Some IIC bus function rely on the passive pullup
configuration, like clock stretch.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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2002\02\28@080053 by Olin Lathrop

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> i have a star topology with serial resistors
> (about600..800 ohms) in the scl and sda wires at each node.
> that works fine!

Don't try this at home, folks.  A number of standard IIC devices may not
work this way, or may even get damaged.  The IIC bus is specified with a
maximum sink current of 3mA on each line.  If you're going to recommend
something like this, please point out that you are only using IIC protocol,
not true IIC.  I assume you are using your own line drivers and not
connecting off the shelf IIC devices to the bus, right?


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, @spam@olinKILLspamspamembedinc.com, http://www.embedinc.com

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2002\02\28@081428 by Michael Rigby-Jones
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> -----Original Message-----
> From: Olin Lathrop [SMTP:KILLspamolin_piclistKILLspamspamEMBEDINC.COM]
> Sent: Thursday, February 28, 2002 12:27 PM
> To:   RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
> Subject:      Re: [PIC]: I2C Questions
>
> > Check the datasheet of an 82B715.
> > This is an I2C driver which has low Z outputs and can
> > drive quite some current.
>
> If you're saying it drives actively high instead of just low, then that is
> non-standard.  Some IIC bus function rely on the passive pullup
> configuration, like clock stretch.
>
The 82B715 has open collector outputs and still requires the use of pull-up
resistors but effectively provide current amplification for the device it is
buffering.  They have to be used at each node.

Mike

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2002\02\28@081832 by Martin Buehler

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i use philips pcf8574 and dallas ds1621 chips on the bus.
remember that the resistive load is really low (i use 4k7 pullups, only
one on each wire).
what you get is high capacitive load. with the serial resistors, you
reduce the switching current to below 10ma.
the 3ma specified is the minimum constant current, a deivce is able to
provide! (absolute max for pcf8574 is 25ma)
a system with about 24 nodes works well now for more than 1.5 years
without any problems.
tino





Olin Lathrop <spamBeGoneolin_piclistspamBeGonespamEMBEDINC.COM>
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28.02.02 13:53
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       An:     RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU
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> i have a star topology with serial resistors
> (about600..800 ohms) in the scl and sda wires at each node.
> that works fine!

Don't try this at home, folks.  A number of standard IIC devices may not
work this way, or may even get damaged.  The IIC bus is specified with a
maximum sink current of 3mA on each line.  If you're going to recommend
something like this, please point out that you are only using IIC
protocol,
not true IIC.  I assume you are using your own line drivers and not
connecting off the shelf IIC devices to the bus, right?


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinEraseMEspam.....embedinc.com, http://www.embedinc.com

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2002\02\28@103437 by Edson Brusque

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Hello PICple,

   have you managed to use fast IIC (400kHz) on a PIC16F877?

   I'm trying to do it with a 'F877 and a 24C64, no sucess... :(

   Best regards,

   Brusque

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2002\02\28@120331 by Herbert Graf

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Perhaps I'm misunderstanding something Olin but to me "serial resistors in
the scl and sda wire" does NOT sound like he/she is using those resistors
for pullups. While out of spec I don't understand how putting a 600ohm
resitor in series with either the scl or sda lines has any chance of harming
an I2C part. TTYL

> {Original Message removed}

2002\02\28@181000 by Olin Lathrop

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>     have you managed to use fast IIC (400kHz) on a PIC16F877?

Yes.

>     I'm trying to do it with a 'F877 and a 24C64, no sucess... :(

The PIC MSSP in IIC master mode has a bug where the ACK bit is sampled on
the wrong edge of clock, causing a race condition.  You may be running into
this.  A small cap on SDA will work around this problem but also limit your
max bit rate.  I consider 400KHz the upper limit with the MSSP in master
mode because of this.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspamembedinc.com, http://www.embedinc.com

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2002\02\28@181019 by Olin Lathrop

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> Perhaps I'm misunderstanding something Olin but to me "serial resistors in
> the scl and sda wire" does NOT sound like he/she is using those resistors
> for pullups. While out of spec I don't understand how putting a 600ohm
> resitor in series with either the scl or sda lines has any chance of
harming
> an I2C part. TTYL

I didn't catch the fact that they were in series.  I thought he was using
600 ohm pullups on each unit.  Sorry.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, RemoveMEolinEraseMEspamEraseMEembedinc.com, http://www.embedinc.com

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