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PICList Thread
'[PIC]: Which mu-C Best For I2C?'
2002\08\15@194544 by Tom Lewis

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Dear PIC List:

My application is strictly I2C, one master, many slaves
100 (+/-) and slow data rates.

After researching the volumes of online information,
I'm no closer to choosing a PIC mu-C than I was three
months ago!

Any suggestions?

Ideally, the chip will be supported by a MicroChip (R)
development environment and programmer.

All replies welcomed!

Tom Lewis
Cypress Data Systems
Chico, CA

spam_OUTtflewisTakeThisOuTspamchico.com

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2002\08\15@202647 by Brendan Moran

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> Dear PIC List:
>
> My application is strictly I2C, one master, many slaves
> 100 (+/-) and slow data rates.

Yowza!! 100 slaves?  Well, I can give you one recommendation right
away.  IIRC the maximum recommended bus length is 10cm.  If you have
that many devices attached, you're going to need some repeater ICs.
There's a 5-port I2C hub made by Philips that may be helpful to you.

> After researching the volumes of online information,
> I'm no closer to choosing a PIC mu-C than I was three
> months ago!
>
> Any suggestions?
>
> Ideally, the chip will be supported by a MicroChip (R)
> development environment and programmer.

Any microchip MCU that has I2C support should fit that bill.  I'd say
for ease of coding, you can go with an 18F part, though if you know
you don't need all that power, you can drop to a 16F part.  If all
you're doing with the master is operating slaves, then you should be
able to drop to a 16F876 or similar, unless code size or RAM are
limiting factors.

Well, that's all I can come up with.  Hope it helps.

- --Brendan

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2002\08\15@220143 by Olin Lathrop

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> My application is strictly I2C, one master, many slaves
> 100 (+/-) and slow data rates.
>
> After researching the volumes of online information,
> I'm no closer to choosing a PIC mu-C than I was three
> months ago!
>
> Any suggestions?
>
> Ideally, the chip will be supported by a MicroChip (R)
> development environment and programmer.
>
> All replies welcomed!

Are you looking for a PIC for the master, the slaves, or both?  What else
does each one need to do?  There are many PICs that have IIC hardware, so
it's mostly the "everything else" that's the deciding factor.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\08\16@040807 by Michael Rigby-Jones

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{Quote hidden}

Any part with the MSSP peripheral should be suitable.  What other
requirements do you have that could narrow the selection (e.g. memory,
timers, analog inputs, PWM etc.)

Regards

Mike

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2002\08\16@080221 by Olin Lathrop

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> Yowza!! 100 slaves?  Well, I can give you one recommendation right
> away.  IIRC the maximum recommended bus length is 10cm.

Recommended in a particular situation perhaps, but IIC has no official
maximum bus length.  The hard limits are the maximum capacitance (400pF if
recall), and the maximum sink current required to lower a bus line to logic
0 (around 3mA, again from memory).  These parameters and the maximum clock
rate will dictate the maximum bus length in your case.  Note that you get to
chose the clock rate.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\08\16@091724 by Michael Rigby-Jones

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> -----Original Message-----
> From: Olin Lathrop [SMTP:.....olin_piclistKILLspamspam.....EMBEDINC.COM]
> Sent: Friday, August 16, 2002 1:03 PM
> To:   EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU
> Subject:      Re: [PIC]: Which mu-C Best For I2C?
>
> > Yowza!! 100 slaves?  Well, I can give you one recommendation right
> > away.  IIRC the maximum recommended bus length is 10cm.
>
> Recommended in a particular situation perhaps, but IIC has no official
> maximum bus length.  The hard limits are the maximum capacitance (400pF if
> recall), and the maximum sink current required to lower a bus line to
> logic
> 0 (around 3mA, again from memory).  These parameters and the maximum clock
> rate will dictate the maximum bus length in your case.  Note that you get
> to
> chose the clock rate.
>
Philips even has an app note showing the use of I2C over very long distances
using their dedicated I2C buffer.  IIRC they managed to get it to work over
a mile.

Mike

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2002\08\16@131641 by Brendan Moran

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> > Yowza!! 100 slaves?  Well, I can give you one recommendation
> > right away.  IIRC the maximum recommended bus length is 10cm.
>
> Recommended in a particular situation perhaps, but IIC has no
> official maximum bus length.  The hard limits are the maximum
> capacitance (400pF if recall), and the maximum sink current
> required to lower a bus line to logic 0 (around 3mA, again from
> memory).  These parameters and the maximum clock rate will dictate
> the maximum bus length in your case.  Note that you get to chose
> the clock rate.

400pF sounds right to me.  I still don't think that suggesting a hub
IC is a bad idea.  It might make things considerably simpler.
Especially since it has select lines, which would enable it to have
up to 4 devices with the same address in the circuit (5 port hub, 1
port goes to master, therefore, only 4 devices with the same address
can be used.) so long the enable lines were used well.

Bus length and clock speed are, of course, the determining factors.
I expect that the 10cm guideline was placed there as a rule of thumb
for what is likely to be about 400pF.  Track placement matters, of
course, etc. etc. etc.

Note on track placement: IIRC the recommended track placement for
longer runs was this: Vcc SCL GND SDA or something like that.

- --Brendan

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