I stand corrected, forgot about clock stretching, probably because I've
never used a device that did it. TTYL

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Barry King
> Sent: Wednesday, March 14, 2001 16:52
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC]: I2C pullups
>
>
> > I'm relatively certain that slaves do NOT drive SCL, mainly because if
> > you had a locked up slave holding SCL low it would prevent ANY
> communication
> > on the bus.
>
> By the spec, slaves MAY drive SCL low, which is called clock
> stretching.  The purpose is that a slow slave can hold SCL low, after
> the master has released it, until the next bit is ready.  The Master
> must wait for the actual rising edge to know the bit is valid.
>
> I have never seen a slave implementation that USES clock stretching
> (example: EEPROMs don't) EXCEPT one that's important to us: PIC's SSP
> in slave mode.  As a slave, it ALWAYS pulls SCL low from the time it
> recognizes its slave read address, until the first byte of the
> response is loaded by the firmware.
>
> So it you are using a PIC as a slave, follow the spec, and provide
> pullups on both lines, and make sure the master is operating "open-
> drain", that is, pull down when presenting a low, and float the line
> and let the pull up present the high level.
>
> >      I'm relatively certain that slaves do NOT drive SCL, mainly
> because if
> > you had a locked up slave holding SCL low it would prevent ANY
> communication
> > on the bus.
>
> TRUE. If you write I2C slave code for a PIC, you've got to be darn
> careful that you always respond to the SSPIF, even if you don't
> "think" the master wants any more data now.  The SSP WILL lock the
> bus if you leave a slave-read (PIC slave sends data to master)
> transfer unfinished.
>
> -Barry.
> ------------
> Barry King
> NRG Systems "Measuring the Wind's Energy"
> http://www.nrgsystems.com
> Check out the accumulated (PIC) wisdom of the ages at:
> PIC/PICList FAQ: http://www.piclist.com/faq
>
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