Huh..?? Not sure about the PIC I2C hardware interface..
    but I2C is NOT synchronous.

    You can slow down I2C comm by holding SCL low (as
    a slave) until you are ready to give the bit data.
    That's why the requirement for an open drain
    bus.

    In fact you can run close to 0 Hz this way as
    I2C specification never specify a timeout for
    the bit handshake.

    SMBus specification, which is a variation of I2C
    added a timeout. But this is understandable
    as SMBus is adapted for managing system devices
    on PC platform.

    The only time an I2C slave have to worry about
    timing is the detection of the I2C start condition,
    which is dependent on the I2C master.

    The last I check, the worst case is at 400kHz or
    2.5us between the SDA going low follow by SCL going low.

    So if you use some external logic to detect the
    start condition (and generate a interrupt signal),
    and a uC that can service an interrupt (and output
    SCL low) within 2.5us, you can run a software
    bit-banged I2C slave on that uC.

Cheers,
Peter Tiang

========================================================

----- Original Message -----
From: "D Lloyd" <dan.lloyd@GB.ABB.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, April 13, 2001 10:31 PM
Subject: Re: [PIC]: I2c Different Speeds


>
> Hi,
>
> No, if you are using the hardware interfaces, because I2C is synchronous.
>
> Dan
>
>
>
>
>  (Embedded     Ronald Cotton <rcotton@COMPRISCORP.COM>@MITVMA.MIT.EDU>
>  image moved   13/04/2001 14:59
>  to file:
>  pic12488.pcx)
>
>
>
>
>
> Please respond to pic microcontroller discussion list
>       <PICLIST@MITVMA.MIT.EDU>
> Sent by:  pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
>
>
> To:   PICLIST@MITVMA.MIT.EDU
> cc:
> Subject:  [PIC]: I2c Different Speeds
>
> Security Level:?         Internal
>
>
> I have 2 PICs, 16f877(20Mhz) and a 16f874(4Mhz).  I have configured the
> 16f874 as the Master and the 877 as the slave.
> Will I have a problem with communications if the crystals are different?
>
> Thanks
>
>
> Ronald Cotton
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics
>
>
>
>
>

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.