Re: [offtopic] Re: I2c was: Cobalt Micro (was Re: Build your own
Geert Uytterhoeven (Geert.Uytterhoeven@cs.kuleuven.ac.be)
Wed, 9 Sep 1998 21:32:31 +0200 (CEST)
On Wed, 9 Sep 1998, Richard B. Johnson wrote:
> On Wed, 9 Sep 1998, Daniel Engstrom wrote:
> > On 8 Sep, David Lang wrote:
> > > it arbitrates the but by listening to the bus as it transmits, if it hears
> > > something different then it is transmitting it stops transmitting and the
> > > other station continues (the first thing it transmits is it's own id # and
> > > the node with more 1 bits in it's address will win as it will transmit a 1
> > > while the other node transmits a 0, the 1 will make it on the bus and the
> > > other node will stop)
> >
> > That would be a CAN bus arbitration....
> > Or maybe I2C uses the same method...
> >
> > /Daniel
>
> The IIC bus has no arbitration capability at all. One checks a busy
> bit to see if it is ready. If not busy, one sends data (address first).
> The receiving machine ACKs every bus state change.
>
> Since two (or more) parties can think the bus is free at the same time,
> clashes are possible. This will hang the IIC bus. Therefore, once the
> bus is crashed, both parties have to reinitialize their controllers to
> make the bus free. Since the possibility exists that both parties will
> detect a crashed bus at the same time, and attempt to reinitialize the
> bus at the same time, deadlocks can (read will) occur.
>
> Therefore, everybody on the bus has to use a pseudo-random back-off-time to
> resolve deadlocks.
Hmm,... very similar to Ethernet.
> Further, since everything is done in software, fast machines can send
> data faster than slow machines can receive it. This means the fast
> machine must artificially slow down data transmission to match the
> slowest receiver.
Yes, but only when talking to the slowest device, if I understand it correct.
When talking to a fast device it can talk faster.
Greetings,
Geert
--
Geert Uytterhoeven Geert.Uytterhoeven@cs.kuleuven.ac.be
Wavelets, Linux/{m68k~Amiga,PPC~CHRP} http://www.cs.kuleuven.ac.be/~geert/
Department of Computer Science -- Katholieke Universiteit Leuven -- Belgium
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