Re: C= and internet

From: Martijn van Buul (
Date: 2001-02-16 10:16:09

Nicolas Welte wrote:
> I have a (German language) book for the C64 that describes a I2C interface on
> the cassette port, it is used to implement a real time clock interface. Since
> the I2C electrical interface is almost the same as the C64 serial bus,

Only if you use short wiring; the bus isn't terminated correctly.

> it should be possible to drive I2C devices just with software. Of course you
> would lose the possibility to use standard serial port devices then, but for
> playing around with I2C it should be fine. The only problem is speed: If the
> C64 is master, there isn't a problem, but if you want to use it as an I2C bus
> logger or slave device, the maximum bus speed of 100 or even 150kHz might be
> too much. 

You're absolutely correct on the data-logger part, but a slave may be 
very slow. I2C acknowledges every single bit, so the bitrate is effectively
determined by the slowest device on the bus. For instance, I played with
an I2C 'analyzer', which attached to the paralell port of a PC. It could
act like slaves and masters too, at the 400kbit/s 'unofficial' standard.
However, when accessed as slave, it streched the clock for more than 3ms
between each byte being sent. Furthermore, I just finished my (C-based)
driver for the 87LPC76x-series of Low-Pin Count processors. These things
have a bit-oriented I2C interface (meaning that every single bit has to
be handled by software; a byte-wise interface would've been a lot better).
Because these processors aren't really fast, you'll never be able to get
a 100kbit transmission rate. 

Having a software-based master talking to a hardware-based slave is no
problem, a hardware-based master talking to a software-based slave 
works fine as well. Things start to get icky if you have a software-based
master talking to a software-based slave, or a hardware-based master
talking to several software-based slaves at the same time.

As a side note, the I2C bus is rather limited because of its master-slave
setup, which is the reason that most I2C busses are used as multi-master
bus these days. Hardware support is *required* for a multi-master setup,
otherwise things go terribly wrong.

I spent the better part of the last 3/4 year developing I2C drivers for
Philips, so I hope I know where I'm talking about..

> But I doubt that this limit is ever reached in real-life, micro
> controller operated applications.

It is. I've seen a few demonstrations about the I2C bus running at +/- 6Mbit/s.

    Martijn van Buul - -
	 Geek code: G--  - Visit OuterSpace: 3333
   Kees J. Bot: The sum of CPU power and user brain power is a constant.
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