TECHNICAL: how fast is fast?

[xstamp]

I have successfully used ‘fast’ I2C on an 18X operating at 8MHz, with a single peripheral device that is rated for a maximum bus speed of 1Mhz.

Have I just been lucky or can we expect reliable operation from a PICAXE running an I2C bus at 800khz? If we can, what might the limitation on pull-up resistance and supply voltage be?

 

[andrewpro]

I've gone all the way to 1.6mhz on a 40X@16mhz with no problems at all. The faster you go, the lower your pullup should be. At that speed I was using ~1.5k and everything was just spiffy. Check this out:

<A href='http://research.plistin.com/picaxe/overclocki2c.html' Target=_Blank>External Web Link</a>

--Andy P

 

[Technical]

The 18X maximum i2c speed is only limited by the speed of operation of the 18X's internal resonator.

If it works, there is no harm in using it, but you will never reach the highest bus speeds possible with an overclocked 28X (external resonator).

 

[xstamp]

Many thanks to you both!