Hi Herbert.

Just question out of curiousity.

Why that impossible to one person to use current source
in place of pull up resistor with the same amount of current
as pull up resistor provides when it's grounded.
Charging up the bus capacitance will become linear
and thus takes less time.


WBR Dmitry.


Herbert Graf wrote:
> 
> On Sun, 2005-07-10 at 17:06 +1200, Jinx wrote:
> > > While I more commonly use lower values, 10k is completely reasonable
> > > in my mind for "normal" I2C speeds which are on the slower side, and
> > > definitely doesn't imply running < 10khz.
> >
> > The major factor I believe is buss capacitance. PIC i/o have pretty fast
> > rise and fall times so I guess they're quite low C. No or short cable
> > wouldn't present too much load
> 
> True, but note that the value of the pull up is more to do with the pull
> up "pulling up" the bus, then the time it takes the device to pull down
> the bus.
> 
> In all applications of I2C where I've plunked down a scope the "pull
> down" time by whatever device is driving is MUCH faster then the "pull
> up" time of the resistors' action.
> 
> If you have a longer bus (or perhaps just an average length bus but iwth
> a crap load of devices), or want to go at a greater speed, then a lower
> pull up is definitely a good idea, but for more average speeds and bus
> configs 10k seems reasonable. However, with all that said, unless power
> is of concern, you almost never loose by using a lower pull up.
> 
> In the end it doesn't matter either way, the op found the problem.
> 
> TTYL
> 
> 
> -----------------------------
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> http://repatch.dyndns.org:8383/pic_stuff/
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