Am 26.09.2013 07:48, schrieb Colin Reese:
>
> This is a very interesting topic. I've used ds9490s forever in
> various topologies wired individually with RJ and never had an issue
> with finding devices, using maxim's drivers on windows and owfs on
> Linux.
>
I do, even with a bus topology. When I scan continously for new chips I
will certainly get misreadings (a certain chip is errorneously not
found) after a few seconds. It gets worse when there are long leaves.
What is your cable length on individual leaves?
> It definitely gets slower as the number of devices increases,
> but no problems other than that. For cost, I've just implemented a
> DS2483 on I2C, which is $0.60 vs. $27 for the DS9490. I have not yet
> tested it extensively. I had assumed that the 2483 would perform
> better as it doesn't rely on the serial bus and is a newer device. Is
> this not the case, and what performance difference and limitations
> should I expect? I'm on raspbian on an RPi.
>
The BCM2835 I²C master has a four-byte fifo which is used for writing
and reading which is at least a bit better than what other boards with
I²C hosts have. The i2c-bcm2708 driver makes use of that fifo.
BUT, as the DS2483 requires busy polling before issuing the next
command, you cannot make much use of that fifo. For writing a single
byte that's only two bytes in a row, then the write-read-turnaround
clears the fifo. Relevant code is at module/owlib/src/c/ow_ds2482.c:646-653
For scanning the bus, this is especially bad, as it requires 64 times
write byte+read byte per chip on the bus, meaning 128 interrupts in 12ms
(64 * 3 onewire bit times of 60µs, I²C timing neglectable). When you
have a network with a dozen chips *and* want to scan a lock on it three
times a seconds for an iButton, this is just an endless stream of
interrupts.
I had this use case and because of it being unfeasibly slowing down the
whole machine, Paul and me implemented the simultaneous/single_ds2400
node just for checking for a single DS2401 chip independly of the other
chips and the scanning mechanism. It's more reliable, too.
Kind regards
Jan
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