On 17.11.2015 06:39, Nigel Titley wrote:
> iButtonlink have confirmed that the Link45/LinkUSB doesn't crawl the OW
> bus but expects the higher level code to do this.
The question is whether it supports the "match ROM + command+data +
search ROM" sequence used to selectively enumerate the buses behind a
DS2409, as required when you use one of the Smart-On commands.
Doing this in higher-level code, i.e. by permanently turning on the main
or aux buses, is not trivial. You'd need (external) locking, and be very
careful about which bus segment you associate newly-discovered devices to.
On the other hand, all things considered I'd expect things to be faster
with a permanent bus connection, since I'd only have to send a bus reset
once per command (plus maybe one to select the correct bus), not per
traversed segment. Similarly for conditional search, which is going to
be a win for multiple DS2409s on a bus (like in a hub).
NB: while we're looking at the DS2409 code ... OWCH:
/* 2409 switch -- branch status -- note that bit value is reversed */
static ZERO_OR_ERROR FS_r_branch(struct one_wire_query *owq)
{
BYTE data = 0 ;
OWQ_U(owq) = (((data) & 0x01) | ((data >> 1) & 0x02)) ^ 0x03;
return GB_to_Z_OR_E(OW_r_control(&data, PN(owq))) ;
}
Wouldn't it make slightly more sense to *first* read data and *then* set
the return value? :-P
Since this code demonstrably cannot have worked since 2010 (Paul did a
"minor cleanup" that broke it, cf. git change fc0c93a31) and nobody
noticed, I propose to replace the currently-useless "branch" two-bit
array with a single value (0: no bus selected, 1:main, 2:aux) that can
be read and written to, in case one wants to (or has to) manually switch
branches in external code.
Any objections? Otherwise I'll prepare a patch.
--
-- Matthias Urlichs
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