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Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.

1	Kernel driver ds2490
2	====================
3	
4	Supported chips:
5	  * Maxim DS2490 based
6	
7	Author: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
8	
9	
10	Description
11	-----------
12	
13	The Maxim/Dallas Semiconductor DS2490 is a chip
14	which allows to build USB <-> W1 bridges.
15	
16	DS9490(R) is a USB <-> W1 bus master device
17	which has 0x81 family ID integrated chip and DS2490
18	low-level operational chip.
19	
20	Notes and limitations.
21	- The weak pullup current is a minimum of 0.9mA and maximum of 6.0mA.
22	- The 5V strong pullup is supported with a minimum of 5.9mA and a
23	  maximum of 30.4 mA.  (From DS2490.pdf)
24	- While the ds2490 supports a hardware search the code doesn't take
25	  advantage of it (in tested case it only returned first device).
26	- The hardware will detect when devices are attached to the bus on the
27	  next bus (reset?) operation, however only a message is printed as
28	  the core w1 code doesn't make use of the information.  Connecting
29	  one device tends to give multiple new device notifications.
30	- The number of USB bus transactions could be reduced if w1_reset_send
31	  was added to the API.  The name is just a suggestion.  It would take
32	  a write buffer and a read buffer (along with sizes) as arguments.
33	  The ds2490 block I/O command supports reset, write buffer, read
34	  buffer, and strong pullup all in one command, instead of the current
35	  1 reset bus, 2 write the match rom command and slave rom id, 3 block
36	  write and read data.  The write buffer needs to have the match rom
37	  command and slave rom id prepended to the front of the requested
38	  write buffer, both of which are known to the driver.
39	- The hardware supports normal, flexible, and overdrive bus
40	  communication speeds, but only the normal is supported.
41	- The registered w1_bus_master functions don't define error
42	  conditions.  If a bus search is in progress and the ds2490 is
43	  removed it can produce a good amount of error output before the bus
44	  search finishes.
45	- The hardware supports detecting some error conditions, such as
46	  short, alarming presence on reset, and no presence on reset, but the
47	  driver doesn't query those values.
48	- The ds2490 specification doesn't cover short bulk in reads in
49	  detail, but my observation is if fewer bytes are requested than are
50	  available, the bulk read will return an error and the hardware will
51	  clear the entire bulk in buffer.  It would be possible to read the
52	  maximum buffer size to not run into this error condition, only extra
53	  bytes in the buffer is a logic error in the driver.  The code should
54	  should match reads and writes as well as data sizes.  Reads and
55	  writes are serialized and the status verifies that the chip is idle
56	  (and data is available) before the read is executed, so it should
57	  not happen.
58	- Running x86_64 2.6.24 UHCI under qemu 0.9.0 under x86_64 2.6.22-rc6
59	  with a OHCI controller, ds2490 running in the guest would operate
60	  normally the first time the module was loaded after qemu attached
61	  the ds2490 hardware, but if the module was unloaded, then reloaded
62	  most of the time one of the bulk out or in, and usually the bulk in
63	  would fail.  qemu sets a 50ms timeout and the bulk in would timeout
64	  even when the status shows data available.  A bulk out write would
65	  show a successful completion, but the ds2490 status register would
66	  show 0 bytes written.  Detaching qemu from the ds2490 hardware and
67	  reattaching would clear the problem.  usbmon output in the guest and
68	  host did not explain the problem.  My guess is a bug in either qemu
69	  or the host OS and more likely the host OS.
70	-- 03-06-2008 David Fries <David@Fries.net>
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