On Mon, Mar 19, 2018 at 08:55:55PM +0100, Peter Rosin wrote: > On 2018-03-19 19:48, Guenter Roeck wrote: > > On Mon, Mar 19, 2018 at 05:47:05PM +0100, Peter Rosin wrote: > >> I also wonder if NXP will ever release a chip with part-id 0 and > >> die-revision 0? If not, an all zero struct i2c_device_identity > >> could be used instead of manufacturer_id 0xffff and that would > >> simplify the pca954x driver code a bit more. But I guess we can > >> never know the answer to that question. And even if we did, the > >> answer might change later. But it would be nice... > >> > > > > That would be nice. You could ask at i2c.support@xxxxxxx, but I guess > > it would always be somewhat risky since the standard doesn't restrict > > its use, and some product manager at NXP might decide in the future > > that a device ID of 0x00 would be "cool". > > No need to bother NXP, PCA9848 has already claimed 0-0-0. Sigh. > > But while I googled that I found old datasheets for the chips PCA9672 > through PCA9675 which use a different layout for the three bytes in > the device id. They have 8 manufacturer bits, 7 category bits, 6 bits > of feature indication and then 3 bits of revision. The top category > bits are zero so it is compatible for NXP chips. But since noone else > has implemented this, it is probably safe, but still a little bit > disturbing. > The PCA9570 datasheet is especially interesting. "9 bits with the part identification, assigned by manufacturer, the 7 MSBs with the category ID and the 6 LSBs with the feature ID (for example PCA9570 4-bit I/O expander)" Maybe there is a magic compression scheme to squash 7 MSBs and 6 LSBs into a 9-bit field, or the category and feature IDs have quite some overlap, or the document would benefit from some proof-reading. > I also found that NXP apparently uses the same part id (0x100) and die > revision (0) for PCA9570 and PCA9670. That seems odd. > ... especially since one has 4 channels and the other has 8 channels. It would be interesting to see if reality and datasheets match; this might as well be a curt-and-paste error. Of course, it might as well be that both chips use the same die and that some pins are just not exposed on the 4 channel version. > Example old datasheet (2006): > https://www.digchip.com/datasheets/download_datasheet.php?id=1098812&part-number=PCA9672 > On the other side this has been corrected in more recent datasheet versions, so I would not be too concerned about that. Guenter
