Re: PCF857x and 16-bit GPIO expanders

[Felipe Balbi <balbi@ti.com>]

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Hi,

On Thu, Sep 19, 2013 at 07:36:33AM -0500, Nishanth Menon wrote:
> On 09/19/2013 07:20 AM, George Cherian wrote:
> > On 9/19/2013 5:37 PM, Nishanth Menon wrote:
> >> On 09/19/2013 03:13 AM, George Cherian wrote:
> >>> On 9/18/2013 11:06 PM, Felipe Balbi wrote:
> >>>> Hi,
> >>>>
> >>>> On Wed, Sep 18, 2013 at 07:18:04PM +0200, Laurent Pinchart wrote:
> >>>>> On Wednesday 18 September 2013 13:16:27 Linus Walleij wrote:
> >>>>>> On Tue, Sep 17, 2013 at 9:07 PM, Felipe Balbi <balbi@ti.com> wrote:
> >>>>>>> has anyone ever successfully using gpio-pcf857x.c driver with 16-bit
> >>>>>>> gpio expanders ? We're having some issues here where toggling the last
> >>>>>>> gpio pin (gpio 15) on a PCF8575 device causes platform to hang and I
> >>>>>>> can't come up with any explanation of why would it hang...
> >>>>>> Bouncing the question to George, Laurent and Kuninori...
> >>>>> I've got a board with a PCF8575 chip, but it uses I/Os 8 to 14 only as far as
> >>>>> I know.
> >>>>>
> >>>>> I can try toggling I/O 15, but that will need to wait until next week as I'm
> >>>>> currently travelling without access to the hardware.
> >>>> alright, that'd help me a lot :-) Just want to make sure if we're having
> >>>> a board issue, or PCF8575 is a bit screwy ;-)
> >>> Is it on dra7x-evm if so which pcf device (i2c address)?
> >>> The pins i were interested were only 1 and 2 I never tried pin 15.
> >>>
> >>> Just tried toggling through sysfs and it works for me.
> >> When I look at the data sheet for PCF8575[1] Page 7, Figure 4 Write
> >> mode (output)
> >> I see the data writes are of the order:
> >> I2c 1's byte: address
> >> I2c 2'nd byte:P[7-0]
> >> I2c 3rd byte:P[17-10]
> > 
> > I read it as an octal numbering.
> >> Note: bits 8,9 are missing not supported.
> > 
> > In octal there is no 8 and 9
> 
> Where is octal coming into play here? P8 and 9 does not exist as per
> the data sheet -> look at the pinout on page 1[1] ->P00-P17 this is
> exactly what is described on page 2[1]:
> "The number of data bytes that can be sent successively is not limited
> After every two bytes, the previous data is overwritten. When the
> PCF8575 receives the pairs of data bytes, the first byte is referred
> to as P07 – P00 and the second byte as P17 – P10. The third byte
> is referred to as P07 – P00, the fourth byte as P17 – P10, and so
> on"
> 
> For someone reading schematics and setting up the P15, if the person
> uses gpios = <&PCF8575 15 OF_GPIO_HIGH>; this will result in offset =
> 15, and as a result 0x80 will be send in byte 3, which from h/w point
> of view is P13 which could be controlling something weird!

you missed one detail only. I said GPIO15, not P15 :-) GPIO15 on that
device is P17 ;-)

-- 
balbi

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