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Re: hal/arm/at91/../var_io.h: HAL_ARM_AT91_GPIO_GET


JÃrgen Lambrecht <J.Lambrecht@televic.com> writes:

> Nick Garnett wrote:
> > JÃrgen Lambrecht <J.Lambrecht@televic.com> writes:
> >
> >
> >> Hello,
> >>
> >> The current implementation of HAL_ARM_AT91_GPIO_GET in
> >> hal/arm/at91/../var_io.h uses the AT91_PIO_PDSR register.
> >> But, "Reading the I/O line levels requires the clock of the PIO
> >> Controller to be enabled". So if you forget to enable the related PIO
> >> clock, this HAL_ARM_AT91_GPIO_GET always fails!
> >> Therefore, I propose to use the AT91_PIO_ODSR register.
> >> Anyhow, the added value of PDSR over ODSR is small I think:
> >> -with ODSR you read the value you want this pin to be
> >> -with PDSR you read the actual (physical) value of this pin
> >> They are only different if there is a hardware problem..
> >>
> >
> > Without reading the documentaion: the ODSR give you the contents of
> > the output latch, while PDSR reads the line value.  ODSR and PDSR
> > should agree for output pins, but for inputs the ODSR may differ from
> > the line level, since it is no longer connected to the pin and pin is
> > being driven to its current level by some external device.
> >
> Indeed. So in general, my proposition is wrong.
> I then propose to make an "OUT" version of that macro.
> > Also, surely if you are using a GPIO port you should be enabling its
> > clock. I would consider not doing that a programming error.
> >
> No, only for an input you must enable the PIO clock: the PIO clock is
> then used to sample the input signal.
> For an output, you just write the output register from your
> application, using the application clock.

OK. In that case a macro that reads back the value set in the output
register might make sense.

Of course one can make the argument that since your code must have set
this value initially, it shouldn't have to read it back to know what
it is. However, I appreciate that code is not always structured to
allow you to do that.


-- 
Nick Garnett                                      eCos Kernel Architect
eCosCentric Limited    http://www.eCosCentric.com      The eCos experts
Barnwell House, Barnwell Drive, Cambridge, UK.     Tel: +44 1223 245571
Registered in England and Wales:                        Reg No: 4422071


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