Two start conditions for PCA9554

Two start conditions for PCA9554

[Hans J. Koch]

I have an embedded board with a PCA9554 on it. It's an 8-bit IO expander
similar to the PCF8574 in mainline. One difference is that when reading
from the inputs, you have to do the following:

- start condition
- send slave address, R/W=0
- send command (0x00 = read input register)
- issue another start condition (!)
- send slave address, R/W=1
- read data
- stop condition

Can such a protocol be handled by the i2c subsystem?

Thanks,
Hans


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Re: Two start conditions for PCA9554

[Trent Piepho]

On Tue, 29 Apr 2008, Hans J. Koch wrote:
> I have an embedded board with a PCA9554 on it. It's an 8-bit IO expander
> similar to the PCF8574 in mainline. One difference is that when reading
> from the inputs, you have to do the following:
>
> - start condition
> - send slave address, R/W=0
> - send command (0x00 = read input register)
> - issue another start condition (!)
> - send slave address, R/W=1
> - read data
> - stop condition
>
> Can such a protocol be handled by the i2c subsystem?

Yes.  That is a normal repeated start transaction, and AFAIK exactly what
all the other PCA, and virtually every other i2c chip for that matter, use.
The smbus read byte data function will do this for you.

If you want multiple stops in a single transaction, the i2c subsystem can't
do it, but I wrote a simple patch that allows it.

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Re: Two start conditions for PCA9554

[David Brownell]

> I have an embedded board with a PCA9554 on it. It's an 8-bit IO expander
> similar to the PCF8574 in mainline.

More like the pca9534 or pca9538.


> Can such a protocol be handled by the i2c subsystem?

Try to tell drivers/gpio/pca953x.c that it's a pca9534;
I think they're register-compatible.

- Dave

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Re: Two start conditions for PCA9554

[Hans J. Koch]

On Wed, Apr 30, 2008 at 11:01:56PM -0700, David Brownell wrote:
> > I have an embedded board with a PCA9554 on it. It's an 8-bit IO expander
> > similar to the PCF8574 in mainline.
> 
> More like the pca9534 or pca9538.
> 
> 
> > Can such a protocol be handled by the i2c subsystem?
> 
> Try to tell drivers/gpio/pca953x.c that it's a pca9534;
> I think they're register-compatible.

Ah, I only looked in drivers/i2c/chips. I'll give it a try.

Thanks a lot for that hint,
Hans

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Re: Two start conditions for PCA9554

[Hans J. Koch]

On Wed, Apr 30, 2008 at 02:26:22PM -0700, Trent Piepho wrote:
> On Tue, 29 Apr 2008, Hans J. Koch wrote:
> > I have an embedded board with a PCA9554 on it. It's an 8-bit IO expander
> > similar to the PCF8574 in mainline. One difference is that when reading
> > from the inputs, you have to do the following:
> >
> > - start condition
> > - send slave address, R/W=0
> > - send command (0x00 = read input register)
> > - issue another start condition (!)
> > - send slave address, R/W=1
> > - read data
> > - stop condition
> >
> > Can such a protocol be handled by the i2c subsystem?
> 
> Yes.  That is a normal repeated start transaction, and AFAIK exactly what
> all the other PCA, and virtually every other i2c chip for that matter, use.
> The smbus read byte data function will do this for you.

OK, thanks. I admit, I would be strange if the i2c subsystem didn't
support that.

> 
> If you want multiple stops in a single transaction, the i2c subsystem can't
> do it, but I wrote a simple patch that allows it.

No need for that now, but I'll remember that...

Thanks,
Hans


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