I'm thinking it would be pretty easy to write a SPI protocol driver
that connects an overo to a bunch of spi devices on the same 3 wires,
(CLK, MOSI, MISO) and use GPIO to select the slaves.

You might not be real fast and all the work queue code in the
omap2_mcspi driver would be wasted, but it seems like it would work.

Is there an electrical limit to the number of slaves on a SPI bus?

The protocol driver would be in userspace.

ads7846.c (and a copy that I tweak, ads1278) is in the kernel.

The first difficulty is detection of 1278 by the init boot.
(init.d, rc0.d, etc...)

I look at how your sample driver in userspace avoids board_overo.c and .modalias.


Ion A. Beza.

On Thu, Feb 25, 2010 at 3:17 PM, Scott Ellis <scottellis.developer@gmail.com> wrote:
I'm thinking it would be pretty easy to write a SPI protocol driver
that connects an overo to a bunch of spi devices on the same 3 wires,
(CLK, MOSI, MISO) and use GPIO to select the slaves.

You might not be real fast and all the work queue code in the
omap2_mcspi driver would be wasted, but it seems like it would work.

Is there an electrical limit to the number of slaves on a SPI bus?


On 2/25/10, Dave Hylands <dhylands@gmail.com> wrote:
> Hi Ion (sorry just noticed you've been signing Ion, but your email says
> Tony),
>
> On Thu, Feb 25, 2010 at 1:54 PM, Tony Oxendahl <toxend@gmail.com> wrote:
>> Good catch, Dave:
>>
>>> SPI_3WIRE refers to Chip-Select, Data, Clock, where both the input
>>> data and output data share the same pin.
>>
>> This means that the omap2_mcspi.c works for 4-wire SPI, might work for
>> 3-wire SPI
>> (as defined by MISO=MOSI, Chip Select, Clock), but not for 3-wire
>> MISO-MOSI-Clock-NoChipSelect
>> which the ADS1278 is.
>
> No - it will work fine for the last case - worst case is that you just
> have to waste a pin for the chip select which doesn't get connected to
> anything.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.DaveHylands.com/
>
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