[MinnowBoard] About SPI Chip Select, MinnowMax.

[Ferchu R]

Hi Michael, thanks for your comments. At this time, my thinking is that
your suggestion is the most sensible walk around to the lack of CS2.

By the way. August, last year, David Anders told about a "MiniDisplay Lure"
within the "Lure plans", stating that "the minidisplay lure will be similar
in nature to this one <http://www.adafruit.com/products/1770> sold from
adafruit". I was wondering if CircuitCo still having plans for
manufacturing this lure, and what interface was going to be the selected
one for Display and Touchscreen.

Thanks.

<http://www.adafruit.com/products/1770>

2015-04-16 17:08 GMT-05:00 Krau, Michael P <michael.p.krau at intel.com>:

> In short terms, you are suggesting the conversion of a single slave SPI
> BUS into an Independent Slave Configuration through the use of the GPIOs.
>
> The thing is that the Chip Select line provided  is part of the interface
> as defined.  The timing of the bus is established in the hardware, and the
> CS1 signal is part of that timing.  So CS1 will go low whenever the bus is
> active.
>
> So, while there are technical ways to provide multiplexing for the chip
> select, but it will most likely require some discrete logic (or two
> GPIOS).  It would be very difficult to use the current CS and the GPIO pin
> directly, as the CS pin will go low whenever the bus is in use.
>
> Method one Example:
> CS1 .AND. GPIOnn == CS.Device1
> CS1 .AND. .NOT. GPIOnn == CS.Device2
>
> Where  the terms .AND. and .NOT. are discrete logic components.
> Basically, using the combination of the two signals to create two new chip
> selects.
>
> GPIO and .NOT. GPIO establishes the device select, while the Chip Select
> line retains its function to regulate the overall chip select in regards to
> the data transfer.  (there is a question of  gate delays of the signal
> which could risk communication, but there are ways around this if it is an
> issue)
>
> Method two: For using GPIOs:
> GPIOnn1 = CS.Device1
> GPIOnn2 = CS.Device2
> Ignore CS1 completely, but use software to handle enabling and disabling
> chip select.
>
> Then the driver to support the SPI IO either has to be modified, or a
> front end (shim) added to the interface procedures to add the parameter for
> device selection and manipulate the GPIO line(s) selected for the purpose
> (device select - device 1 or 2) in the mix.   How this works depends upon
> the method.
>
> Also, remember doing this is playing around with bus timings.  Method one
> will delay the chip select to the device by the time delay of the gate
> involved.  Method two will most likely enable the slave device before the
> master interface actually signals the slave enable (making the
> communication susceptible to bus noise during the expanded window) and keep
> it enabled longer..
>
> That is the technical (academic) overview of the general question
>
> Michael Krau
>
> While I am an Intel employee, I do not represent Intel and am not
> authorized to speak for Intel.
> -----Original Message-----
> From: elinux-MinnowBoard [mailto:
> elinux-minnowboard-bounces at lists.elinux.org] On Behalf Of John Hawley
> Sent: Thursday, April 16, 2015 1:43 PM
> To: MinnowBoard Development and Community Discussion
> Subject: Re: [MinnowBoard] About SPI Chip Select, MinnowMax.
>
> My understanding is that the GPIOs can be used as a chip select, however
> the exact steps needed to make them work is something I haven't looked into.
>
> - John
>
> On 4/9/2015 11:59 AM, Ferchu R wrote:
> > Hi.
> >
> > I want to interface a display/touch screen (ILI9325/ADS7843 based)
> > with Minnowboard Max. It uses the SPI port, but it has to be shared
> > between the LCD controller and the touch controller. However it seems
> > that Minnowboard has only one "SPI Chip Select 1". Could other GPIO be
> > used as "SPI Chip Select 2"?. What changes could imply this, regarding
> software?.
> >
> > Thanks.
> >
> >
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> >
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