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From: BruceMcF on 6 Apr 2008 14:26
(Now you can edit the subject to SPI/I2C, but I don't want to worry
about the precise timing required by I2C, so I'm sticking to SPI).

On Apr 6, 1:09 pm, christianlott1 <christianlo...(a)yahoo.com> wrote:
> > On the side of the SPI chip, I only want to solve the connector
> > problem once. Since the simplest way to plug one in to experiment with
> > it is into a joystick port (imagine being able to plug a fast modem
> > into a C64DTV via a joystick port), I'm looking at putting SPI chips
> > into joystick dongles, and then any other SPI ports will end with a
> > DB9 male wired for the SPI chip in the same joystick dongle.

> not interested in dtv but whatever's easiest for you to put together.
> don't let my delusions of grandeur slow you down. they only get more
> involved from here (even if i got the above correct :)

By the same token, for testing out an SPI dongle, without investing in
any other hardware (User port, datasette port, expansion port), the
easiest way is to plug it into the joystick port, load the routine
that accesses the SPI part on that joystick connector, and away you
go.

Indeed, I am thinking of using the two free lines in the DB-9 (the
paddle inputs) to specify a four-port hub, just so someone who wants
to only have one connector dedicated can use more than one SPI device
at a time.

The way it would work is the complement of the "SPI device number": 0
is 11, 1 is 10, 2 is 01, 3 is 00. That means that if you do not touch
the I/O setting for the paddle lines, they are set as input, which
read as 1's for output, so every device would SELECT on the wired-or
of the Select and Device lines (in the C64 ... AFAIU, C64DVC lines are
open collector, so it would need pull-up resisters added on the
joystick lines) ...

And then the hub needs two invert gates, and puts the appropriate
original line or complement line on the four DB-9's in the hub.

The power these SPI's normally draw is very small (a lot of them are
for embedded applications), so a four way hub is not a serious power
problem. If a dongle violates that, I would want it to use the 5V line
only as a reference voltage for pull-up resisters, and have its own
power supply for its power needs.

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