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- # Building a SPI relay for the SMS
-
- The I/O space on the SMS is, sadly, entirely taken. If you had
- the idea of somehow plugging a SPI relay that is similar the one
- on the RC2014, you can forget about it. Only A7, A6 and A0 are
- considered by the 8 builtin peripherals on the SMS and trying to
- do an IN or OUT to any address is going to end up conflicting
- with one of them.
-
- What we can do to achieve SPI communication with the SMS is to
- use the B controller port. It can already do bit banging. It's
- slow, but it works.
-
- One problem we have, however, is that only 2 pins can be set as
- output. We need 3. What I did, and it works with SD cards, is
- to hard-wire CS to GND so that it's always turned on. The
- downside of this is that if you go out-of-sync with the SPI
- device, you have to physically disconnect it and reconnect it
- to solve the sync problem.
-
- The advantage of using port B is that the connector is really
- simple, you don't even need a schematic:
-
- * CLK to TH
- * DI to TR
- * DO to Up
- * CS to GND
-
- Add pull-downs to CLK and DI to avoid messing up with your
- device (it's always on, remember).
-
- # Building the binary
-
- The SPI driver is in B622, which depends on controller port
- routines at B625-B626. A ready-to-use xcomp unit is at
- arch/z80/sms/xcompsdc.fs.
-
- The SMS emulator has support for a SPI relay based on the B
- controller port and can emulate a SD card plugged in it with the
- "-c" argument. If it works in the emulator, it has good chances
- of running on the real thing.
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