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