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75 lines
2.8 KiB
Plaintext
75 lines
2.8 KiB
Plaintext
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# Building a SPI relay for the z80
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In this recipe, we build a SPI relay (see /doc/hw/spi.txt) for
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a RC2014.
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# Gathering parts
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* A RC2014 Classic
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* A proto board + header pins with 39 positions so we can make
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a RC2014 card.
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* Diodes, resistors and stuff
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* 40106 (Inverter gates)
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* 74xx138 (Decoder)
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* 74xx375 (Latches)
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* 74xx125 (Buffer)
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* 74xx161 (Binary counter)
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* 74xx165 (Parallel input shift register)
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* 74xx595 (Shift register)
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# Building the SPI relay
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The schematic (img/spirelay.jpg) works well with the SD Card
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subsystem (B420). Of course, it's not the only possible design
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that works, but I think it's one of the most straighforwards.
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This relay communicates through the z80 bus with 2 ports, DATA
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and CTL and allows up to 4 devices to be connected to it at
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once, although only one device can ever be active at once. This
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schema only has 2 (and the real prototype I've built from it),
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but the '375 has room for 4. In this schema, DATA is port 4, CTL
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is port 5.
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We activate a device by sending a bitmask to CTL, this will end
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up in the '375 latches and activate the SS pin of one of the
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device, or deactivate them all if 0 is sent.
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You then initiate a SPI exchange by sending a byte to send to
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the DATA port. This byte will end up in the '165 and the '161
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counter will be activated, triggering a clock for the SPI
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exchange. At each clock, a bit is sent to MOSI from the '161 and
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received from MISO into the '595, which is the byte sent to the
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z80 bus when we read from DATA.
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When the '161 is wired to the system clock, as it is in the
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schema, two NOPs are a sufficient delay between your DATA write
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and subsequent DATA read.
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However, if you build yourself some kind of clock override and
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run the '161 at something slower than the system clock, those 2
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NOPs will be too quick. That's where that '125 comes into play.
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When reading CTL, it spits RUNNING into D0. This allows you to
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know when the result of the SPI exchange is ready to be fetched.
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Make sure you AND away other bits, because they'll be garbage.
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The '138 is to determine our current IORQ mode (DATA/CTL and
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WR/RO), the '106 is to provide for those NOTs sprinkled around.
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Please note that this design is inspired by
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https://www.ecstaticlyrics.com/electronics/SPI/fast_z80_interface.html
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Advice 1: Make SCK polarity configurable at all 3 endpoints (the
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595, the 165 and SPI connector). Those jumpers will be useful
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when you need to mess with polarity in your many tinkering
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sessions to come.
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Advice 2: Make input CLK override-able. SD cards are plenty fast
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enough for us to use the system clock, but you might want to
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interact with devices that require a slower clock.
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# Driving the relay
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There is a provider for the SPI protocol (doc/protocol.txt) that
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work with this device in B418. It needs SPI_DATA and SPI_CTL
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constants which in this case are 4 and 5 respectively.
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