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135 lines
5.1 KiB
Markdown
135 lines
5.1 KiB
Markdown
# Accessing a MicroSD card
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SD cards are great because they are accessible directly. No supporting IC is
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necessary. The easiest way to access them is through the SPI protocol.
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Due to the way IO works in z80, implementing SPI through it as a bit awkward:
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You can't really keep pins high and low on an IO line. You need some kind of
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intermediary between z80 IOs and SPI.
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There are many ways to achieve this. This recipe explains how to build your own
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hacked off SPI relay for the RC2014. It can then be used with the SD Card
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subsystem (B420) to drive a SD card.
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## Gathering parts
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* A RC2014 Classic
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* A MicroSD breakout board. I use Adafruit's.
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* A proto board + header pins with 39 positions so we can make 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 above works well with the SD Card subsystem (B420). Of course,
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it's not the only possible design that works, but I think it's one of the most
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straighforwards.
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This relay communicates through the z80 bus with 2 ports, `DATA` and `CTL` and
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allows up to 4 devices to be connected to it at once, although only one device
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can ever be active at once. This schema only has 2 (and the real prototype I've
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built from it), but the '375 has room for 4. In this schema, `DATA` is port 4,
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`CTL` is port `5`.
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We activate a device by sending a bitmask to `CTL`, this will end up in the
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'375 latches and activate the `SS` pin of one of the device, or deactivate them
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all if `0` is sent.
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You then initiate a SPI exchange by sending a byte to send to the `DATA` port.
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This byte will end up in the '165 and the '161 counter will be activated,
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triggering a clock for the SPI exchange. At each clock, a bit is sent to `MOSI`
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from the '161 and received from `MISO` into the '595, which is the byte sent to
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the 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 schema, two `NOP`s
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are a sufficient delay between your `DATA` write and subsequent `DATA` read.
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However, if you build yourself some kind of clock override and run the '161 at
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something slower than the system clock, those 2 `NOP`s will be too quick. That's
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where that '125 comes into play. When reading `CTL`, it spits `RUNNING` into
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`D0`. This allows you to know when the result of the SPI exchange is ready to be
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fetched. 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 `WR/RO`), the
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'106 is to provide for those `NOT`s sprinkled around.
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Please note that this design is inspired by [this design][inspiration].
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Advice 1: Make `SCK` polarity configurable at all 3 endpoints (the 595, the 165
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and SPI connector). Those jumpers will be useful when you need to mess with
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polarity in your many tinkering sessions to come.
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Advice 2: Make input `CLK` override-able. SD cards are plenty fast enough for
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us to use the system clock, but you might want to interact with devices that
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require a slower clock.
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## Building your binary
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The binary built in the base recipe doesn't have SDC drivers. You'll need to
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assemble a binary with those drivers. To do so, you'll modify the xcomp unit
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of the base recipe. Look at `xcomp.fs`, you'll see that we load a block. That's
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our xcomp block (likely, B599). Open it.
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First, we need drivers for the SPI relay. This is done by declaring `SPI_DATA`
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and `SPI_CTL`, which are respectively `4` and `5` in our relay design.
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You also need to tell the SDC subsystem which SPI device to activate by defining
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the `SDC_DEVID` (1, 2, 4, 8 for device 0, 1, 2 or 3)
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You can then load the driver with `616 LOAD`. This driver provides
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`(spix)` and `(spie)` which are then used in the SDC driver.
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The SDC driver is at B420. It gives you a load range. This means that what
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you need to insert in `xcomp` will look like:
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423 436 LOADR ( sdc )
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You also need to add `BLK$` to the init sequence.
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Build it (run `make pack` in `cvm/` first to ensure an up-to-date blkfs) and
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write it to EEPROM.
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## Testing in the emulator
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The RC2014 emulator includes SDC emulation. You can attach a SD card image to
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it by invoking it with a second argument:
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../../../emul/hw/rc2014/classic os.bin ../../../cvm/blkfs
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You will then run with a SD card having the contents from `/blk`.
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## Usage
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First, the SD card needs to be initialized
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SDC$
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If there is no error message, we're fine. Then, we need to hook `BLK@*` and
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`BLK!*` into the SDC driver:
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' SDC@ BLK@* !
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' SDC! BLK!* !
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And thats it! You have full access to disk block mechanism:
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105 LOAD
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BROWSE
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(at this moment, the driver is a bit slow though...)
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## How do I fill my SD card with Collapse OS' FS?
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Very easy. You see that `/cvm/blkfs` file? You dump it to your raw device.
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For example, if the device you get when you insert your SD card is `/dev/sdb`,
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then you type `cat emul/blkfs | sudo tee /dev/sdb > /dev/null`.
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[inspiration]: https://www.ecstaticlyrics.com/electronics/SPI/fast_z80_interface.html
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