97 lines
3.6 KiB
Markdown
97 lines
3.6 KiB
Markdown
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# Accessing a MicroSD card
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**Status: work in progress.**
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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 `sdc.asm` to
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drive a SD card.
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## Goal
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Read and write to a SD card from Collapse OS using a SPI relay of our own
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design.
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## Gathering parts
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* A RC2014 with Collapse OS with these features:
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* shell
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* blockdev
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* sdc
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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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* 4011 (NAND gates)
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* 74xx139 (Decoder)
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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][schematic] supplied with this recipe works well with `sdc.asm`.
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Of course, it's not the only possible design that works, but I think it's one
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of the most straighforwards.
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The basic idea with this relay is to have one shift register used as input,
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loaded in parallel mode from the z80 bus and a shift register that takes the
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serial input from `MISO` and has its output wired to the z80 bus.
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These two shift registers are clocked by a binary counter that clocks exactly
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8 times whenever a write operation on port `4` occurs. Those 8 clocks send
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data we've just received in the `74xx165` into `MOSI` and get `MISO` into the
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`74xx595`.
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The `74xx139` then takes care of activating the right ICs on the right
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combinations of `IORQ/WR/RD/Axx`.
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The rest of the ICs is fluff around this all.
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My first idea was to implement the relay with an AVR microcontroller to
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minimize the number of ICs, but it's too slow. We have to be able to respond
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within 300ns! Following that, it became necessary to add a 595 and a 165, but
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if we're going to add that, why not go the extra mile and get rid of the
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microcontroller?
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To that end, I was heavily inspired by [this design][inspiration].
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This board uses port `4` for SPI data, port `5` to pull `CS` low and port `6`
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to pull it high. Port `7` is unused but monopolized by the card.
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Little advice: If you make your own design, double check propagation delays!
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Some NAND gates, such as the 4093, are too slow to properly respond within
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a 300ns limit. For example, in my own prototype, I use a 4093 because that's
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what I have in inventory. For the `CS` flip-flop, the propagation delay doesn't
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matter. However, it *does* matter for the `SELECT` line, so I don't follow my
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own schematic with regards to the `M1` and `A2` lines and use two inverters
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instead.
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## Building the kernel
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To be able to work with your SPI relay and communicate with the card, you
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should have [glue code that looks like this](glue.asm).
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Initially, when you don't know if things work well yet, you should comment out
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the block creation part.
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## Testing CD card initialization
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To test that you can properly initialize a SD card, you can compile this [user
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program](sdinit.asm) (see [Makefile](Makefile)) and then
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[run it from memory][run-from-mem]. Success means the card is initialized.
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## Create a block device from the SD card reader
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TODO
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[schematic]: spirelay/spirelay.pdf
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[inspiration]: https://www.ecstaticlyrics.com/electronics/SPI/fast_z80_interface.html
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[run-from-mem]: ../../../doc/load-run-code.md
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