mirror of
https://github.com/hsoft/collapseos.git
synced 2024-11-10 02:48:05 +11:00
132 lines
4.7 KiB
Markdown
132 lines
4.7 KiB
Markdown
# Accessing a MicroSD card
|
|
|
|
SD cards are great because they are accessible directly. No supporting IC is
|
|
necessary. The easiest way to access them is through the SPI protocol.
|
|
|
|
Due to the way IO works in z80, implementing SPI through it as a bit awkward:
|
|
You can't really keep pins high and low on an IO line. You need some kind of
|
|
intermediary between z80 IOs and SPI.
|
|
|
|
There are many ways to achieve this. This recipe explains how to build your own
|
|
hacked off SPI relay for the RC2014. It can then be used with `sdc.fs` to
|
|
drive a SD card.
|
|
|
|
## Goal
|
|
|
|
Read and write to a SD card from Collapse OS using a SPI relay of our own
|
|
design.
|
|
|
|
## Gathering parts
|
|
|
|
* A RC2014 Classic
|
|
* A MicroSD breakout board. I use Adafruit's.
|
|
* A proto board + header pins with 39 positions so we can make a RC2014 card.
|
|
* Diodes, resistors and stuff
|
|
* 40106 (Inverter gates)
|
|
* 4011 (NAND gates)
|
|
* 74xx139 (Decoder)
|
|
* 74xx161 (Binary counter)
|
|
* 74xx165 (Parallel input shift register)
|
|
* 74xx595 (Shift register)
|
|
|
|
## Building the SPI relay
|
|
|
|
The [schematic][schematic] supplied with this recipe works well with `sdc.fs`.
|
|
Of course, it's not the only possible design that works, but I think it's one
|
|
of the most straighforwards.
|
|
|
|
The basic idea with this relay is to have one shift register used as input,
|
|
loaded in parallel mode from the z80 bus and a shift register that takes the
|
|
serial input from `MISO` and has its output wired to the z80 bus.
|
|
|
|
These two shift registers are clocked by a binary counter that clocks exactly
|
|
8 times whenever a write operation on port `4` occurs. Those 8 clocks send
|
|
data we've just received in the `74xx165` into `MOSI` and get `MISO` into the
|
|
`74xx595`.
|
|
|
|
The `74xx139` then takes care of activating the right ICs on the right
|
|
combinations of `IORQ/WR/RD/Axx`.
|
|
|
|
The rest of the ICs is fluff around this all.
|
|
|
|
My first idea was to implement the relay with an AVR microcontroller to
|
|
minimize the number of ICs, but it's too slow. We have to be able to respond
|
|
within 300ns! Following that, it became necessary to add a 595 and a 165, but
|
|
if we're going to add that, why not go the extra mile and get rid of the
|
|
microcontroller?
|
|
|
|
To that end, I was heavily inspired by [this design][inspiration].
|
|
|
|
This board uses port `4` for SPI data, port `5` to pull `CS` low and port `6`
|
|
to pull it high. Port `7` is unused but monopolized by the card.
|
|
|
|
Little advice: If you make your own design, double check propagation delays!
|
|
Some NAND gates, such as the 4093, are too slow to properly respond within
|
|
a 300ns limit. For example, in my own prototype, I use a 4093 because that's
|
|
what I have in inventory. For the `CS` flip-flop, the propagation delay doesn't
|
|
matter. However, it *does* matter for the `SELECT` line, so I don't follow my
|
|
own schematic with regards to the `M1` and `A2` lines and use two inverters
|
|
instead.
|
|
|
|
## Building your binary
|
|
|
|
Your Collapse OS binary needs the SDC drivers which need to be inserted during
|
|
Cross Compilation, which needs you need to recompile it from stage 1. First,
|
|
look at B600. You'll see that it indicates a block range for the driver. That
|
|
needs to be loaded.
|
|
|
|
Open xcomp.fs from base recipe and locate acia loading. You'll insert a line
|
|
right after that that will look like:
|
|
|
|
602 616 LOADR ( sdc )
|
|
|
|
Normally, that's all you need to do. However, you have a little problem: You're
|
|
busting the 8K ROM limit. But it's ok, you can remove the linker's XPACKing
|
|
line: because you'll have access to the blkfs from SD card, you can load it
|
|
from there!
|
|
|
|
Removing the linker from XPACKing will free enough space for your binary to fit
|
|
in 8K. You also have to add `BLK$` to initialization routine.
|
|
|
|
Build it and write it to EEPROM.
|
|
|
|
If you want, once you're all set with the SD card, you can relink core words
|
|
like you did in the base recipe for optimal resource usage.
|
|
|
|
## Testing in the emulator
|
|
|
|
The RC2014 emulator includes SDC emulation. You can attach a SD card image to
|
|
it by invoking it with a second argument:
|
|
|
|
../../../emul/hw/rc2014/classic stage3.bin ../../../emul/blkfs
|
|
|
|
You will then run with a SD card having the contents from `/blk`.
|
|
|
|
## Usage
|
|
|
|
First, the SD card needs to be initialized
|
|
|
|
SDC$
|
|
|
|
If there is no error message, we're fine. Then, we need to hook `BLK@*` and
|
|
`BLK!*` into the SDC driver:
|
|
|
|
' SDC@ BLK@* !
|
|
' SDC! BLK!* !
|
|
|
|
And thats it! You have full access to disk block mechanism:
|
|
|
|
102 LOAD
|
|
BROWSE
|
|
|
|
(at this moment, the driver is a bit slow though...)
|
|
|
|
## How do I fill my SD card with Collapse OS' FS?
|
|
|
|
Very easy. You see that `/emul/blkfs` file? You dump it to your raw device.
|
|
For example, if the device you get when you insert your SD card is `/dev/sdb`,
|
|
then you type `cat emul/blkfs | sudo tee /dev/sdb > /dev/null`.
|
|
|
|
[schematic]: spirelay/spirelay.pdf
|
|
[inspiration]: https://www.ecstaticlyrics.com/electronics/SPI/fast_z80_interface.html
|