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
stage3.bin from the base recipe
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 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.

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 stage 4

Using the same technique as you used for building your stage 3, you can append required words to your boot binary. Required units are forth/blk.fs and drv/sdc.fs. You also need drv/sdc.z80 but to save you the troubles of rebuilding from stage 1 for this recipe, we took the liberty of already having included it in the base recipe.

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 stage4.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...)

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