Writing to a AT28 from Collapse OS

Goal

Write in an AT28 EEPROM from within Collapse OS so that you can have it update itself.

Gathering parts

A RC2014 Classic that could install the base recipe
An extra AT28C64B
1x 40106 inverter gates
Proto board, RC2014 header pins, wires, IC sockets, etc.

Building the EEPROM holder

The AT28 is SRAM compatible so you could use a RAM module for it. However, there is only one RAM module with the Classic version of the RC2014 and we need it to run Collapse OS.

You could probably use the 64K RAM module for this purpose, but I don't have one and I haven't tried it. For this recipe, I built my own module which is the same as the regular ROM module but with WR wired and geared for address range 0x2000-0x3fff.

If you're tempted by the idea of hacking your existing RC2014 ROM module by wiring WR and write directly to the range 0x0000-0x1fff while running it, be aware that it's not that easy. I was also tempted by this idea, tried it, but on bootup, it seems that some random WR triggers happen and it corrupts the EEPROM contents. Theoretically, we could go around that by putting the AT28 in write protection mode, but I preferred building my own module.

I don't think you need a schematic. It's really simple.

Building the kernel

For this recipe to work, we need a block device for the at28w program to read from. The easiest way to go around would be to use a SD card, but maybe you haven't built a SPI relay yet and it's quite a challenge to do so.

Therefore, for this recipe, we'll have at28w read from a memory map and we'll upload contents to write to memory through our serial link.

at28w is designed to be ran as a "user application", but in this case, because we run from a kernel without a filesystem and that pgm can't run without it, we'll integrate at28w directly in our kernel and expose it as an extra shell command (renaming it to a28w to fit the 4 chars limit).

For all this to work, you'll need glue code that looks like this. Running make in this directory will produce a os.bin with that glue code that you can install in the same way you did with the basic RC2014 recipe.

If your range is different than 0x2000-0x3fff, you'll have to modify AT28W_MEMSTART before you build.

Writing contents to the AT28

The memory map is configured to start at 0xd000. The first step is to upload contents at that address as documented in "Load code in RAM and run it".

You have to know the size of the contents you've loaded because you'll pass it as at argument to a28w. You can run:

Collapse OS
> bsel 0
> seek 00 0000
> a28w <size-of-contents>

It takes a little while to write. About 1 second per 0x100 bytes (soon, I'll implement page writing which should make it much faster).

If the program doesn't report an error, you're all good! The program takes care of verifying each byte, so everything should be in place. You can verify yourself by peek-ing around the 0x2000-0x3fff range.

Note that to write a single byte to the AT28 eeprom, you don't need a special program. You can, while you're in the 0x2000-0x3fff range, run poke 1 and send an arbitrary char. It will work. The problem is with writing multiple bytes: you have to wait until the eeprom is finished writing before writing to a new address, something a regular poke doesn't do but at28w does.

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