With DRAM implemented, the next step for Mackerel-10 is to add support for IDE storage. Mackerel-08 has rudimentary persistent storage in the form of an SD card connected to the XR68C681's GPIO. The bitbang SPI protocol technically works, but it is incredibly slow. It takes about 4 minutes to load the 1.5MB kernel into RAM from the SD card. For Mackerel-10, I want something faster and more robust.

IDE is actually somewhat of a natural choice for the 68000. Although the protocol has its roots in the x86 world, it's a fairly simple 16-bit memory-mapped register interface. It's possible to connect the full 16 data lines, 3 address lines, and a handful of control signals from an IDE drive directly to the 68000 with only a small amount of glue logic. I did just that.

Ignoring interrupts and DMA for now, the only signals that don't map directly between the 68000 and the IDE interface are the chip selects and the read/write lines. IDE splits read and write into two pins instead of the single /RW pin of the 68000. I updated the glue logic in the CPLD to create the required signals and I put together some really basic C code to talk to the IDE device.

The IDE protocol is actually really in depth, but for basic functionality, there's only a few pieces that matter. There are registers for data, status, error, and setting sector values and there's a command register with a list of supported commands. I've implemented two of these commands: DEVICE_IDENTIFY (0xEC) and READ_SECTOR (0x20).

This is enough to read device info, e.g. model, firmware, and capacity details and to read arbitrary sectors of data from the drive. Sectors are transferred as 256x16 bit words. The only sticking point is that IDE, like x86, is little-endian and 68000 is big-endian so each word has to have the high and low bytes swapped. I've chosen to do this in code for now, but I've seen other projects implement this in hardware by simply wiring the data bus connection between the CPU and the IDE drive with the swap built in.

It would have been nice to combine this IDE prototype and the DRAM breakout board to get a sense of what the full Mackerel-10 SBC will look like, but I did not think to design that option into the prototype PCBs. Oh well. I'm feeling good about this IDE test and my DRAM board has been very stable as well. I think it's time to jump back into KiCAD and start building the Mackerel-10 v1 PCB.