Linux on breadboard!

It is just amazing what people with skills can do. Check this guy, not only he build a computer on a breadboard but he also made it capable of running a light version of linux. And all this with a Motorola 68008 cpu, 512k(!) Ram and 512kb Rom. The cpu is running at the mind blowing speed of 2Mhz. The operating system is μClinux, a special distribution designed for microcontrollers. I suggest you go and check the whole project. I wish I had the skills and knowledge to do this kind of projects myself.

Here’s a look at the final version of the hardware. It took about a week to assemble and wire up all the parts on a solderless breadboard. The heart of the system is a Motorola 68008 CPU, a low-cost variant of the more common 68000, with fewer address pins and an 8-bit data bus. The CPU has 20 address pins, allowing for 1 MB of total address space. It’s paired with a 512K 8-bit SRAM, and a 512K Flash ROM (of which 480K is addressable – the remaining 32K is memory-mapped I/O devices).

The standard 68000 CPU has a 16-bit data bus, so it normally requires at least two 8-bit RAM chips and two 8-bit ROM chips. The 68008 requires fewer memory chips thanks to its 8-bit data bus, but the trade-off is that memory bandwidth is only half that of the 68000. Neither chip has any on-board cache, so half the memory bandwidth leads to roughly half the performance. My 68008 runs at 2 MHz (it was unstable when tested at 4 MHz), providing similar performance to a 1 MHz 68000. That’s pretty slow, even in comparison to 68000 systems from the early 1980′s, which were typically 8 MHz or faster.

An FT245 USB-to-FIFO module provides a communication link to another computer. On the external PC, it appears as a virtual serial port. Windows Hyperterm or another similar terminal program can be used to communicate with it, like an old VT100 terminal. On the 68 Katy side, the FT245 appears as a byte-wide I/O register mapped into the CPU’s address space. Reading from its address fetches the next incoming byte from the PC, and writing to the address sends a byte out to the PC. The FT245 has an internal 256-byte buffer, which helps smooth out the communication. When there’s an incoming byte waiting in the buffer, it triggers a CPU interrupt.

A 555 timer provides the only other interrupt source, generating a regular series of CPU interrupts at roughly 100 Hz. The initial version of the hardware had no timer interrupt, but I later discovered it was essential in order to get Linux working correctly.

The protoboard has eight LEDs for debugging, which are driven from a memory-mapped 74LS377 register. The rest of the protoboard is filled with assorted 7400 series parts and one PAL, which are used for address decoding, interrupt arbitration, and other basic glue logic.

Schematics? Forget it. Everything was built incrementally, one wire at a time, while staring at chip datasheets. It’s an organic creation.

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