Gecko's CPU Library

Motorola 68000 processors

Introduction: 1979

As the new design reached production a trade name had to be selected. The name "68000" was selected largely to provide some naming and marketing continuity with the earlier 6800, although there was little in common between the two designs. The name was justified by claiming the chip featured about 68,000 transistors, but in fact the count was closer to 70,000.

On average, the 68000's instructions in typical program code did more work per instruction than competing Intel processors, which meant that 68000 designs both needed less RAM to store machine code and were faster. Additionally, the 68000 offered a "flat" 24-bit addressing system supporting up to 16MB of memory; at the time, this was a very large memory space. Intel chips instead used a segmented system which was much more difficult to program. Only in 1986 the 80386 allowed flat addressing. Thus the 68000 was easier to learn and work with, which led many engineers to prefer it to the Intel designs.

The original MC68000 was fabricated using an HMOS process with a 3.5-micron feature size. Initial engineering samples were released in late 1979. Production chips were available by 1980, with initial speed grades of 4, 6, and 8MHz. 10MHz chips became available during 1981, and 12.5MHz chips during 1982. The 16.67MHz "12F" version of the MC68000, the fastest version of the original HMOS chip, was not produced until the late 1980s.

To support low-cost systems with smaller memory sizes, Motorola introduced the MC68008 in 1982. This was a 68000 with an 8-bit data bus and a smaller address bus. The 68012 later brought a 32-bit address bus.

The 68HC000, the CMOS version of the 68000, was jointly introduced by Motorola and Hitachi in 1985. Motorola's version was called the MC68HC000, while Hitachi's was the HD68HC000. The 68HC000 was eventually offered at speeds from 8MHz to 20MHz. Except for using CMOS circuitry, it behaved identically to the HMOS MC68000, but the change to CMOS greatly reduced its power consumption. The original HMOS MC68000 consumed around 1.35 watts at an ambient temperature of 25 °C, regardless of clock speed, while the MC68HC000 consumed only 0.13 watts at 8MHz and 0.38 watts at 20MHz.

Motorola introduced the MC68HC001 in 1990. This chip resembled the 68HC000 in most respects, but its data bus could operate in either 16-bit or 8-bit mode, depending on the value of an input pin at reset. Thus, like the 68008, it could be used in systems with cheaper 8-bit memories.

Several other companies were second-source manufacturers of the HMOS 68000. These included Hitachi (HD68000), Mostek (MK68000), Rockwell (R68000), Signetics (SCN68000), Thomson/SGS-Thomson (originally EF68000 and later TS68000), and Toshiba (TMP68000). Toshiba was also a second-source maker of the CMOS 68HC000 (TMP68HC000).

The 68000 became the foundation for a large number of microcontrollers and embedded processors. In 1989, Motorola introduced the MC68302 communications processor, its first microcontroller to use a 68000 CPU core. This core was based on the CMOS 68HC000 but removed support for 8-bit 6800 peripherals. In 1991 Motorola introduced a separate processor chip based on this core, the MC68EC000.

Motorola went on to make several additional microcontrollers with the 68EC000 core, including the MC68306 and MC68307 general-purpose microcontrollers, the MC68322 "Bandit" printer controller, the MC68356 modem data pump, and the MC68328 DragonBall, designed for portable devices. Other microcontrollers in the 683XX family used the more powerful CPU32 processor core.

Several of the 68EC000-based 683XX microcontrollers used a static version of the 68EC000 core; the processor clock of this core could be slowed or stopped to save power. In 1996 Motorola introduced this static core as a separate processor, the MC68SEC000.

Motorola ceased production of the HMOS MC68000 and MC68008 in 1996, but its spin-off company, Freescale Semiconductor, is still producing the MC68HC000, MC68HC001, MC68EC000, and MC68SEC000, as well as the MC68302 and MC68306 microcontrollers and later versions of the DragonBall family. The 68000's architectural descendants, the 680x0, CPU32, and Coldfire families, are also still in production.

Source: Wikipedia, the free encyclopedia.