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Tuesday, December 28, 2010

Motorola 6800

Motorola MC6800 Microprocessor.
The 6800 was an 8-bit microprocessor designed and manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System that also included serial and parallel interface ICs, RAM, ROM and other support chips. A significant design feature was that the M6800 family of ICs required only a single five-volt power supply; other microprocessors required three voltages. The M6800 Microcomputer System was announced in March 1974 and was in full production by the end of that year.The 6800 architecture and instruction set were influenced by the then popular Digital Equipment Corporation PDP-11 mini computer.The 6800 had a 16-bit address bus address that could access 64 KB of memory and a 8-bit bi-directional data bus. It has 72 instructions with seven addressing modes for a total of 192 opcodes. The original MC6800 could have a clock rate of up to 1 MHz. Later versions had a maximum clock rate of 2 MHz.In addition to the ICs, Motorola also provided a complete assembly language development system. The customer could use the software on a remote timeshare computer or on an in-house mini-computer system. The Motorola EXORciser was a desktop computer built with the M6800 ICs that could be used for prototyping and debugging new designs. An expansive documentation package included datasheets on all ICs, two assembly language programming manuals, and a 700 page application manual that showed how to design a point-of-sale computer terminal.The 6800 was popular in computer peripherals, test equipment applications and point-of-sale terminals. The MC6802, introduced in 1977, included 128 byte of RAM and an internal clock oscillator on chip. The MC6801 and MC6805 included with RAM, ROM and I/O on a single chip were popular in automotive applications.

Motorola's history in semiconductors

Motorola transistor radio, 1960.
Galvin Manufacturing Corporation was founded in 1928 in Chicago, Illinois and produced their first Motorola brand car radio in 1930. The company name was changed to Motorola in 1947. A research and development center was opened in Phoenix, Arizona in 1949 and they began commercial production of transistors at a new $1.5 million facility in Phoenix in 1955. In 1966 an integrated circuit facility was built in the Phoenix suburb of Mesa.In 1973, they announced plans to build a new plant in Austin, Texas to manufacture MOS integrated circuits. The microprocessor group moved from Mesa to Austin in 1975.Motorola's transistors and integrated circuits were used in-house for their communication, military, automotive and consumer products and they were also sold to other companies. In the 1960s, Motorola sold a full line of digital ICs that were used in computers, test equipment and industrial controls. By 1970 they were producing metal-oxide semiconductors (MOS) LSI chips as standard products and custom circuits. They also produced semiconductor memories that were used in calculators, point-of-sale terminals and computers. Motorola's sales had reached $1.2 billon in 1973 and they had 64,000 employees. The Semiconductor Products Division (SPD) had sales of $419 million and was the second largest semiconductor company after Texas Instruments.Intel was founded in 1968 and had sales of $66 million in 1973.In the early 1970s Motorola started a project that developed their first microprocessor, the MC6800. This was followed by single chip microcontrollers such as the MC6801 and MC6805. In 1979 the 16-bit version of MC68000 microprocessor was introduced, this processor was later extended to 32 bits.
In 1999 Motorola spun off their analog IC, digital IC and transistor business to ON Semiconductor of Phoenix Arizona. In 2004 they spun off their microprocessor business to Freescale Semiconductor of Austin, Texas.

Development team

Motorola did not chronicle the development of the 6800 microprocessor the way that Intel did for their microprocessors. In 2008 the Computer History Museum interviewed four members of the 6800 microprocessor design team. Their recollections can be confirmed and expanded by magazine and journal articles written at the time.

The Motorola microprocessor project began in 1971 with a team composed of designer Tom Bennett, engineering director Jeff LaVell, product marketer Link Young and systems designers Mike Wiles, Gene Schriber and Doug Powell. They were all located in Mesa, Arizona. By the time the project was finished, Bennett had 17 chip designers and layout people working on five chips. LaVell had 15 to 20 system engineers and there was another applications engineering group of similar size.

Tom Bennett had a background in industrial controls and had worked for Victor Comptometer in the 1960s designing the first electronic calculator to use MOS ICs, the Victor 3900. In May 1969 Ted Hoff showed Bennett early diagrams of the Intel 4004 to see if it would meet their calculator needs. Bennett joined Motorola in 1971 to design calculator ICs. He was soon assigned as the chief architect of the microprocessor project that produced the 6800. Others have taken credit for designing the 6800. In September 1975 Robert Cushman, EDN magazine's microprocessor editor, interviewed Chuck Peddle about MOS Technology's new 6502 microprocessor. Cushman then asked "Tom Bennett, master architect of the 6800," to comment about this new competitor. After the 6800 project Bennett worked on automotive applications and Motorola became a major supplier of microprocessors used in automobiles.

Jeff LaVell joined Motorola in 1966 and worked in the computer industry marketing organization. Jeff had previously worked for Collins Radio on their C8500 computer that was built with small scale ECL ICs. In 1971 he led a group that examined the needs of their existing customers such as: Hewlett Packard, National Cash Register, National Data Corporation (CDC), and Digital Equipment Corporation. They would study the customer's products and tried to identify functions that could be implemented in larger integrated circuits at a lower cost. The result of the survey was a family of 15 building bocks; each could be implemented in an integrated circuit. Some of these blocks were implemented in the initial M6800 release and more were added over the next few years. To evaluate the 6800 architecture while the chip was being designed, Jeff's team built an equivalent circuit using 451 small scale TTL ICs on five 10 by 10 inch (25 by 25 cm) circuit boards. Later they reduced this to 114 ICs on one board by using ROMs and MSI logic devices.

John Buchanan was a memory designer at Motorola when Bennett asked him to design a voltage doubler to for the 6800. Typical n-channel MOS IC's required three power supplies: -5 volts, +5 volts and +12 volts. The M6800 family was to use only one, +5 volts. It was easy to eliminate the -5 volt supply but the MOS transistors needed a supply of 10 to 12 volts. This on-chip voltage doubler would supply the higher voltage and Buchanan did the circuit design, analysis and layout for the 6800 microprocessor. He received patents on the voltage doubler and the 6800 chip layout. Rod Orgill assisted Buchanan with analyses and 6800 chip layout. Later Orgill would design the MOS Technology 6501 microprocessor that was socket compatible with the 6800.

Bill Lattin joined Motorola in 1969 and his group provided the computer simulation tools for characterizing the new MOS circuits in the 6800. Lattin and Frank Jenkins had both attended UC Berkley and studied computer circuit simulators under Donald Pederson, the designer of the SPICE circuit simulator. Motorola's simulator, MTIME, was an advanced version of the TIME circuit simulator that Jenkins had developed at Berkley. The group published a technical paper, "MOS-device modeling for computer implementation" in 1973 describing a "5-V single-supply n-channel technology" operating at 1 MHz. They could simulate a 50 MOSFET circuit on an IBM 370/165 mainframe computer. In November 1975, Lattin joined Intel to work on their next generation microprocessor.

Bill Mensch joined Motorola in 1971 after graduating from the University of Arizona. He had worked several years as an electronics technician before earning his BSEE degree. The first year at Motorola was a series of three month rotations through four different areas. Memsch did a flowchart for a modem that would become the 6860. He also worked the application group that was defining the M6800 system. After this training year, he was assigned to the 6820 Parallel Interface Adapter (PIA) development team. Mensch was a major contributor to the design of this chip and received a patent on the IC layout and was named as a co-inventor of seven other M6800 system patents. Later Mensch would design the MOS Technology 6502 microprocessor.

Mike Wiles was a design engineer in Jeff LaVell's group and made numerous customer visits with Tom Bennett during 6800 product definition phase. He is listed as an inventor on eighteen 6800 patents but is best known for a computer program, MIKBUG.This was a rudimentary BIOS for a 6800 computer system that allowed the user to examine the contents of RAM and to save or load programs to tape. This 512 byte program occupied half of a MCM6830 ROM. This ROM was used in the Motorola MEK6800 design evaluation kit and early hobby computer kits. Wiles stayed with Motorola, moved to Austin and helped design the MC6801 microcontroller that was released in 1978.

Chuck Peddle joined the design team in 1973 after the 6800 processor design was done but he contributed to overall system design and to several peripheral chips, particularly the 6820 (PIA) parallel interface. Peddle is listed as an inventor on sixteen Motorola patents, most have six or more co-inventors. Like the other engineers of the team, Peddle visited potential customers and solicited their feedback. Peddle and John Buchanan built one of the earliest 6800 demonstration boards. In August 1974 Chuck Peddle left Motorola and joined a small semiconductor company in Pennsylvania, MOS Technology. There he led the team that designed the 6500 microprocessor family.
MIKBUG was part of the extensive M6800 microcomputer support developed by Motorola's Application Engineering Group.

MC6800 microprocessor design

The Motorola 6800 and the Intel 8080 were designed at the same time and were similar in function. The 8080 was a superset of the Intel 8008 which was based on the Datapoint 2200 processor. The 6800 architecture was modeled after the DEC PDP-11 processor. Both are TTL compatible, have an 8-bit bidirectional data bus, a 16-bit stack pointer, a 16-bit address bus that can address 64 KB of memory and come in a 40-pin DIP package. The 6800 has two accumulators and a 16-bit index register. The Direct Addressing mode allows fast access the first 256 bytes of memory. I/O devices are addressed as memory so there are no special I/O instructions. The 8080 has more internal registers and instructions for dedicated I/O ports. When the 8080 was reset, the program counter was cleared and the processor started at memory location 0000. The 6800 loaded the program counter from the highest address and started at the memory location stored there. The 6800's has a three-state control that will disable the address bus to allow another device direct memory access. A disk controller could transfer data into memory with no load on the processor. It is even possible to have two 6800 processors access the same memory.

MOS ICs typically used dual clock signals (a two-phase clock) in the 1970s. These were generated externally for both the 6800 and the 8080. The next generation of microprocessors incorporated the clock generation on chip. The 8080 had a 2 MHz clock but the processing throughput was similar to the 1 MHz 6800. The 8080 require more clock cycles to execute a processor instruction. The 6800 had a minimum clock rate of 100 kHz while the 8080 could be halted. Higher speed versions of both microprocessors were released by 1976.
A Motorola MC6800 microprocessor registers and I/O lines.

Other divisions in Motorola developed components for the M6800 family. The Components Products Department designed the MC6870 two-phase clock IC; the Memory Products group provided a full line of ROMs and RAMs. The CMOS group's MC14411 Bit Rate Generator provided a 75 to 9600 baud clock for the MC6850 serial interface. The buffers for address and data buses were standard Motorola products. Motorola could supply every IC, transistor and diode necessary to build a MC6800 based computer.

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