The scope of this article is to introduce the reader to a famous, old (but still under production manufactured by Intersil Corporation) microprocessor, which contribution to many challenging and valuable electronics projects and applications has been widely recognized: the RCA CDP1802 (also known as COSMAC)

Forty years ago, the Apollo 11 space mission with the two astronauts N.Armstrong and E.Aldrin achieved an outstanding result: for the first time a man could put his feet on the moon surface. That events has been recalled in several ways, also focusing on the aspects strictly related to the electronics equipments (mainly navigation and guidance systems) which strongly contributed to the mission success. Space missions have always been using top level technologies, not only in the electronic field, so one could think that the electronic equipments and computers which fly in space are always based on microprocessors able to provide the best level of performance. Actually, this is not always true. In order to fly in space, an electronic device must not only be compliant with extended temperature range (the same thing happens for military equipments), but it must also tolerate the high level of radiation (due to the presence of cosmic rays) which can be found in the space. As a result, there might be the case that the processor of your desktop computer is newer and better performing than a model used in aerospace devices. The scope of this article is to introduce you to a famous, old (but still under production manufactured by Intersil Corporation) microprocessor, which contribution to many challenging and valuable electronics projects and applications has been widely recognized: the RCA CDP1802 (also known as COSMAC) visible in the following picture. If a book related to the history of microprocessors were written, for sure a whole chapter would be reserved to this antique but still actual microprocessor.

The RCA CDP1802 COSMAC is an 8-bit CMOS central processor unit introduced in early 1976, and it has been the first RCA’s single chip microprocessor. The term COSMAC is the acronym for Complementary Symmetry Monolithic Array Computer, and it is how RCA used to define its first CMOS (COS/MOS) semiconductor technology. The 1802 is derived from the previous version, the 1801, which was a two chips microprocessor and is based on the same silicon gate CMOS technology. Compared to its predecessor, the 1802 features more powerful and complete features, such as:

• The 1801 was able to work initially at a clock frequency of 2 MHz, and subsequently at 4 MHz; the 1802 was able to work at a clock frequency of 6.4 MHz (at 10V, very fast for those years). Moreover, since the 1802 uses static CMOS circuitry, it has no minimum clock frequency; that means it can be run at very slow clock frequency, with minimal power consumption
• The 1802 has a wider instruction set than the 1801 (91 against 59 operative codes) including math and branch instructions; the 1802 is however backward compatible with the 1801

The architecture

The architecture of the 1802 is based on an 8-bit parallel bus with a bidirectional data bus and a multiplexed address bus: the high and low order bytes access the bus in different clock cycles in order to form a 16-bit word able to address a memory space of 64 KB. The register set consists of:

• 16 16-bit general-purpose registers which could be accessed as 32 8-bit registers
• an accumulator D used for arithmetic and memory access (from memory to D, and then from D to registers and vice versa, using one 16-bit register as address)
• a 4-bit control register P to select one of the 32 registers as program counter, and control registers X and N to select registers for I/O index and the operand for the current instruction
• a single bit programmable output port and a 4 input pins

The COSMAC is considered as a microprocessor with an 8-bit architecture, since the accumulator is 8 bits wide. However, its registers were 16-bit (and 8-bit) and it was able to address external memory with 16-bit registers. This architecture register-oriented made the COSMAC a very efficient processor, with a clean and easy to understand programming. As mentioned before, the COSMAC was designed using the CMOS chip technology, which helped to deliver the following features:

• the chip can run at very low frequencies, and even be suspended, under some circumstances, in order to preserve the battery life
• it can operate in extended temperature range
• it can well tolerate electronic interference
• it can operate under the radiation which can be found in the outer space

All these characteristics contributed to make COSMAC the first microprocessor to be used in space missions. The 1801 model was used in 1978 as the main controller of a satellite (OSCAR), and other outstanding space applications adopted the COSMAC as microprocessor: Voyager (1802), Viking, Galileo, and also the Space Shuttle (COSMAC was responsible of the TV systems). It has to be mentioned that the COSMAC units which flew in space were special versions hardened to tolerate space radiation and electrostatic discharge (ESD), and they were developed together with the Sandia National Laboratories. In particular, they were based on the CMOS/SOS chip technology (Complementary Metal Oxide Semiconductor / Silicon On Sapphire), a radiation hardened chip technology initially developed by the
department of defense and then put in production by RCA.
The 1802 is also the furthest microprocessor from Earth, since it has been used on the Voyager spacecraft, the furthest human-made object from Earth, with a distance of about 100 A.U. (Astronomical Unit, equivalent to 149598000 kilometers). The following picture shows the cover of the User Manual for the CDP1802 COSMAC Microprocessor

The COSMAC ELF

In 1976, the famous magazine Popular Electronics published a series of articles where the 1802 was used to create the "COSMAC Elf" (see a photo below), one of the first hobby computers also available in kit. In the same period, several commercial companies started offering similar products based on the COSMAC processor.