Embedded design

As measured by units shipped, most CPUs are embedded in other machinery, such as telephones, clocks, appliances, vehicles, and infrastructure. Embedded processors sell in the volume of many billions of units per year, however, mostly at much lower price points than that of the general purpose processors.
These single-function devices differ from the more familiar general-purpose CPUs in several ways:
Low cost is of utmost importance.
It is important to maintain a low power dissipation as embedded devices often have a limited battery life and it is often impractical to include cooling fans.
To give lower system cost, peripherals are integrated with the processor on the same silicon chip.
Keeping peripherals on-chip also reduces power consumption as external GPIO ports typically require buffering so that they can source or sink the relatively high current loads that are required to maintain a strong signal outside of the chip.
Many embedded applications have a limited amount of physical space for circuitry; keeping peripherals on-chip will reduce the space required for the circuit board.
The program and data memories are often integrated on the same chip. When the only allowed program memory is ROM, the device is known as a microcontroller.
For many embedded applications, interrupt latency will be more critical than in some general-purpose processors.
[edit] Embedded processor economics
As of 2009, more CPUs are produced using the ARM architecture instruction set than any other 32-bit instruction set. The ARM architecture and the first ARM chip were designed in about one and a half years and 5 man years of work time.[1]
The 32-bit Parallax Propeller microcontroller architecture and the first chip were designed by two people in about 10 man years of work time.[2]
It is believed[weasel words] that the 8-bit AVR architecture and first AVR microcontroller was conceived and designed by two students at the Norwegian Institute of Technology.
The 8-bit 6502 architecture and the first 6502 chip were designed in 13 months by a group of about 9 people.[3]