AMULET is a series of microprocessors that implement the ARM

ARM architecture

ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...

 processor architecture. Developed by the Advanced Processor Technologies group under the University of Manchester

University of Manchester

The University of Manchester is a public research university located in Manchester, United Kingdom. It is a "red brick" university and a member of the Russell Group of research-intensive British universities and the N8 Group...

's computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

 school (formerly the AMULET and PAL groups based at the same institution), AMULET is unique from other ARM implementations in being an asynchronous microprocessor, not making use of a square wave

Square wave

A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...

 clock signal

Clock signal

In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...

 for data synchronization and movement.

List of AMULET microprocessors

• AMULET1 — Designed in 1990 and first fabricated in 1993. Its estimated performance is approximately 70% of that of a comparably-sized synchronous ARM6 running at 20 MHz.
• AMULET2 — A re-implementation of AMULET1 first fabricated in 1996. Features on-chip memory that can be used either as processor cache or mapped RAM. The APT group estimates AMULET2 to have a similar power dissipation/performance ratio as ARM8. One very notable feature due to the asynchronous design is the drop of power dissipation to 3 μW when not in use (assuming the on-board timer, which handles DRAM refresh, is also inactive).
• AMULET3 — This was a redesigned architecture aiming at higher performance than the previous AMULET processors whilst retaining low power dissipation. Fabricated in 2000, it supported the ARM level 4 instruction set compatibility, alongside support Thumb mode (i.e. ARM9TM). Performance and power dissipation were approximately the same as an ARM9 fabricated on the same technology. AMULET3 was employed in a commercial prototype DECT
  Digital Enhanced Cordless Telecommunications
  Digital Enhanced Cordless Telecommunications , usually known by the acronym DECT, is a digital communication standard, which is primarily used for creating cordless phone systems...
  
   device because of its inherent low electromagnetic interference
  Electromagnetic interference
  Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
  
  characteristics. This did not go into manufacture for non-technical reasons.