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Unformatted text preview: nous microprocessor developed elsewhere which support other instruction set architectures, but only the AMULET processors implement the ARM instruction set. Self-timed signalling Asynchronous design is a complex discipline with many different facets and many different approaches. It is outside the scope of this book to offer any general introduction to asynchronous design, but a few basic concepts should enable the reader to come to grips with the most important features of the AMULET cores. The foremost of these concepts is the idea of asynchronous communication. How is the flow of data controlled in the absence of any reference clock? The AMULET designs both use forms of the Request--Acknowledge handshake to control the flow of data. The sequence of actions comprising the communication of data from the Sender to the Receiver is as follows: 1. The Sender places a valid data value onto a bus. 2. 3. 4. 5. The Sender then issues a Request event. The Receiver accepts the data when it is ready to do so. The Receiver issues an Acknowledge event to the Sender. The Sender may then remove the data from the bus and begin the next communi cation when it is ready to do so. The data is passed along the bus using a conventional binary encoding, but there are two ways that the Request and Acknowledge events may be signalled: Transition signalling AMULET 1 uses transition encoding where a change in level (either high to low or low to high) signals an event; this is illustrated in Figure 14.1. Figure 14.1 Transition-signalling communication protocol. AMULET1 377 Level signalling AMULET2 and AMULETS use level encoding where a rising edge signals an event and a return-to-zero phase must occur before the next event can be signalled; this is illustrated in Figure 14.2. Figure 14.2 Level-signalling communication protocol. Transition signalling was used on AMULET 1 since it is conceptually cleaner; every transition has a role and its timing is therefore determined by the circuit's function. It also uses the minimum number of transitions, and should therefore be...
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- Spring '09