Contributions to Asynchronous Communication Ports for GALS Systems

University dissertation from Institutionen för systemteknik

Abstract: Digital systems commonly use a single global clock signal to synchronize the whole system. This is not always possible and it can be more advantageously to divide the system into separate clock domains, where each clock domain can operate with its own clock frequency. Communication between the different clock domains are not trivial and must be handled with care. Several schemes can be used depending on the relation between the clock frequencies of the communicating clock domains. This thesis focuses on the Globally Asynchronous Locally Synchronous (GALS) scheme, in which all communications between clock domains are handled using dedicated communication channels. These communication channels use asynchronous handshaking protocols to transfer information between clock domains. No global clock signal is used and the clock signal is instead local for each clock domain.An efficient design flow for GALS system has been developed, which allows a designer to implement GALS systems without prior knowledge of asynchronous circuits. The GALS design flow starts with a high-level model of the system behavior and ends with an implementation in an FPGA or an ASIC. The design flow can also increase the design efficiency for GALS system since the flow alleviates the design and placement of the asynchronous circuits for the designer. A tool that handles the asynchronous circuits in the design flow has been developed.Two types of communication ports have been developed to handle the communication between clock domains. Both of these ports can be used in systems with static schedule or dynamic schedule of transactions. One of the communication ports can easily be migrated to a new CMOS process, since it only uses standard-cells that care provided by most vendors of CMOS processes. A clock gating circuit has been developed to allow a clock domain to use an external stable clock signal to create an internal stoppable clock signal. A stoppable local clock is used to eliminate problems with metastability when transferring data between clock domains with arbitrary clock frequencies.In order to validate the design flow and proposed circuitry, has an integrated circuit for 2-dimensional Discrete Cosine Transform been implemented using the GALS scheme and one of the proposed communication ports. The circuit has been implemented using a standard-cell library in a 0.35 mm CMOS process. A few possible improvements to the implementation are also discussed in the thesis.The GALS design flow with the asynchronous wrapper generation tool has been used to implement the digital baseband processing in the physical layer of the IEEE 802.11a transmitter. The transmitter is built using multiple clock domains. The transmitter has been implemented and tested in a Stratix II FPGA.

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