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The testing of VLSI circuits is becoming progressively more difficult as device densities increase. This has brought about several proposals for designing VLSI circuits with testability built in. A method is presented in the paper for the design of easily testable VLSI circuits with a view to producing fault tolerant systems. A microprocessor datapath is used to illustrate the technique. The method used for checking the VLSI devices is an error detecting code, in this case a residue code. Residue codes offer several advantages over linear block codes for providing testability in a wide range of VLSI circuits. A detailed evaluation of the increase in chip area required to produce a self testing chip is also given in the paper.