Importance

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Abstract

A data processing system includes a microcomputer (1) provided with an address latch (5). The microcomputer is provided with a watch-dog arrangement which comprises the usual clocked counter (17) an output (Qn) of which is coupled to a reset input (RST) of the microcomputer. In order to increase the number of malfunctions to which the watch-dog responds the microcomputer is arranged to repeatedly generate reset signals each in the form of a complete byte X or Y which alternate. These are applied to an input (30) of a comparator (10) which compares them with identical bytes fed to a further input 31 via a switched multiplexer (13), the comparator output (37) being connected to a reset input (RS) of the counter. Each time this occurs the microcomputer strobes the watch-dog by applying its address to a further input (46, 22). If the byte applied to the comparator input (30) is incorrect and/or if this application does not occur within a time window defined by a period during which a further output (Qn-1) of the counter is logic ' 1' the resulting level at the comparator output (37), stored in a flip-flop (15), resets the microcomputer. The state of the signal at the further output (Qn-1) of the counter is periodically ascertained via a buffer (18). Each of the bytes X and Y is obtained by complementing the previous such byte, stored in RAM, further program steps being carried out between each such complementing operation and the preceding and succeeding generation of a reset signal.

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First Claim