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Unformatted text preview: h and then hold it before the combinatorial logic has time to respond to the new values. However, in practice this turns out to be a very hard way to build a reliable circuit. Edge-triggered latches There are various ways to build more reliable latching circuits, most of which require each signal to pass through two transparent latches per clock cycle. In the simplest of these, the second latch is placed in series with the first and operates with the inverse enable function. At all times, one latch or the other is holding and the other is transparent. On one clock edge, where the first latch goes opaque and the second goes transparent, the input data value propagates through to the output. It is then held through the full cycle until the same" edge in the following cycle. This is therefore an edge-triggered latch; its logic symbol, circuit and sequence table are shown in Figure A.6 on page 404. (In the table an 'x' in an input column indicates that the input has no effect on the output.) Edge-triggered latches require very careful design, since they are not simply combinatorial logic circuits and their function depends critically on the dynamic properties of the circuit elements. If constructed, as suggested above, from two transparent latches in series, there are various race conditions which must be avoided. But, with good tools, reliable latches can be designed, and once a reliable latch is available, sequential circuits of arbitrary complexity can be constructed. A set of edge-triggered (or equivalent) latches which jointly store the state of a binary value through a clock cycle is termed a register. A flexible form of register is connected to a free-running clock and has a 'clock enable' control input so that control logic can decide on a cycle-by-cycle basis whether or not to update the register's contents. A simple way to build such a register is to add a gate on the common clock Registers 404 Appendix: Computer Logic Figure A.6 A D-type edge-triggered latch symbol, circuit and sequence table. line to the edge-triggered latches. If we use negative-edge triggered latches, an AN...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09