Changes

== The block diagram ==

== The block diagram ==

Having defined your block box, you need to fill in your black box.  But before doing that, we need to note the different between two types of logic:

Having defined your block box, you need to fill in your black box.  But before doing that, we need to note the difference between two types of logic:

* ''Combinational logic'' merely recombines lines into new lines.  For example, signal Q may be the logical AND of signals A, B, and C.  There is no reference to a clock in combinational logic.

* ''Combinational logic'' merely recombines lines into new lines.  For example, signal Q may be the logical AND of signals A, B, and C.  There is no reference to a clock in combinational logic.

* ''Sequential logic'' is any logic that makes use of a clock for latches, flip-flops, registers, or other devices.  Sequential logic changes only when the clock changes.  Often circuits are wired so that all sequential logic changes together, either on a ''falling edge'' of a clock or a ''rising edge'' of a clock.  Advanced designs can change some components on a rising edge and other components on a falling edge, but this is significantly more difficult due to the tighter timing restrictions imposed.

* ''Sequential logic'' is any logic that makes use of a clock for latches, flip-flops, registers, or other devices.  Sequential logic changes only when the clock changes.  Often circuits are wired so that all sequential logic changes together, either on a ''falling edge'' of a clock or a ''rising edge'' of a clock.  Advanced designs can change some components on a rising edge and other components on a falling edge, but this is significantly more difficult due to the tighter timing restrictions imposed.