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Circuit Description

A digital clock with display for hours, minutes and seconds. The circuit is built from a cascade of six asynchronous, falling-edge triggered BCD counters with a simple trick to reset the counters at the corresponding times.

The counter for the lower seconds digit simply runs continously. The upper seconds digit counter, however, needs to be reset after 59 seconds to avoid counting to 99 seconds. This is achieved by connecting the asynchronous reset input of the counter block to its Z2 and Z1 outputs. Therefore, as soon as the counter reaches 60, it is immediately asynchronously reset and at the same time generates the carry to increment the minutes counter - which uses the same structure as the seconds counter.

The hours counters use the same trick to immediately asynchronously reset the hours after the hours reach the value 24. Because the reset is asynchronous and very fast, the user has no chance to see the wrong values (like 00:00:60 or 24:00:00) before these are corrected to the right values.