two-frequency clock

27MHz I=1 3 rd overtone 00115 Many computer systems use one clock signal, from which all other timing signals are derived. The fre- quency of the clock signal determines, among others, the maximum number of characters per line the video con- troller can display on the monitor screen. This is normally 32 or 40. If more characters per line are required, the clock frequency has to be increas- ed. The clock generator described here makes it possible to switch between frequencies which are related in a ratio of 2:3. The switching is car- ried out synchronously, so that no bits are lost. The clock oscillator, T,, is controlled by an inexpensive 3rd overtone 27 MHz crystal, XL1. The LC circuit connected to the collector of T1is tuned to 54 MHz. The 54 MHz signal is converted to logic bits by field- effect transistor T2 which are then applied to the Q inputs of dual J-K bistable IC1( = FFi /FF2). The ring counter formed by these bistables can be changed over by T3. When T3 is on, the J in...