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TOPIC 6TIMING CIRCUITS
TOPIC 7COMPUTER DATA TYPES AND FORMATS

Fire Controlman Volume 03-Digital Data Systems
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Figure 4-39.—Example of a delay line oscillator. rectangular waveforms are normally used to turn such circuits on and off because the  sharp leading  and trailing  edges  make  them  ideal  for  timing  purposes. The  components  used  to  accomplish  this  depend  on  the complexity of the computer. The components you will most  frequently  encounter  in  timing  circuits  are oscillators and multivibrators. Oscillators Oscillators are used in computer timing circuits for their output and frequency stability   characteristics. The more important quality of the two for use in computers is their frequency stability. The speed of a clock   is   determined   by   the   oscillator   using   a resistance-capacitance (RC) or inductance-capacitance (LC) network and/or crystal combination. An oscillator can use bipolar or MOS technology. Crystal-controlled oscillators are used in computers because they are stable even at extremely high frequencies. Master clocks in computers often use an oscillator with a delay line to deliver the basic clock phase and any additional clock phases. See figure 4-39. Multivibrators Three types of multivibrators are used in timing circuits.  They  are  astable  (free  running),  bistable (flip-flop),  and  monostable  (one-shot)  multivibrators. We have already discussed bistable flip-flops and their uses in a computer. How they are used will depend on the technology of the computer. Generally speaking, when used for timing circuits, we can say: Astable  (free  running)  multivibrators  provide the   voltage   pulse   to   trigger   a   one-shot multivibrator. Monostable (one-shot) multivibrators shape the pulse to be used to enable and disable circuits, logic gates, and special registers. They can be used in single- or multiple-phase systems. Bistable (flip-flops) multivibrators are used as a special register to count clock pulses from a one-shot multivibrator or an oscillator. TIMING CIRCUIT FUNCTIONS The uses of astable and monostable multivibrators depend  on  the  complexity  of  the  computer.  The multivibrator can be used to provide the pulse and/or pulse shaper. Let’s discuss their two types of uses. They   are   single-phase   clock   systems   and multiple-phase clock systems. SINGLE-PHASE  CLOCK  SYSTEMS.  —A single-phase clock system consists of a free running multivibrator and a single-shot multivibrator. A free running  multivibrator  provides  the  pulse  and  the single-shot   multivibrator   shapes   the   pulse.   An oscillator could also be used to provide the trigger pulse for a single-shot multivibrator. The pulse is the output of the pulse shaper, which is then used to enable and disable circuits in whatever sequence is necessary to properly  execute  the  computer  program.  Refer  to  figure 4-40 for a simple diagram (block and timing) of a single-phase clock system. MULTIPLE-PHASE  CLOCK  SYSTEMS.  —A multiple-phase  clock  system  on  the  other  hand provides  multiple  pulses  that  can  be  used  to alternately  enable  and  disable  circuits.  This  permits functions involving more than one operation to be completed during a given clock cycle, or a given operation to be extended over more than one clock cycle. A multiple phase clock system can consist of an oscillator or free running flip-flop, and single- shot  multivibrator  combination,  or  a  delay  line oscillator and flip-flop combination. Remember a crystal-controlled   oscillator   will   provide   better Figure  4-40.—Example  of  a  single-phase  clock  system. 4-23






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