Especially helpful for developing real-time application programs, the circuit in Fig 1 plugs into a PC printer port and measures the execution time of a piece of software. The CD4536 is a 16/24-bit binary counter with a built-in oscillator. This counter has an 8-bit prescaler, which the chip bypasses if the 8_BY pin is high. When this is the case, the CD4536 is a 16-bit counter, and its A through D inputs select which bit is connected to output DO. If the 8_BY pin is low, the CD4536 is a 24-bit counter and the inputs select which 9 to 24 bits connect to output DO. Setting the R pin high clears the counter, and setting CINH high inhibits the counter. With the components values shown in the figure, the oscillation frequency is around 100 kHz. The printer port can directly power the CD4536 because it needs only several milliamps….
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