10_MHz_FREQUENCY_COUNTER

Published:2009/6/15 20:51:00 Author:May | From:SeekIC

The circuit consists of an ICM7208 seven-decade counter (U1), an ICM7207A oscillator controller (U2), and a CA3130 biFET op amp (U3). Integrated circuit UI counts input signals, decodes them to 7-segment format, and outputs signals that are used to drive a 7-digit display. Integrated cir-cuit U2 provides the timing for UI, while U3 conditions the input signal to provide a suitable wave-form for input to UI. The 5.24288-MHz crystal frequency is divided by U2 to produce a 1280-Hz multiplexing signal at pin 12 of U2. That signal is input to UI at pin 16 and is used to scan the display digits in sequence. The cathodes of each digit are taken to ground several times each second, acti-vating any segments of the digits whose anodes are high as the result of decoding by UI. The crystal frequency is further divided to produce a short store pulse at pin 2 of U2, followed (after about 0.4 ms) by a short reset pulse at pin 14 of U2. The frequency of the pulses is determined by the state of U2 pin 11.When pin 11 of U2 is taken to ground through S1, the pulses occur every 2 seconds and cause U2 pin to go high for one second, which prevents additional input signals from entering UI. That causes the count latched in UI's internal counters to be transferred to the display.Integrated circuit U2 pin 13 then goes low for one second, allowing a new count to be entered into the seven decade counters of UI. That cycle is repeated, continuously updating the display every two seconds.When U2 pin 11 is taken to the positive supply rail (+5 V), the store and reset pulses occur at 0.2-s intervals, resulting in a 0.1-s count-period. Ten input pulses must be counted in order for a 1 to appear on the first digit, Dl, so that the frequency being measured is obviously 10 times larger than the frequency that is shown on the display. In that mode, the decimal points are driven by M and visually indicate that the 0.1-s count period is being used.The display must have at least seven 7-segment common-cathode multiplexed LED digits. Any common-cathode seven-segment display can be used; no particular display is specified.



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