10Mhz Frequency Counter Circuit

Posted on Jan 9, 2013

The circuit consists of an ICM7208 seven-decade counter (Ul), an ICM7207A oscillator controller (U2), and a CA3130 biFET op amp (U3). Integrated circuit Ul counts input signals, decodes them to 7-segment format, and outputs signals that are used to drive a 7-digit display. Integrated circuit U2 provides the timing for Ul, wliile U3 conditions the input signal to provide a suitable waveform for input to Ul. 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 Ul 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, activating any segments of the digits whose anodes are high as the result of decoding by Ul.

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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 SI, the pulses occur every 2 seconds and cause U2 pin to go high for one second, which prevents additional input signals from entering Ul. That causes the count latched in Ul`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 Ul. 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 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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