June Birth Magoncia

user Electronics Engineer

city Innovuze Solutions, Inc.


  • 24 Seconds Shot Clock

  • Created: Sep 21, 2016

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Summary

Logic gates, counters, and decoders are some of the basic components of a logic circuit and a microcontroller. Without them, digital systems couldn’t operate. Recently, microcontrollers have really become more popular. The programmability of a microcontroller makes it preferable to be used instead of basic logic components. However, in applications that would require more digital pins and more tasks, it is better to support the microcontroller with external logic components. This 24 seconds shot clock reference design shows an example of basic logic components supporting an ATTiny microcontroller.


This reference design features a logic circuit that can be used for a 24 seconds shot clock application. The circuit is composed of 74LVC2G02 NOR gates, 74LVC2G08 AND gates, CD74HC192NSR counters, SN74LS47NSR 7-segment drivers, ATTINY85 microcontroller, passive components, switches, and headers. The two CD74HC192NSR counters (U5 and U6) are set to number “2” and “4”, respectively, through its BCD or binary preset pins (P0-P3). Its BCD outputs (Q0-Q3) are decoded by the SN74LS47NSR (U7 and U8) into an output that can be used to drive 7-segment displays. The CD74HC192NSR counter (U6) waits for a clock signal on its clock down pin (CPD). The task of the ATTiny85 microcontroller is to generate a 1Hz clock signal with 50% duty cycle and send it to the counter (U6). When switch S1 is closed, the counter (U6) starts to count down to zero. When the counter (U6) reaches zero, its borrow pin (TCD) will send a half a clock period LOW signal to the CPD pin of counter (U5) to decrement the number at counter (U5).


When both of the counters (U5 and U6) reach zero, all of its outputs are LOW. These outputs are connected to the inputs of the two 74LVC2G02 NOR gates (U1 and U2), which outputs a HIGH signal. The HIGH outputs of the NOR gates make the 74LVC2G08 AND gates (U3 and U4) outputs a HIGH signal. The two final outputs at U4 are used to trigger a buzzer and give a HIGH signal to the master reset pin (MR) of the two CD74HC192NSR counters. This HIGH signal applied to master reset pin of the counters stops the countdown at zero. To reload the 24 seconds shot clock, both of the CLEAR and LOAD pushbuttons must be pushed simultaneously.

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