Through a few of my previous articles I have tried to cover the working principles of a couple of versatile ICs like the 4060 and 4017. Unlike ordinary CMOS ICs (which has also been discussed in many of my previous articles), the above ICs are equipped with many different built-in features which may be accessed or activated by just applying the appropriate logic inputs or components to their relevant pin-outs. For example, the IC 4060 has a timer/counter/oscillator which can be triggered just by connecting a couple of passive components to its pin no 9, 10, and 11. Similarly the IC 4017 requires only an external clock signal to make its outputs swing into action and produce interesting logic sequences.
Another IC whose efficiency can be compared to the above devices is the IC 74LS164, the initial digits “74" indicates that this device is from the TTL family. Obviously being from the TTL family means that it strictly operates at 5 volts DC supply.
Let’s try to understand the various parameters involved to make its output switch interesting logic patterns having diverse application potentials.
Referring to the figure on the left we see that externally it looks just like an ordinary 14-pin dual in-line chip. The extreme top left pin from the printed side is the first pin in the order, and the pin just opposite to it on the other row is the last or the 14th pin.
The following data gives its pin-out specifications:
Pin #14 is supply positive input,
Pin #7 is the negative or the ground pin of the IC,
Pin #8 is the clock input and receives externally applied oscillations,
Pin #9 is the reset input,
Pin-out 1 and 2 are the serial inputs of the IC.
The remaining pin-outs are all the outputs of the IC.
Basically in response to every clock input’s rising edge, the outputs serially produce one after the other a sequencing logic high at their pin-outs in the order 3-4-5-6-10-11-12-13 until all of them go high.
The serial inputs pin 1 and 2 of the IC has a special purpose of resetting the above procedure in an interesting pattern. For example if the pin – outs 1 and 2 are permanently connected to the positive supply and the reset pin is joined to the last output of the IC (pin #13), then, as discussed earlier, once all the outputs become high the last pin #13 instantly resets the whole circuit, shutting the whole array of outputs, and the cycle repeats.
Now suppose if through a simple wiring pins 1 and 2 are configured in such a way that when the last pin (#13) of the IC goes high while sequencing serially, it grounds the serial input (#1 and 2). The whole output array, instead of shutting down all together, now starts receding sequentially one-next-to-the-other, until all of them are shut down to begin the cycle all over again. This opposite sequencing also takes place in accordance to the rising pulse of the input clocks.
I hope it wasn’t all that difficult to understand the above functioning details of the IC 74LS164. Hopefully you will get to see another article pretty soon, which will explain the practical utility of this wonderful IC.