 Professor, Electronics and Telecommunication Engineering, Walton Institute of Technology, Solapur. Today, we will discuss time period measurement using IC-74C926. Learning outcome, at the end of this session, students will be able to describe time measurement scheme using IC-74C926. Contents. We will first study the block diagram of time measurement scheme. Then we will take one example on time period measurement scheme. And finally, we will see what is the resolution in a measurement. Here, we start with the block diagram of time period measurement scheme. As usual, we have this 74C926, which is a counter plus display driver circuitry. This consists of four decade counters. So we require four seven segment displays, which are displayed using multiplexing technique. Here, this counter counts the clock pulses. Counter data is latched at the end of the counting period. And the counter is reset by using this reset input to make this counter ready for the next measuring cycle. And this display select pin is connected to ground so that this latched data into this 74C926 will be displayed on this display system. So the measurement, time period measurement is done by using this block diagram. Here, we require a crystal, say, of 1 megahertz. Then we require some frequency divider network. And then this signal is given to the end gate input. Unknown low frequency signal is applied here. It is passed through this input interface circuit, which is used for signal conditioning of the input signal. It is then passed to the flip-flop so that the whole time period, this output of the flip-flop will be on for whole time period of the unknown low frequency so that whatever the clock pulses given by this crystal oscillator are counted for the whole time period of the unknown frequency signal. The same signal is given to the control signal to generate the latch enable input to this 74C926 at the end of the counting. And after the data is latched, we should make this 74C926 ready for the next measurement cycle by generating this reset signal. So these two signals are generated by this control signal generating block. Time period measurement can be better understood by using, see, the example. Consider a low frequency whose time period is to be measured as 50 hertz. From this, we can find out the time period t is equal to 1 by 50, which is 20 millisecond. So time period of this input signal frequency is 20 millisecond. So this we should get on the display device. Using IC74C926, we can display this value maximum because this IC74C926 consists four-decade counters. So four-digit display with this number will be 2,000 for above example, that is this 50 hertz signal. That is the count value maximum is 2,000. So count value, how this count value is obtained? That is this oscillator frequency signal is counted for 20 milliseconds. That means how many clock pulses of this oscillator frequency should consist in 20 milliseconds to get the count is equal to 2,000. So since here it is a 2,000, so put the value. By putting the value, the t oscillator we found as 20 millisecond divided by 2,000. That is 0.01 millisecond. Therefore, the f oscillator frequency given for counting is 100 kilohertz. If you look at the previous diagram, here what is required is here we require 100 kilohertz. But we have 1 megahertz. So this frequency divider network must be of value 10 so that we get from 1 megahertz the 100 kilohertz. And for this 50 hertz signal we get the maximum count is equal to 2,000. That means 2,000 we can say count is there. Now we can adjust the decimal point. That means we can activate this. They get 20.00 means it is a 20 millisecond. Unit of measurement will be millisecond. So this will vary. This count will vary if the input frequency varies. So this way we get a time period measurement scheme. Then we resolution in frequency measurement. So first resolution is a measure of the number of digits in the reading of the frequency. So in frequency measurement resolution is determined by the gate time. In previous video we saw that this gate time we can adjust for changing the resolution. So for example, if gate time is 1 second then resolution will be 1 hertz. That means resolution will be here less. 1 hertz means error in measurement will be plus or minus 1 hertz. If you want resolution to be good then you can increase the gate time to 10 fold. That is 10 second. Then resolution will be improved and it will be 0.1 hertz. So now here resolution is so error in the measurement will be just 0.1 hertz. But here error in the measurement is 1 hertz. So to decrease the error in measurement or to increase the resolution what we have to do is we have to increase the gate time. That's why in frequency measurement resolution is determined by the gate time. This is the time for which the counter is counting the number of pulses. We know already this one. Now let us see resolution in time period measurement. The resolution of the time period measurement is better than frequency measurement. Since in a frequency measurement the count will be out by plus or minus 1 hertz. As we saw already here this resolution will be gate time is 1 second. Resolution will be 1 hertz. So in frequency measurement the resolution will be plus or minus 1 hertz for gate time is equal to 1 second. Whereas in time counting it will be out by plus or minus 1 oscillator clock frequency which is higher value. That's why the resolution using time period measurement will be better than the frequency measurement. Let us derive a formula which decides when to go for time period measurement. Consider a f oscillator frequency of 1 megahertz as usual. fx is an unknown frequency. ft is a time-based frequency generally which is 1 hertz. And n is a count value which is displayed on the display device. Now in frequency measurement n that is count value how it is found that is unknown frequency fx divided by time-based frequency ft. But always ft is 1 that's why n is equal to fx. This is equation number 1. And in period measurement n is equal to f oscillator frequency divided by fx. So here in time period measurement what is counted is oscillator frequency signal is counted for a time period of unknown frequency signal. So this is equation number 2. We can equate equation 1 and 2. So we get fx is equal to f oscillator divided by fx. So fx is equal to under root f oscillator. Since f oscillator we have chosen as 1 megahertz. So we get 1 kilohertz. That means if unknown frequency fx is 1 kilohertz or less than 1 kilohertz it is better to go for time period measurement instead of frequency measurement. And if the input frequency fx is greater than 1 kilohertz it is better to go for frequency measurement. Let us have a question. Which method of measurement gives better resolution? Whether frequency measurement or time period measurement. You pause the video and answer the question. The answer will be as discussed earlier it depends upon the input signal frequency. If the input signal frequency is less than square root of oscillator frequency then it is better to go for time period measurement. And if the input frequency is greater than square root of oscillator frequency it is better to go for frequency measurement. References. Electronic system designed by Bhavti Tarate Electric Tech Publications Satara. Thank you.