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FIGURE 16.14 A case of a bad power supply with the potential to destroy a 5-Vrated processor like the AT90S2343. The trace illustrates the output voltage surging to 12 V when it is switched off.
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AVR PROJECT 8: A PULSE FREQUENCY COUNTER WITH AN RS-232 INTERFACE
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17.1 At a Glance
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In this chapter we look at:
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What a frequency/period counter is Design of a frequency counter Design of a period counter A working design of an AT90S2323-based frequency counter for the PC RS-232 serial port 5. A practical application for the frequency counter
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1. 2. 3. 4.
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17.2 Introduction
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Measuring frequency can be a frequent requirement. You often need a frequency counter either as a general-purpose test instrument or as a gadget for a specific application. With a general-purpose test instrument, you may want to measure the frequency of a signal, and with a gadget you may want to measure the number of people passing a gate in a minute,
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292 AVR PROJECT 8: A PULSE FREQUENCY COUNTER WITH AN RS-232 INTERFACE
for example. Or you may want to measure the wind speed or the speed of rotation of the wheel of your motorbike. Measuring wind speed or the rotation speed of a wheel may seem unconnected to the matter of measuring frequency, however that is not so. Consider a scheme where you install a magnet of the wheel spokes and a hall-effect sensor on the wheel fork. When the magnet passes the sensor, the sensor output changes state during that time. By measuring the number of such pulses in a unit of time, you calculate the speed of rotation of of the wheel. A similar mechanism could be created for measuring the wind speed using a magnet and a hall-effect sensor on a wind vane. Similarly, you would also want to measure time between events, which would need a timer. Measuring time and frequency are related issues. Once you can measure time, you can calculate frequency, and vice-versa because time and frequency are inversely related; all you need is some means to process the information.
17.3 How Does a Frequency Counter Work
Figure 17.1 illustrates the block diagram of a simple frequency counter. This frequency counter is designed to handle analog signals. The analog input is amplified by an amplifier and then passed through a waveshaper circuit to produce digital waves that have the same frequency as the original analog signal. The circuit has a time-base generator that generates precise pulses of required duration. This gate pulse enables a gate (that is why it is called a gate pulse) for the duration of the gate pulse, and the incoming waves are counted by a counter. Just before the incoming pulses are clocked into the counter, the
Input Amplifier and Wave shaper
Controlled Gate
Counter Chain
Display or Transmitter
Gate Control T seconds
Reset Counters
Time-base generator and Control circuit
FIGURE 17.1 A frequency counter.
HOW DOES A PERIOD COUNTER WORK 293
counters are cleared, and at the end of the gate pulse, the contents can be displayed or further manipulated. If the gate pulse period is set to one second, the count accumulated by the counter is exactly the frequency of the incoming wave in hertz. It is obvious that the frequency to be counted must be greater than the period of the gate pulse. If that is not the case, then either increase the gate period or use the period counter as described in the next section and then calculate the frequency. Figure 17.2 illustrates the timing diagram of a frequency counter. The time-base signal is passed through a D-type filp-flop to get the gating signal (which is the gate pulse). This results in a symmetrical square wave of 2-Hz frequency with an on period of 1s and an off period of 1s. Other control signals to clear the counters and display the count are not illustrated.
17.4 How Does a Period Counter Work
The period counter is built in a similar fashion as the frequency counter. Here, instead of the gating pulse gating the incoming wave, the incoming wave is used to gate the pulses generated by the time-base generator. This method is used if the period of the incoming wave is large and measuring frequency is a problem. Consider a situation when the incoming wave has a period of 2 seconds, i.e., a frequency of 0.5 Hz. To resolve the frequency of this wave to some accuracy, we will need to have a time-base gate pulse of say 50 seconds. However, if we choose to measure the period of the pulse, we will get better resolution in a smaller measurement time. In a period measurement device, the time-base generator generates high-frequency pulses of precise value. These pulses are gated into the counter for one time period of the incoming wave. Figure 17.3 illustrates the block diagram for a period counter. Figure 17.4
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