"tachs" - 1 õppematerjal

Book Analog Interfacing to Embedded Microprocessors

to a port bit or decoded address strobe so the microprocessor can reset it. Finally, the output of the flip-flop is connected to a digital input on the processor. In operation, the microprocessor will periodically check the input and clear the output. If the fan is running, it will eventually (in a few milliseconds) set the flip-flop again. This does not measure fan speed, but it does give an indication that the fan is running. One problem with built-in fan tachs is that they have to run from the fan supply voltage (+V in Figure 6.8). The output is clamped with a zener. But what happens if the fan is plugged in while power is on and the +V and output connections are made before the ground connection is made (Figure 6.9)? Since the zener ground is floating, you instantly get the fan power supply 150 Analog Interfacing to Embedded Microprocessors Figure 6.9 Protecting against fan-tach overvoltage.