Книга: Introduction to Microprocessors and Microcontrollers

How much noise can we put up with?

The 10-finger system that we use is called a ‘denary’ or ‘decimal’ system. We have seen that a 5 V supply would accommodate a 10-digit counting system if each digit was separated by 0.5 V or, using the more modern choice of 3.3 V, the digits would be separated by only 0.33 V.

Question: Using a 5 V supply and a denary system, what is the highest noise voltage that can be tolerated?

Answer: Each digit is separated by only 5 V/10 = 0.5 V. The number 6 for example would have a value of 3 V and the number 7 would be represented by 3.5 V. If the noise voltage were to increase the 3 V to over 3.25 V, the number is likely to be misread as 7. The highest acceptable noise level would therefore be 0.25 V. This is not very high and errors would be common. If we used a supply voltage of 3.3 V, the situation would get even worse.

So why don’t we just increase the operating voltage to say, 10 V, or 100 V? The higher the supply voltage the less likely it is that electrical noise would be a problem. This is true but the effect of increasing the supply would be to require thicker insulation and would increase the physical size of the microprocessor and reduce its speed. More about this in Chapter 11.

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