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MAKING THE NUMBERS WORK FOR YOU

by George Martin

Start ý Stating Our Case ý Working the Wiring ý A Case Study in Timer Interrupts ý Sources and PDF

WORKING THE WIRING

Now, letýs assume the A/D converter is measuring acceleration. The readings have a range of 3 to ý3 g. Letýs also assume that the signal conditioning produces an electrical signal of ý5 V for ý3 g, 0 V for 0 g, and 5 V for 3 g. You could wire the 12-bit A/D a couple different ways. Hereýs one example:

The bits marked s are wired to Bit 11, the sign bit. Table 2 is the signed number system values.

In the twoýs complement number system, there are 2048 positive numbers and 2048 negative numbers. Iýve wrestled with how to scale the bits. Is it 3 g per 2047 or 3 g per 2048? Iýll leave that up to you to decide.

WORKING THE WIRING WITH OVERFLOW

If you add readings together, you could stand 16 full-scale (either positive or negative) readings before you overflow the register in the CPU. So, averaging eight readings could be done without concern for overflow. If youýve got a system that needs some filtering, consider this input mapping:

The bits marked x are wired as zero. The signed number system values can be seen in Table 3.

Each bit on the A/D converter has a binary weight of 16 in the CPU. In the twoýs complement number system there are 2048 positive numbers and 2048 negative numbers.

In this scaling, you cannot start adding numbers together. You probably would overflow (or underflow) immediately. But if the first step was a scaling that reduced the magnitude, youýre set up for it.

And, of course, you can wire the A/D inputs to any bit starting with 15 through 11. You would then have a scaling between the two examples Iýve described.

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