Re: [avr-chat] Comparisson with negative values

[Rick Altherr]

Sure. A few things are actually dubious here. volatile is reserved for cases where the contents of the variable might change without direct intervention from the code. Examples would be if the variable was a pointer referencing a register. In your case, the values should be set somewhere in your code, so volatile is not necessary.

So, the corrected snippet of code would be:
uint16_t reference;
uint16_t feedback;
int16_t error;

error = reference - feedback;

if (error>0) set_pwm(pwm+1);
if (error<0) set_pwm(pmw-1);



Rick


On Nov 19, 2007, at 9:30 AM, John wrote:

Second, you have specified error to be a uint16_t. This means that error will be a unsigned 16-bit integer. So, when error is set to reference - feedback, the negative number is implicitly converted to an unsigned number. A side effect of the way signed numbers are stored in computers is that when they are treated as unsigned number, they become very large. Let's consider this in hexadecimal for a bit to illustrate the point. A value of 1 would be 0x0001. Now, if we subtract 1 from 0, we would logically expect the result to be -1. In the world of computer arithmetic, however, you get 0x0000 - 0x0001 = 0xFFFF. In the case of a uint16_t, this is interpreted as 65535. Now, if you define the variable as an int16_t, the number is considered to be in 2s compliment which correctly translates 0xFFFF to -1. Beware that there are side effects here. If the result of reference - feedback is a large positive number (>32767), the result will be a large negative number. So, make sure that reference - feedback always results in a number between -32768 and 32767.

My application is just a development from largedemo.
I have two potentiometers with the idea of developing a controller.
These are named reference and feedback.

volatile uint16_t reference;
volatile uint16_t feedback;
volatile uint16_t error;

// this works

if (reference > feedback) set_pwm(pwm + 1);
if (reference < feedback) set_pwm(pwm - 1);

// this does not work

error = reference - feedback;

if (error > 0) set_pwm(pwm + 1);
if (error < 0) set_pwm(pwm -1);

In the later case error can take positive or negative values.
In the former case only positive values are involved.

Would someone be as good as to send me the correct syntax
for a comparisson with negative numbers.


John
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Rick Altherr
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