On Sun, 21 Mar 2010, yamanoor sairam wrote:

> Hello Sergio,
>         Thanks for your mail. 
> 
> Should the above read
> > Could this be a problem with my code?
> >                  I did the following steps in my code using MikroC complier:
> > adc_rd = adc_read(0); // read voltage from AN0
> > t1 = 900-(float)(1.0625*adc_rd);//t1 is declared as float. I am doing this subtraction because my opamp gives an output of 0V for 900mV and 4.139V for 0mV.
> > t3long = (long)t1long;
> > //I am proceeding to LCD display section thereafter. 
> 
> Should the above read
> 
> t1 = 900 - (float)(adc_rd) * 1.0625;
> t3long = (long)t1;
> 
> 
> The second line was a typo. Am I wrong in the first line?

Think about it. If you NEED to convert "1.0625*adc_rd" to a float AFTER 
the multiplication, then you are going to have problems.

You shouldn't "need" to convert adc_rd to a float because the compiler 
should do that automatically for you before the multiplication because you 
are multiplying an integer with a float. The problem is I don't know how 
well behaved the MikroC complier is (I have not used it) when it comes to 
promoting integers to floats in conjunction with floating point constants. 
I've seen lots of issues in the past with other compilers so I would play 
safe and rewrite it as I've shown (and even then I wouldn't like to 
guarentee it).

I haven't done the math but you should check yours (that adc_rd * 1.0625 
cannot produce a result greater than 900) otherwise you'll get negative 
numbers.

Also something else that comes to mind, is adc_rd a 16 bit integer 
variable or an 8 bit integer variable and does adc_read(0) return a 16 or 
8 bit value? If adc_read(0) returns a 16 bit value and adc_rd is an 8 bit 
integer, you could be seeing a high value (above 255) that is dropping to 
zero and you are seeing it jump about because you are losing the high 
bits.

e.g.

	900 = 0x384 -> 0x84 = 132
	800 = 0x320 -> 0x20 = 32
	700 = 0x2bc -> 0xbc = 188
	600 = 0x258 -> 0x58 = 88
	500 = 0x1f4 -> 0xf4 = 244
	400 = 0x190 -> 0x90 = 144
	300 = 0x12c -> 0x2c = 44
	200 = 0xc8  -> 0xc8 = 200

Regards
Sergio Masci
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