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kel
Joined: 17 Oct 2005
Posts: 68
Location: Brisbane
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| Why is max set to 0 and min to 255 in the code? |
 Posted: Tue Dec 27, 2005 10:11 pm |
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from the following code, can some exolain to me why min is set 255 and max set 0 initially...i.e. min = 255; //and displays the min and max = 0; //values for that 100ms period
Thanks budds..
Happy chrismass
| Code: | [/////////////////////////////////////////////////////////////////////////
#include<16f877a.h>
#fuses HS,NOLVP,NOWDT,PUT
#use delay(clock=20000000)
#use rs232(baud=9600,xmit=PIN_C6,rcv=PIN_C7)
void main() {
int i, value, min, max;
printf("Sampling:");
setup_adc_ports( RA0_ANALOG );
setup_adc( ADC_CLOCK_INTERNAL );
set_adc_channel( 0 );
do { //Takes 30 samples from pin A0
min = 255; //and displays the min and max
max = 0; //values for that 100ms period
for(i = 0; i <= 30; ++i) {
delay_ms(100);
value = read_adc();
if(value < min)
min = value;
if(value > max)
max = value;
}
printf("\n\rMin:%x MAX: %x", min, max);
} while (TRUE);
}
] |
[/code] |
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jecottrell
Joined: 16 Jan 2005
Posts: 559
Location: Tucson, AZ
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| Posted: Tue Dec 27, 2005 10:42 pm |
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Those are "seed" values for the first comparison of the values of interest. If "min" was initially defined as 100 any value between 100 and 255 would not be registered as a new min.
Good luck,
John |
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kel
Joined: 17 Oct 2005
Posts: 68
Location: Brisbane
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| why not the otherway around? |
| Posted: Tue Dec 27, 2005 11:08 pm |
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| Why not the min=0; and max=255 instead? |
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jecottrell
Joined: 16 Jan 2005
Posts: 559
Location: Tucson, AZ
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| Posted: Tue Dec 27, 2005 11:40 pm |
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If you initialized "min" with zero and "max" with 255 everytime through the MAIN DO LOOP you'd get:
(I think that is the correct formatting for %x)
Look at the comparisons and give yourself a couple of "for instance" ADC readings and see how the code works. Remember you'll never get a value smaller than 0 or greater than 255.... The new min value would never get put into the min variable and the new max value would never get put into the max variable.... |
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newguy
Joined: 24 Jun 2004
Posts: 1903
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| Posted: Tue Dec 27, 2005 11:53 pm |
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I have to agree with jecottrell here. If you want your code to work as you intended, you need to do an initial read of the A/D, right after you initialize it, and just before your do.....while loop.
i.e.:
| Code: | min = read_adc();
max = min; |
This sets your max and min to be the same, but more importantly within the range you'll see. |
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ckielstra
Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands
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| Posted: Wed Dec 28, 2005 2:33 am |
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| newguy wrote: | I have to agree with jecottrell here. If you want your code to work as you intended, you need to do an initial read of the A/D, right after you initialize it, and just before your do.....while loop.
i.e.:
| Code: | min = read_adc();
max = min; |
This sets your max and min to be the same, but more importantly within the range you'll see. |
The code as given is correct. Your suggested change is just another method of getting the same result at the cost of an extra call to read_adc(). |
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jecottrell
Joined: 16 Jan 2005
Posts: 559
Location: Tucson, AZ
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| Posted: Wed Dec 28, 2005 7:20 am |
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Upon re-reading your original post I see I missed a subtle but important point that may be confusing you.
| kel wrote: | | //and displays the min and max = 0; //values for that 100ms period |
It appears your ADC may be reading 0 constantly and you're assuming that the code is behind getting a min and max = 0? (HINT: The code is not the problem.)
| Code: | do { //Takes 30 samples from pin A0
min = 255; //and displays the min and max
max = 0; //values for that 100ms period
for(i = 0; i <= 30; ++i) {
delay_ms(100);
value = read_adc();
if(value < min)
min = value;
if(value > max)
max = value;
}
printf("\n\rMin:%x MAX: %x", min, max);
} while (TRUE); |
Also, the comment about taking thirty samples and reporting for a 100ms period is incorrect. The 100mS delay is within the FOR loop. (HINT: 30 x 100mS = ?)
Newguy, my suggestion was not to change the code, it was to sit down with pencil and paper and what if some values through the code to see how it reacted. I'm afraid I'm a victim of my own unclear explanation... I also agree with ckielstra that the initial ADC is not required.
Maybe our biggest problem here is translating between Australian, American, Canadian, and Dutch English?
I will reiterate my original suggestion. Give yourself some "made-up" ADC readings between 0 and 255 and step through the code and see how it reacts. Make a column for "min" and "max" and note their values each time through and I think you'll see what I'm talking about.
Good luck,
John |
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newguy
Joined: 24 Jun 2004
Posts: 1903
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| Posted: Wed Dec 28, 2005 10:47 am |
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| jecottrell wrote: | | Newguy, my suggestion was not to change the code, it was to sit down with pencil and paper and what if some values through the code to see how it reacted. I'm afraid I'm a victim of my own unclear explanation... I also agree with ckielstra that the initial ADC is not required. |
Doh!
I hate it when I miss things! I just now realized that he was setting 'min' to 255 and 'max' to 0. When I first read that, my mind switched them around. You're right - setting the variables like this is proper, no extra read is required.
| jecottrell wrote: | | Maybe our biggest problem here is translating between Australian, American, Canadian, and Dutch English? |
You're probably right! I've run into this type of thing before......just so you know, the north american definition of 'fanny' is vastly different from the British definition!  |
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