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wedilo
Joined: 16 Sep 2003
Posts: 71
Location: Moers, Germany
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| Unbelievable result of an subtraction |
 Posted: Mon Mar 22, 2004 2:29 am |
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Hello folks,
Please help me by this strange problem. I can't find my mistake...
The following code is to determine if an pulse found or not. If not then show a message. For the present...
For info, all variables lg... are long and global
| Code: | if (lgCounter - lgLastPulse >= lgTimer_Calls_Per_Sec * 3)
{
bgSignal_OK = False;
printf("No signal detected - %lu-%lu >= %lu * 3 --> %lu >= %lu\r\n"_
, lgCounter, lgLastPulse, lgTimer_Calls_Per_Sec, lgCounter - lgLastPulse, lgTimer_Calls_Per_Sec * 3);
output_b(~0x02);
}
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Most of the time it works, but after some minutes I get a message like this:
'No signal detected - 2048-2044 >= 20 * 3 --> 4 >= 60'
I'm sure there is no overflow or anything else.
I CAN'T believe, that the subtraction of '2048-2044' is greater than '20 * 3'
I'm driving crazy... Please help
73 Sven |
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Pete Smith
Joined: 17 Sep 2003
Posts: 55
Location: Chester, UK
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| Re: Unbelievable result of an subtraction |
| Posted: Mon Mar 22, 2004 3:35 am |
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| wedilo wrote: | Hello folks,
Please help me by this strange problem. I can't find my mistake...
The following code is to determine if an pulse found or not. If not then show a message. For the present...
For info, all variables lg... are long and global
| Code: | if (lgCounter - lgLastPulse >= lgTimer_Calls_Per_Sec * 3)
{
bgSignal_OK = False;
printf("No signal detected - %lu-%lu >= %lu * 3 --> %lu >= %lu\r\n"_
, lgCounter, lgLastPulse, lgTimer_Calls_Per_Sec, lgCounter - lgLastPulse, lgTimer_Calls_Per_Sec * 3);
output_b(~0x02);
}
|
Most of the time it works, but after some minutes I get a message like this:
'No signal detected - 2048-2044 >= 20 * 3 --> 4 >= 60'
I'm sure there is no overflow or anything else.
I CAN'T believe, that the subtraction of '2048-2044' is greater than '20 * 3'
I'm driving crazy... Please help
73 Sven |
Print out (before the if statement) the values of each of the terms, and the results of the maths.
Something along the lines of
printf("%lu %lu %lu %lu",lgCounter,lgLastPulse ,lgTimer_Calls_Per_Sec * 3, (lgCounter-lgLastPulse));
Also, check that all your variables really are long. It could even be that by doing *3, you're casting the lgTimer_Calls_per_sec to an int.
Pete. |
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Ttelmah
Guest
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| Re: Unbelievable result of an subtraction |
| Posted: Mon Mar 22, 2004 6:02 am |
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| wedilo wrote: | Hello folks,
Please help me by this strange problem. I can't find my mistake...
The following code is to determine if an pulse found or not. If not then show a message. For the present...
For info, all variables lg... are long and global
| Code: | if (lgCounter - lgLastPulse >= lgTimer_Calls_Per_Sec * 3)
{
bgSignal_OK = False;
printf("No signal detected - %lu-%lu >= %lu * 3 --> %lu >= %lu\r\n"_
, lgCounter, lgLastPulse, lgTimer_Calls_Per_Sec, lgCounter - lgLastPulse, lgTimer_Calls_Per_Sec * 3);
output_b(~0x02);
}
|
Most of the time it works, but after some minutes I get a message like this:
'No signal detected - 2048-2044 >= 20 * 3 --> 4 >= 60'
I'm sure there is no overflow or anything else.
I CAN'T believe, that the subtraction of '2048-2044' is greater than '20 * 3'
I'm driving crazy... Please help
73 Sven |
My suspicion is that lgCounter is being changed in an interrupt driven event?. If so, then the problem is that you are encountering an interrupt between reading the first and second byte in the comparison. Note that the value after the comparison, is an even binary value (2048). Hence the counter will have jumped from 07FF to 0800 at the last interrupt. At the moment of the comparison, assume that the LSB had been read, and then the interrupt occurred before the MSB was read. The comparison would then be using the value 08FF, which would result in a 'true' test.
When comparing 16 bit values, that are changed in an interrupt, you need to either disable the interrupt for the duration of the comparison, or copy the value into a temporary register, with the interrupts disabled for the copy.
Best Wishes |
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wedilo
Joined: 16 Sep 2003
Posts: 71
Location: Moers, Germany
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| Posted: Mon Mar 22, 2004 6:57 am |
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Hello Pete, hello Ttelmah,
Yeah, I think that's the problem. The interupt occur during the read of high and low byte.
But I thought until yet, something like this is not possible. Oops!
I will try to backup lgCounter before the duration of the comparison.
Thank you very MUCH...
73 Sven |
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Ttelmah
Guest
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| Posted: Mon Mar 22, 2004 7:07 am |
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| wedilo wrote: | Hello Pete, hello Ttelmah,
Yeah, I think that's the problem. The interupt occur during the read of high and low byte.
But I thought until yet, something like this is not possible. Oops!
I will try to backup lgCounter before the duration of the comparison.
Thank you very MUCH...
73 Sven |
Remember to disable the interrupts when you save the copy of the counter, or the same problem can still 'catch'!...
Best Wishes |
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