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lsteele
Joined: 02 Jan 2007
Posts: 18
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| simple floating point in an interrupt |
 Posted: Tue Jan 02, 2007 4:00 pm |
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Hello,
I want to do some simple floating point in an ISR, like this:
| Code: |
#INT_TIMER1 // Gyro calculation ISR
void timer1_isr()
{
isrGyroVal+=currGyroVal;
}
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isrGyroVal & currGyroVal are both float's.
Here's the setup code for the timer 1 interrupt:
| Code: | setup_timer_1(T1_INTERNAL | T1_DIV_BY_1);
set_timer1(0);
enable_interrupts(INT_TIMER1);
enable_interrupts(GLOBAL0); |
If I've done my sums right this interrupt should be triggering at approx 76Hz.
Intuitively I would have thought that I would have been asking for trouble using floating point in a ISR, but I've tried it, and everything seems to be peachy. Am I saving up trouble for myself further along, or can I get away with this?
Thanks,
Luke |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue Jan 02, 2007 4:55 pm |
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It's hard to say exactly how long the floating point addition will take,
because the code branches, depending upon the data.
The following program, running on a PicDem2-Plus board, takes about
290 us (max) to execute the line that does the floating point operation.
This is after it's been running for a few minutes. It starts at about 200 us
and the duration slowly increases in length as the operands increase
in size. I measured this with a scope. There are probably better ways
to do this, but this will give you a ballpark figure.
| Code: |
#include <18F452.h>
#fuses XT, NOWDT, PUT, BROWNOUT, NOLVP
#use delay(clock=4000000)
//============================
void main()
{
float isrGyroVal = 0;
float currGyroVal = 0.1234567;
while(1)
{
output_high(PIN_B0);
isrGyroVal += currGyroVal;
output_low(PIN_B0);
delay_us(500);
}
} |
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