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jecottrell
Joined: 16 Jan 2005
Posts: 559
Location: Tucson, AZ
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| Time Measurement Between Events |
 Posted: Sun Jul 03, 2005 10:16 pm |
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Hello All,
I have searched the forum for posts that were similar to my problem without any luck.
I need to measure the time between one flag being set and a second flag being set in my code. My initial attempt at resolution should be around 1ms. Part is a 18F2525 and TIMER_0 (RTCC) is currently being used as a counter for a firmware RTC. A 4Mhz crystal is being used.
My initial approach would be to configure TIMER_1 and start it upon the first flag being set and read the number of ticks when the second flag is set..... However, my inexperience with timers doesn't make this as easy as it sounds.
Here are some specific questions that may help me understand:
1. I assume you probably can't enable(start and stop) timers within code. I would imagine that I will have to generate an ISR for the timer and then use the ISR to generate a start time, a finish time, and then subtract the two in the main-line code reach a period?
2. How do I extract the value of the timer to allow me to subtract one reading from the next? (Now that I think of it, I guess I could just use an ISR to add ticks to a counter. Zero the counter at the first flag and read the counter at the second flag?)
3. Is there a better way to do this?
4. Is there a post that anyone knows of that already solves this problem?
Thanks in advance for the help,
John |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Jul 03, 2005 10:58 pm |
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Do a test program.
The program below displays the following output on the terminal window:
| Code: | #include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
//========================
void main()
{
int16 value;
set_timer1(0);
// Turn on the timer and let it run for 1 ms.
setup_timer_1(T1_INTERNAL | T1_DIV_BY_1);
delay_ms(1);
// Turn it off, read it and display it.
setup_timer_1(T1_DISABLED);
value = get_timer1();
printf("%lu\n\r", value);
// Turn it on again for 1 ms.
setup_timer_1(T1_INTERNAL | T1_DIV_BY_1);
delay_ms(1);
// Turn it off, read it and display it.
setup_timer_1(T1_DISABLED);
value = get_timer1();
printf("%lu\n\r", value);
while(1);
} |
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Mark
Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA
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| Re: Time Measurement Between Events |
| Posted: Mon Jul 04, 2005 12:48 am |
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| Quote: |
1. I assume you probably can't enable(start and stop) timers within code. I would imagine that I will have to generate an ISR for the timer and then use the ISR to generate a start time, a finish time, and then subtract the two in the main-line code reach a period?
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Nope you can. Read the datasheet on the pic.
| Quote: |
2. How do I extract the value of the timer to allow me to subtract one reading from the next? (Now that I think of it, I guess I could just use an ISR to add ticks to a counter. Zero the counter at the first flag and read the counter at the second flag?)
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Well if you start at 0 and you stop it then just read the timer value
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3. Is there a better way to do this?
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yeah, toggle a pin and measure the signal with a scope.
| Quote: |
4. Is there a post that anyone knows of that already solves this problem?
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lots of them if you search with the correct terms. |
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jecottrell
Joined: 16 Jan 2005
Posts: 559
Location: Tucson, AZ
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| Posted: Mon Jul 04, 2005 11:35 am |
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Thanks,
It looks like PCM's suggestion is just what I need.
To explain in a litle more detail what I'm attempting might clarify things. I have two SPI sensors that return readings. When the readings excede a preset value a flag is set. So I'm attempting to determine velocity by measuring the time between the two flags.
John |
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