Post by thread:[PIC]: Measuring a decreasing frequency.

Greetings, I'm using a PIC16F876, and would like to measure time from a steady state frequency to 0. To clarify, I have a motor that will be running somewhere from 500 to 10000 HZ (RPM, but I will convert the RPM to HZ using a 60 slot wheel and LED pickup). I will be stopping this motor quickly, and wish to measure the time required to stop. This total time will be somewhere around 50 milliseconds. In order to measure this, I was thinking of monitoring the frequency, then when it started to drop, set a timer, then waited for it to reach 0 or even a slow enough frequency to call it 0. This is my first project using a PIC, and I've messed around with one enough to be able to use TMR0 and basic I/0. Does this PIC have the capability to do what I need? Are there enough TIMERS/COUNTERS to do this? Any ideas would be greatly appreciated. Thanks, Jeffrey Scharpf Senior Engineering Technician Milwaukee Electric Tool Corporation Technical Center Brookfield, Wisconsin, 53005 (262) 783-8681 _______________________________________________________ Are you a Techie? Get Your Free Tech Email Address Now! Many to choose from! Visit http://www.TechEmail.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]: PIC only [EE]: engineering [OT]: off topic [AD]: advertisements

> I'm using a PIC16F876, and would like to measure time from a steady state frequency to 0. To clarify, I have a motor that will be running somewhere from 500 to 10000 HZ (RPM, but I will convert the RPM to HZ using a 60 slot wheel and LED pickup). > I will be stopping this motor quickly, and wish to measure the time required to stop. This total time will be somewhere around 50 milliseconds. In order to measure this, I was thinking of monitoring the frequency, then when it started to drop, set a timer, > Does this PIC have the capability to do what I need? Are there enough TIMERS/COUNTERS to do this? Any ideas would be greatly appreciated. Yes, the 16F876 can do this because it as a CCP module. The capture feature is well suited to measuring the period of a pulse train. I've done these kind of tachometer applications a number of times using CCP modules in capture mode. The trickiest part is dealing with long periods that exceed the 16 bit timer 1 wrap period. You have to sample timer 1 more often than it could possibly wrap. When you do this, you accumulate an additional delta since the last pulse in a separate variable, then reset your saved timer 1 start value for this interval. When the capture happens, subtract the saved timer 1 start value and add the accumulated delta. Now you've got the period which you can use directly or invert to get the frequency. That's the basic strategy, but there are lots of details to work out, and some shortcuts. For example, you don't really need to sample both bytes of timer 1 when just compensating for wrap arounds. The high byte is enough, but you have to think carefully about how your saved start time is adjusted. Also keep in mind that if you do an unsigned subtract of a new timer value minus an old value, you get the positive delta regardless of whether the number wrapped in between - as long as there weren't more than 65535 ticks. ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, spam_OUTolinTakeThisOuTcognivis.com, http://www.cognivis.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Fri, 11 August 2000, Olin Lathrop wrote: Olin, Thank you very much for your response. I will look at the strategy you have shown. Would you by any chance happen to have any sample source code to start me out? I am relatively new, and any help would be appreciated. Thank you for your response. Jeff {Quote hidden}> > Yes, the 16F876 can do this because it as a CCP module. The capture feature > is well suited to measuring the period of a pulse train. I've done these > kind of tachometer applications a number of times using CCP modules in > capture mode. > > The trickiest part is dealing with long periods that exceed the 16 bit timer > 1 wrap period. You have to sample timer 1 more often than it could possibly > wrap. When you do this, you accumulate an additional delta since the last > pulse in a separate variable, then reset your saved timer 1 start value for > this interval. When the capture happens, subtract the saved timer 1 start > value and add the accumulated delta. Now you've got the period which you > can use directly or invert to get the frequency. > > That's the basic strategy, but there are lots of details to work out, and > some shortcuts. For example, you don't really need to sample both bytes of > timer 1 when just compensating for wrap arounds. The high byte is enough, > but you have to think carefully about how your saved start time is adjusted. > Also keep in mind that if you do an unsigned subtract of a new timer value > minus an old value, you get the positive delta regardless of whether the > number wrapped in between - as long as there weren't more than 65535 > ticks. > > > ***************************************************************** > Olin Lathrop, embedded systems consultant in Devens Massachusetts > (978) 772-3129, .....olinKILLspam@spam@cognivis.com, http://www.cognivis.com > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics Jeffrey Scharpf Senior Engineering Technician Milwaukee Electric Tool Corporation Technical Center Brookfield, Wisconsin, 53005 (262) 783-8681 _______________________________________________________ Are you a Techie? Get Your Free Tech Email Address Now! Many to choose from! Visit http://www.TechEmail.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

part 1 771 bytes content-type:text/plain; (decoded 7bit) >> Thank you very much for your response. I will look at the strategy you have shown. Would you by any chance happen to have any sample source code to start me out? I am relatively new, and any help would be appreciated. << OK, I've attached some code, but it probably won't help as much as you think. I use a lot of macros for subroutine linkage, bank switching, and the like which of course you won't find in the assembler documentation. Also, every tachometer implementation I've done has its own unique custom issues. The one I included the code snippet for is trying to measure the speed of a gasoline engine, but also determine whether the engine should be assume to be off or not. Anyway, for what it's worth... part 2 15046 bytes content-type:application/octet-stream; (decode) part 3 335 bytes ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, olinKILLspamcognivis.com, http://www.cognivis.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics