calculate time between pulses

[hell_pk]

signal is ttl from spindle of an old CNC machine.i want to measure rpm.max rpm are 2400 so max frequency is 50 Hz.the code i wrote above is now working correctly to calculate period.now iwant to calculate rpm.
rmp=60/period
should do the trick i think
but how to perform this division?
accuracy is not a problem now.

[Acetronics2]

Hi,

You were near it in your question ...

rmp=60/period * 1 000 000

But ...

care to the units of "period" , and overflow issues ...

if you Xtal is 4 Mhz, then, period will be in µS ... the minimum "correct" display will be ~ 1100 rpm due to an overflow of your timer ... ( Period > 65535 ...)

Then, the simplest way is to use a prescaler value of 8 ( 137 rpm ) or, if possible, 16 ( 68 rpm minimum )

period will be in 8µS ( or 16 µS ) steps

so,your result will then be :

rpm = 60*1 000 000 /period*prescaler_value

due to PBP limitations you'll have to use the DIV32 "function"

So it will be Written :


intermediate = 60 * 1000 / prescaler_value ' Compulsory !!!
' ( no "intermediate CON xxx" allowed )

' Disable interrupts ... if some used !!!

Dummyvar = 1000 * intermediate

rpm = DIV32 period

' re-enable interrupts allowed

Alain



Alain

[GioppY]

if your period counter has 1uS resolution

asm
move?cb 00h,r2 ; load dividend 60e6
move?cb 87h,r2+1
move?cb 93h,r0
move?cb 03h,r0+1
endasm
TmpW = div32 Period
lcdout dec Tmpw

N.B. div32 is limited to 31bit / 15bit unsigned integer
so Period must be < 32767

Regards
Gianni

[Acetronics2]

Hi,

Gioppy is bright right ...

so let's add :



IF period.7 then

Period = period/2
scale = 2

ELSE

scale = 1

ENDIF


intermediate = 60 * 1000 / prescaler_value ' Compulsory !!!
' ( no "intermediate CON xxx" allowed )

' Disable interrupts ... if some used !!!

Dummyvar = 1000 * intermediate

rpm = DIV32 period

rpm = rpm / scale

Alain

[hell_pk]

thanks for ur help.
I am also interested in what Mr. Wolf mentioned:using PULSEIN with flipflop.would u shed some light on it also.
thanks

[J. Mark Wolf]

In short, what the flip-flop does for you, when measuring frequency, such as in a tachometer application, is it negates the need to measure the duration of the high portion of the period, then switch context and measure the duration of the low portion of the period. The time it takes to switch contexts (jump to another section of code) will be lost, and your measurement will be innacurate.

The flip-flop makes one long pulse, spanning the entire period of the tach signal, so that a single PULSEIN command can accurately measure the duration of the period.

For an explanation of the flip-flop idea go to the following Parallax URL:

http://www.parallax.com/dl/appnt/stamps/bs1appnotes.pdf

Scroll down to the "Practical Pulse Measurements" section.

For the complete blow-by-blow as to how I implemented rotor tachometers on my helicopter go to the following URL:

http://www.basicmicro.com/downloads/docs/RRPM.pdf

[Charles Linquis]

I have a PIC reliably reading 10 fan tachometers while it is doing other things. Something to keep in mind is that if your input signal is at all noisy, a simple period measurement may not result in a stable reading, since noise can easily affect a single reading. It is best to take a an average over a period.

My method involves running a timer interrupt with a period shorter than the shortest expected "high" or "low" tachometer time. The interrupt service routine reads the port bits, and XORs them with the last reading of those bits. If the bit has changed, a counter is incremented. There is one counter for each fan to be monitored. The interrupt service routine keeps track of how many times it has run, so after a period of time (say a quarter or a half-second) it stores the individual counts. Simply multiplying the number of counts by a fixed value yields the RPM.

The ISR is written in assembly, but this approach has several advantages:

It doesn't require any external hardware
It averages the counts over a short period, so the readings are stable even in the presence of noise.
It works when the duty cycle is far from 50%.
It runs in the background, so you can process other tasks at the same time.

If you choose this approach, I can send you the code.