calculate time between pulses

[hell_pk]

Ok here is a code for CCP but for some reason it is not working:
Define LCD_DREG PORTD ' Define LCD connections
Define LCD_DBIT 4
Define LCD_RSREG PORTD
Define LCD_RSBIT 2
Define LCD_EREG PORTD
Define LCD_EBIT 3



Lcdout $fe, $80, " Pulse Reader"

Pause 2000
Lcdout $fe, $1





Period var Word
Capture var PIR1.2
Overflow var PIR1.0 CCP1CON = %00000100
T1CON = %00000001
Loop:
While Capture = 0 : Wend

Period.lowbyte = CCPR1L Period.highbyte = CCPR1H
If Overflow = 0 Then Lcdout $fe, $80, "Input Period: " , #Period , "uS"
Endif
Capture = 0 ' Clear the capture flag
Rst:
While Capture = 0 : Wend
TMR1L = 0 TMR1H = 0
Capture = 0
Overflow = 0
Goto loop

[J. Mark Wolf]

I've used the PULSEIN command as a tachometer for years.

One trick is to employ a flip-flop in the signal chain that acts as a frequency divider. This has the effect of generating a single high pulse between two succesive edges, allowing the PULSEIN command to work, without having to switch context to time the high portion of the period and then the low portion of the period.

[Acetronics2]

Hi, Hell_pk

<< I want to know how to calculate exact time between two pulses. >>

The question 1 ) is : How exact must be your "exact time" ??? : +/- 1 ms ? +/- 1µS ... better accuracy ???

The Question 2) is : Which is the period ( approximate ... ) of your signal.

Could you also tell us what is your "signal" ???


Then we will be able to show you ideas ...

Alain

[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