16F877@4Mhz calculate Pulsin to RPM

[Darrel Taylor]

Hi Leigh,

From your two values of 165 and 168, I calculate that the engine was running at 4504 RPM. So, the following formula may be what your looking for.

Pulse_High  Var  word
Pulse_Low   Var  word
Pulse_Total Var  Word
RPM         Var  Word

Pulse_High = 165
Pulse_Low  = 168

Pulse_Total = Pulse_High + Pulse_Low
RPM = 1000
RPM = RPM * RPM           ' Preload internal registers with 1,000,000
RPM = DIV32 Pulse_Total   ' 1,000,000 / Pulse_Total
RPM = RPM * 60            ' Per minute
RPM = Div32 40            ' 4 pulses per rev

lcdout $FE,1,dec RPM

In normal math it might look like this:
(1,000,000/(Pulse_High+Pulse_Low))*60/40

A better way to take the measurements might be to use the Capture mode of the CCP module. Set the prescaler to 2:1 and have it count 4 rising edges of the input and then generate an interrupt. The Timer value would contain 1/2 the number of uS it takes for 1 revolution, which should be fairly easy to convert to RPM. This way, the processor can continue doing other things while it's taking the measurement. This should work from 500-10,000 RPM

That's a bit more complicated, but may be worth looking into.

HTH,
   Darrel

[montree]

Hi Darrel Taylor

You said the ccp should work from 500 - 10,000 rpm. Please kindly advice how to measure rpm 0-10,000 rpm by use ccp module on [email protected] you have source code please show me. Thank you.

[Darrel Taylor]

You'll need to count the timer overflows and use them as the high word of the period after a capture.
Then change all the math, because the values will be larger than 16-bits.

Since you are using an 877A, you won't have the luxury of using LONG variables.

[montree]

Thank you for your advice but i still not clear please kindly show example about your advice above. For my program show as below
PIC16F877A
DEFINE OSC 4
Define LCD_DREG PORTD
Define LCD_DBIT 4
Define LCD_RSREG PORTE
Define LCD_RSBIT 2
Define LCD_EREG PORTD
Define LCD_EBIT 1


Low PORTE.2
PIE1.2=1

Symbol Capture = PIR1.2 ' CCP1 capture flag
T1 VAR WORD ' 1st capture value
PW VAR WORD ' 2nd capture value & ultimately final pulse width



intermediate VAR WORD
Dummyvar VAR WORD


rpm VAR WORD

TRISC.2 = 1 ' CCP1 input pin (Capture input)
INTCON = 0 ' Interrupts off

ReLoad:
CCP1CON = %00000101 ' Capture mode, 4capture on rising edge
T1CON = 0 ' TMR1 prescale=1, clock=Fosc/4, TMR1=off (200nS per count @20MHz)
TMR1H = 0 ' Clear high byte of TMR1 counter
TMR1L = 0 ' Clear low byte
T1CON.0 = 1 ' Turn TMR1 on here


Capture = 0 ' Clear capture int flag bit
While (!Capture) ' Wait here until capture on rising edge
Wend

' Rising edge detected / stuff Timer1 value in T1
T1.HighByte = CCPR1H
T1.LowByte = CCPR1L

CCP1CON = %00000100 ' Configure capture for falling edge now
Capture = 0 ' Clear capture interrupt flag bit

While !Capture ' While here until capture on falling edge
Wend

' Falling edge detected / stuff Timer1 value in PW
PW.HighByte = CCPR1H
PW.LowByte = CCPR1L

PW = PW-T1 ' High pulse width = PW-T1


intermediate = 60 * 1000 / 4 ' Compulsory !!!
' ( no "intermediate CON xxx" allowed )


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


Dummyvar = 1000 * intermediate


rpm = DIV32 PW


' re-enable interrupts allowed
lcdout $fe,1,"TCH",$fe,$84,DEC RPM

GOTO ReLoad

END