Pic Speed / Instruction Speed etc

[jamie_s]

Hi all,

I've got some code im testing, basically to decode a trigger wheel, and output 3 pulses per rotation of the wheel which has 35 teeth.

CMCON = 7						' set portA to digital
TRISA = %00100000				' Port A all output but MCLR
TRISB = %00001000				' Port B all output but 3 (CCP)
InPIN	VAR	PORTB.3				' Define input pin
OutPIN 	VAR	PORTB.5				' Define output pin
LastState   VAR	BIT				' Last Pin State
PulseCount	VAR BYTE			' Number of input pulses
	

PulseCount = 0
Low OutPin

MainLoop:
if InPIN <> LastState then  ' If Input State Changed 

	LastState = InPIN    
	
	PulseCount = PulseCount + 1
					
		Select Case PulseCount
		
			Case 1
			High OutPin
			Case 4
			Low Outpin
			
			Case 12
			High OutPin
			Case 15
			Low Outpin
			
			Case 24
			High Outpin
			Case 27
			Low Outpin
			
			Case 35
			PulseCount = 0
			Low OutPin
			
			Case is > 35
			PulseCount = 0
			Low OutPin
			
			Case Else
			
		End Select

endif
GOTO MainLoop

at maxium wheel rotation speed, the pic input frequency is approx 5khz,
now, at 4mhz, should the pic be able to keep up, or does anyone have any alternative code ideas to help improve the performance?

[Acetronics2]

Hi, Jamie

@ first ... there is a small thing I do not understand so well ...

you count the teeth ... oK ... but the way you use rising AND falling edges make 2 transitions per tooth ... or 70 transitions per turn ...

the way the program is written - and taking 70 transitions into account ... I get a 13200 rpm counting limit @ 4 Mhz...

Alain

[Bruce]

Use a hardware counter like Timer0.

Here's one example; http://www.picbasic.co.uk/forum/showthread.php?t=10608

If you get 2 pulses per tooth just set a 1:2 prescaler for 35 counts per rev.

[jamie_s]

alain - yeah i found that out when i tested with the scope - i just doubled the numbers in the case statements and it worked fine. As the max speed is around 8500rpm, testing it seemed fine on the bench.

[Acetronics2]

Hi, Jamie

... having 3 pulses per turn and a 8500 rpm limit ... make me think of a somewhat "unusual" engine ...

could you tell me which engine it is ... may be I can be VERY interested in ...

Alain

[jamie_s]

suzuki v6, and i want to decode a crank trigger to run an aftermarket tacho.
it probably will never see 8500rpm, but i want to ensure the pic is stable at least to 7000rpm

[enauman]

I know this is an old post but I made a tach adapter using a PIC 16F1824. It basiclly takes a single cylinder engine signal (1 pulse /720 degrees) and multiplies it by 4 to use a 4 cylinder tach. Works real well if you are interested, I can send you the code. It can be adapted to other situations as well. I also designed a real stable interface to the coil if you don't have a crank trigger.