In SX Microcontrollers, SX/B Compiler and SX-Key Tool, kgracey wrote:

Hey SXers:
 

This is my first time attempting to use the SX's interrupt stack. This portion of the project consists of a Honda engine magneto connected to an SX, with a servo for throttle control. The bigger project is that I've got an alternator connected to the engine for a high-powered robot power plant, but this is where I'm stuck. Basically, I need to control the servo to set the engine's RPM. The signals I'm dealing with look like this:

 

( insert screen capture of USB Scope) 

 

Every 20-50 ms I generate the servo pulse, and meanwhile I'd like to be counting magneto pulses for 1 second. The scope capture above shows the engine in an idle state, so the pulse frequency increases with RPM. I'm able to control the servo and count pulses accurately, but putting these two tasks together is where I need some help. 

 

This is my schematic:

 

(insert schematic)

 

It looks like I can generate a periodic interrupt (every 50 ms) to pulse the servo with SX/B's INTERRUPT and RATE parameters. I created some psuedo-code which compiles, though the use of PULSOUT in the interrupt routine will wreck the timing. How can I incorporate the adjustment of a servo timing pulse within the ISR? It seems like I'd use the tix counter to adjust servo pulses. It seems like I need to draw a relationship between ServoVal and tix variables, so that I use tix and HIGH/LOW in the ISR to control the servo, but I'm not certain. I would benefit from a couple more hints. Preliminary psuedo-code is below, so you can understand the approach I'm trying to take (to format this code I used [url=http://www.phipi.com/format/]http://www.phipi.com/format/[/url]).  

 

[code]
' -------------------------------------------------------------------------
' Throttle Feedback Control.SXB
' Magneto counting for Honda Engine / alternator coupling 
' to set an RPM between two values
' -------------------------------------------------------------------------
DEVICE          SX28, OSCXT2, TURBO, STACKX, OPTIONX
FREQ            4_000_000
' -------------------------------------------------------------------------
' I/O Pins
' -------------------------------------------------------------------------
Servo   PIN RA.1  OUTPUT
Magneto   PIN RB.4  INPUT CMOS
RCCircuit  PIN RC.3  INPUT  ' not used in this code 
' -------------------------------------------------------------------------
' Constants
' -------------------------------------------------------------------------
HighThrottle  [b]CON[/b] 130
LowThrottle  [b]CON[/b] 190
' -------------------------------------------------------------------------
' Variables
' -------------------------------------------------------------------------
ServoVal  [b]VAR[/b] [b]Word[/b]
analog   [b]VAR[/b] [b]Word[/b]
RPM   [b]VAR[/b] [b]Word[/b]
MagnetoPulses  [b]VAR[/b] [b]Byte[/b]
tix   [b]VAR[/b] [b]Byte[/b]

' -------------------------------------------------------------------------
  INTERRUPT 200
' -------------------------------------------------------------------------
ISR_Start:
  INC tix                                       ' update tix counter
  [b]IF[/b] tix = 10 THEN                              ' every 50 ms
    tix = 0
    PULSOUT Servo, ServoVal   
  ENDIF
ISR_Exit:
  RETURNINT  

' =========================================================================
  PROGRAM Start
' =========================================================================
Start:
  ServoVal = LowThrottle
CountRPMs:
  COUNT Magneto, 1000, MagnetoPulses
  RPM = MagnetoPulses * 60
  [b]IF[/b] RPM <  3000 THEN
    ServoVal = ServoVal - 5    ' increase throttle
    [b]IF[/b] RPM > 4000 THEN
    ServoVal = ServoVal    ' hold throttle
      [b]IF[/b] RPM > 5000 THEN
      ServoVal = ServoVal + 5   ' decrease throttle 
      ENDIF
    ENDIF
  ENDIF
GOTO CountRPMs

[/code]


<FONT face=Verdana>Thanks for any help ahead of time. </FONT>

 

<FONT face=Verdana>Ken Gracey
Parallax, Inc.  
</FONT>

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