Asynchronous PWM for Pumps, Glow Plug Drive, etc


Any turbojet ECU will require as a minimum a controllable fuel pump, and sometimes an adjustable glow plug driver. The traditional technique for varying a DC motor is by using PWM (Pulse Width Modulation), whereby a pulsed signal's duty cycle is varied, causing the device (in this case an advanced MOSFET) to switch on and off very rapidly. This in turn effectively varies the voltage which the target device sees. A benefit, especially for a motor, is that torque at low speeds is improved over a resistive setup, and the efficiency is much greater.

A microcontroller can produce PWM in 2 ways... it can be dedicated to PWM output, which means that it will have essentially no additional time to process other parameters. Or, it can have contained within it a Timer module which allows continuous PWM output while the processor clock can effectively perform other functions without causing gross interruptions in the PWM output.

The PIC16F876 conveniently has two PWM output modules, one of which can control a pump, the other controlling a glow plug driver. The PWM output is fed through a diode and resistor and delivered to the gate of an N-MOSFET capable of "turning on" at logic voltage levels. The MOSFET must additionally have a resistor from gate to source (ground), or the MOSFET will latch "on" with the first pulse from the uController.

This code, when compiled and loaded into the PIC, generates a PWM output suitable for both a Speed280/300 style pump or a glow plug.

'This program is designed to explore the PWM characteristics
'of the 16F876 for motor or Glow excitation.  Buttons one
'and 2 are Active Low and are wired to PortB 3 and 4 using
'2 10K pullup resistors.  The LCD is a Scott Adams Serial LCD
'module.  Compiled with PicBasic Pro and MPASM.

'For MOSFETs, I highly recommend the compact and efficient International
'Rectifier IRLU3103.  For gate drive for a Speed 300/280 type pump, use
'a 10K resistor, with a 12K resistor gate to ground.  Higher gate to ground
'resistance reduces resolution of the byte Duty Cycle variable.

'Clamp the Motor with a 10 or 12V 1/2W Zener.
'Experimentation is always in order.

INCLUDE "modedefs.bas"

PumpDC		var		CCPR1L
PumpOut		var		PORTC.2
PumpOn		con		%1111110	'16 Pre/Post scale
PumpOff		con		%1111010
PumpEnable	var		T2CON		'Timer2 ControlRegister

'See Scott Adams Serial LCD sheets for these commands
LCDOutPin		VAR		PORTB.2		'23 --> LCD
LCDCmd		CON		254		'LCD Command prefix
LCDHome		CON		2		'Move cursor home, Pause
LCDClr		CON		1		'Clear LCD screen, Pause
LCDLine1		CON		128		'First cell line 1
LCDLine2		CON		192		'First cell line 2

'Buttons, select any convenient I/O port.  Buttons will change
'(and scroll) the Duty Cycle of the PWM from 0 to 255.
Btn1		var		PORTB.3
Btn2		var		PORTB.4

Gosub EnablePump		'Set required PWM/Timer2 registers
Gosub DisplayDC		'Initialize LCD


'Buttons, set for pull up, active low
TRISB.3 = 1: TRISB.4 = 1


'The buttons will increment or decrement the duty cycle
'by one within a range of 0 to 255.
SetDC:
	if Btn1 = 0 then
		if PumpDC < 255 then PumpDC = PumpDC + 1
		Gosub DisplayDC				'Update LCD
		Pause 150				'Button debounce/scroll rate
	Endif
 	if Btn2 = 0 then
		if PumpDC then PumpDC = PumpDC - 1
		Gosub DisplayDC
		Pause 150
	Endif
Goto SetDC		'Loop endlessly polling for button presses
		




'Pump subroutines --------------------------------------------------

PumpEmergencyOff:				'Sets throttle to zero, Kills timer2, and sets
	PumpDC = 0			'Pump Drive pin to Low
	PumpEnable = PumpOff
	Low PumpOut			
Return

DisplayDC:						'Display the current Duty Cycle of PWM on LCD
	SerOut LCDOutPin, N9600, [LCDCmd, LCDClr]	'Clear LCD
	Pause 2
	SerOut LCDOutPin, N9600, [LCDCmd, LCDLine1, "DC: ", #PumpDC]
Return

EnablePump:			'Call once to enable PWM 
	PR2 = $FF		'PWM Period
	PumpDC = 0		'Set DC to 0
	TRISC.2 = 0		'Set pin to output
	CCP1CON = %001100	'Set CCP Register for PWM
	PumpEnable = PumpOn	'Turn On timer
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