Muti_SPWM (Multiple Slow speed PWM)

Multiple Slow speed Software PWM  (for PicBasic Pro)

OK, I finally got it working, Yahoo.

After writing the SSPWM program.  The first thing that everyone said was... "How can I use more than one SSPWM on the same chip?"  Of course the answer was.. "You can't".  SSPWM was written as a minimum impact Background process.  This means that it uses Timer1 to determine when the output needs to change states, and uses very little processing time to accomplish it.  That way the main program can continue on without much disruption.  Therefore, to have more than 1 SSPWM output on the same chip would require 2 16-bit timers, one for each output.  Since there aren't any 16F chips with 2 16-bit timers, this became a problem for everyone.  So, back to the proto-board I went

The result, is now called Multi_SPWM.pbp and has the following features

• up to 8  Slow Speed PWM outputs simultaneously on 1 chip
• Works on just about any chip. 12F, 16F, 18F
• Each SPWM output can be assigned to ANY General I/O pin on any Port
• PWM is 8-bit (0-255)  0=Always Low,  255=Always High
• Can be compiled with either MPASM or PM
• It's an Include file, for easy implementation in your program.
• Error checking is built in, and supplies Messages detailing the problems.

The downside to this version is that it uses a lot of processor time.  This leaves very little for the rest of the program.  Depending on how it is configured, it may use 75% or more of the available processing time.  The reason it  uses so much is due to the large number of interrupts required.  Instead of only 2 interrupts per cycle like SSPWM, Multi_SPWM needs 256 interrupts per cycle to be able to handle multiple outputs at various DutyCycles. So if the PWM frequency is say 100Hz, it takes 25,600 interrupts per second.  With each output using around 20 instructions per interrupt, it adds up to about 512,000 instructions per second, per output.  Yowsa.  Needless to say, you probably won't want to run this on a 4Mhz processor.

Here's what it takes to use the program. 

• Define the PWM frequency

define SPWMFREQ  100           ' PWM frequency in Hz    

• For each output that you will be using, create a BYTE sized variable to hold the Dutycycle value.  They can be named anything you wish, but for the example I'll just stick with the obvious.

DutyCycle1   VAR  byte    ' 0-255  0=Idle Low   255=Idle High
DutyCycle2   VAR  byte 
DutyCycle3   VAR  byte     

• Designate the Output Pin for each PWM channel, and tell Multi_SPWM what veriable holds the Dutycycle for that channel by using a define.

SPWM1PIN  VAR PORTB.0          ' SPWM channel 1
define SPWM1VAR  _DutyCycle1

SPWM2PIN  VAR PORTB.1          ' SPWM channel 2
define SPWM2VAR  _DutyCycle2

SPWM3PIN  VAR PORTB.2          ' SPWM channel 3
define SPWM3VAR  _DutyCycle3

• And finally, include the Multi_SPWM.PBP file

Include "Multi_SPWM.pbp"

Congratulations, you now have 3 PWM channels running at 100hz, ready and waiting for you to set the dutycycle.  All PWM channels automatically configure the pin as an output, and set the output to the LOW state.  It will IDLE there until the Dutycycle is changed.

• The Dutycycle can be changed at any time by simply changing the variable.  The PWM output will be automatically updated (if nescessary) on the next interrupt.  There are no subroutines to call.  It all "Just Happens".

DutyCycle1 = 0
DutyCycle2 = 50
DutyCycle3 = 255

That's all there is to it.

Here's the complete example program.

define OSC 20

define SPWMFREQ  100           ' PWM frequency in Hz    

DutyCycle1   VAR  byte    ' 0-255  0=Idle Low   255=Idle High
DutyCycle2   VAR  byte 
DutyCycle3   VAR  byte    

SPWM1PIN  VAR PORTB.0          ' SPWM channel 1
define SPWM1VAR  _DutyCycle1

SPWM2PIN  VAR PORTB.1          ' SPWM channel 2
define SPWM2VAR  _DutyCycle2

SPWM3PIN  VAR PORTB.2          ' SPWM channel 3
define SPWM3VAR  _DutyCycle3
 
Include "Multi_SPWM.pbp"

DutyCycle1 = 0
DutyCycle2 = 127
DutyCycle3 = 255

Loop:
   Pause 100
Goto Loop

This is a new program as of 5/16/2004 so I wouldn't be surprised if there's a problem somewhere.  So, if you have any problem, please let me know, as it will help everyone involved.  Please feel free to post a comment if you'd like at the bottom of this page.

Best regards,
   Darrel Taylor