I imagine you can do this in MPASM with just a digital output pin and
some pulses during an interrupt, if you have no PWM pin available.

--Bob A


Robin Abbott wrote:
> Soft start for powered peripherals
> ==================================
>
> I have just solved an issue on an application which may bite others so I'm
> dropping it here for interest. The system has an audio amplifier, lighting
> and LED's which are all controlled from a single transistor switch on a PIC
> output. The intention being to turn them all off when the PIC enters sleep.
> This group of peripherals has a single 22uF capacitor decoupling it. 
>
> The issue was that when the transistor turned on the instantaneous current
> rush into the peripherals and the local decoupling capacitor caused a glitch
> on the PIC supply which caused it to reset, or jump or lose RAM conents.
> Undoubtedly it could have been fixed with better decoupling or PCB tracking
> but we were stuck with a PCB layout.
>
> The solution was a "soft" turn on of the transistor, effectively using a PWM
> signal to step the voltage to the peripherals in 6 steps over about 100mS.
> The  C code to do this is shown below - it is quite short (it is written for
> the FED WIZ-C compiler, other compilers may need a change to the first line
> which defines the pin to which the transistor switch is connected). The
> function provided turns power on or off according to the parameter. This
> completely solved the issue and the system is now rock solid.
>
> This function takes around 100mS which is fine in an application like ours
> where the peripherals are turned on at PIC reset and only turned off when
> sleep mode is entered. For systems where the on/off signal is required more
> frequently and the system cannot afford to wait for 100mS, then an
> enhancement would be to drive this code in a timer interrupt. It will only
> ever work when the peripherals can cope with a slow Vdd pin rise and have
> suitable decoupling capacitors to avoid the 60KHz PWM signal hitting the
> power line. Finally if one of the CCP outputs is not being used then the
> internal PWM could be used to simplify the code.
>
> #define AuxPower bRC5.		// Operate on the RC5 pin 
>                                 // (may need to change for different
> compiler)
> 					// Defined automatically with App
> Wizard
>
> void SetAuxPower(unsigned char On)
> {
>  unsigned int j;
>  unsigned char i,k;
>  
>  if (On)	// Turning power on
>  {
>   for(i=0; i<5; i++)	// Turn on power in  5 voltage steps
>    for(j=0; j<1000; j++)	// This loop controls the time in each step
>     for(k=0; k<6; k++)		// This loop controls the PWM output
>     {
>      if (k<=i) 		// Approx 60KHz here with 20MHz clock
>      	AuxPower=1;		// In this example output high turns on the
> transistor (NPN)
>      else 
>      	AuxPower=0;
>     }
>  }
>  AuxPower=On;		// Finally turn it on (or off) all the way
> }
>
>
>
> Robin Abbott
> Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front
> end
> robin.abbott@fored.co.uk
> www.fored.co.uk 
>  
>
>   

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