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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