Vee (negative power supply) (for LCD etc...) from PIC PWM

David Covick [dac@WEST.NET]

I designed this little circuit a few months ago using SPICE. Just vary the PWM and you are home free. You don't need any external negative voltage!! It produces negative voltage on its own using a positive voltage in a charge pumping type method off the PIC PWM.

The attached virus free jpg schematic will fill you in.

Have at it! Sample ref code:
Connect PORTC.2 of 16F877 to PWM IN.
Connect Vee on LCD to Vee Out.

'**************** LCD Display Negative Voltage ********************
'********************* 16F877 @ 20 MHz ****************************

PR2=255              ' Set PR2 to 19.53 kHz
CCPR1L=230           ' Set Pulse width - CCPR1L must be less than PR2.
                     ' Lower number(pulse width)=higher view angle
                     ' PR2 value is 100% duty cycle.
                     ' PR2/2 = 50% duty cycle.
TRISC.2=0            ' Set CCP1 bit as output
T2CON=%00000100      ' Timer on prescale is 1
CCP1CON=%00001100    ' Set PWM Mode
'******************************************************************

See

  .