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Although the previous program does what it should do and produces a note of
varying frequency, it isn’t exactly pleasing to the ear. What’s required is a form
of envelope shaper in order to give the note a more mellow sound. This is
achieved by taking advantage of a well known method of charging a capacitor
and allowing it’s voltage to decay naturally. While the capacitor is fully charged,
the note is at full volume but as the capacitor discharges, the note’s volume
will decrease until it is silent. This
will form a rather pleasant chime effect. In order to accomplish this we require a few changes to our previous program
and circuit, but the interrupt driven note generator (with minor changes) is still
at the heart of the process. The circuit to produce a chime effect from pin
PORTB.0 is shown below. The circuit above also shows the approximate wave shapes produced on each
pin and the listing below shows the program needed to create the chime effect. ' Program PLAY_CHIME.BAS
' Produce two Chimes from PORTB.0 using a TMR1 interrupt
' Device= 18F452 ' Use a PIC18F452 device XTAL= 20 ' With a 20MHz crystal/resonator DimNOTE_COUNTER asWordSYSTEM ' Determines when the pin is toggled DimNOTE asWordSYSTEM ' Determines the pitch of the note DimTIMER1asTMR1L.Word ' Combine TMR1L/TMR1H as a 16-bit word SymbolTRIGGER = PORTB.4 ON_INTERRUPTGotoNOTE_INT ' Point interrupts to our interrupt handler Delayms400 ' Wait for PICmicro to stabilise ALL_DIGITAL= True ' Set PORTA and PORTE to digital GotoOVER_INTERRUPT ' Jump over the interrupt handler R1
220W R2
100kW R3
100kW C1
4.7uF/10V RB0 RB4 18F452
PICmicro Output toAmplifier
