Page 270 - ARM Based Microcontroller Projects Using MBED

256 8. INTERMEDIATE LEVEL PROJECTS

k = 3;
break;
case 'g': // Is it 'g' ?
k = 4;
break;
case 'h': // Is it 'h' ?
k = 5;
break;
case 'j': // Is it 'j' ?
k = 6;
break;
case 'k': // Is it 'k' ?
k = 7;
break;
}

pwm.period(Periods[k]); // Start the PWM
pwm.pulsewidth(DutyCycle[k]); // 50% Duty Cycle
wait_ms(200); // Play for 200ms
pwm.pulsewidth(0.0); // Stop PWM
}

int main()
{
MyPC.attach(&ISR, Serial::RxIrq); // Attach Serial Int

//
// Calculate the periods (in seconds), Duty Cycles (50%) before
// entering the loop
//
for(int k = 0; k < MaxNotes; k++)
{
Periods[k] = 1.0f / Notes[k];
DutyCycle[k] = Periods[k] / 2.0f;
}

while(1) // Wait for ISR
{
}
}

FIG. 8.124, CONT’D

use a PWM waveform with the frequency set to about 50Hz (period¼20ms), then the
viewer will not notice the flashing effect. As the duty cycle is varied, the brightness of
the LED will change. For example, a higher duty cycle will result in brighter light since
the average voltage is higher. Similarly, a lower duty cycle will result in dimmer light. The
PDL of the project is shown in Fig. 8.125.