Sample code for low-pass filter …...

		case 3:						// Function 4: Derivative
			data1 = data0;			// Store previous data points
			data0 = ReadADC();		// Read A/D and save the present sample in data0
			dummy = (int)data0 - data1 + 128;	// Take derivative and shift to middle
			PORTD = (unsigned char)dummy;		// Output to D/A
			break;
		case 4:						// Function 5: Low-pass filter
			data2 = data1;			// Store previous data points
			data1 = data0;
			data0 = ReadADC();		// Read A/D and save the present sample in data0
			dummy = ((int)data0 + data1 + data1 + data2) / 4;	// smoother
			PORTD = (unsigned char)dummy;		// Output to D/A
			break;

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