6.3.2 Adding String Markers

#include <avr/io.h>
#include <avr/interrupt.h>
const char* message_on = "NIGHT MODE ON";
const char* message_off = "NIGHT MODE OFF";
uint16_t adc_value = 0;
uint8_t nightmode_threshold = 40;
uint8_t nightmode_active = 0;
void adc_init(void){
	// Internal 1.5V reference, ADC0 as single ended input
	ADMUX = (1 << REFS1);
	// Enable the ADC,
	ADCSRA |= (1<<ADEN);
	// Check that the reference is OK
	while (0x00 == (ADCSRB & (1 << REFOK)));
}
uint16_t adc_sample(void){
	// Trigger an ADC conversion
	ADCSRA |= (1<<ADSC);
	// Wait for conversion to finish
	while (0x00 == (ADCSRA & (1 << ADIF)));	
	// Clear the interrupt flag
	ADCSRA |= (1 << ADIF);	
	return (ADC);
}
void spi_init(void){
	// Slave select (PB0), MOSI (PB2) and SCK (PB1) as output
	DDRB |= (1<<PINB0) | (1<<PINB2) | (1<<PINB1);
	//Slave select high (inactive)
	PORTB |= (1<<PINB0);
	// Master mode, enable SPI module. 
	// Clock polarity and phase is kept at default (sample on rising edge)
	SPCR = (1<<SPE) | (1<<MSTR);
}
void spi_send(uint8_t data){
	// Slave select low
	PORTB &= ~(1<<PINB0);
	// Write data to shift register
	SPDR = data;
	// Wait for the transmission to complete
	while (0x00 == (SPSR & (1<<SPIF)));
	// Slave select high
	PORTB |= (1<<PINB0);
}
void cdc_init(void){
	// Baud rate 9600 based on 8 MHz CPU clock
	UBRR1 = 51;
	// Enable the transmitter and receiver, 8 bit character size,
	// receive interrupts enabled
	UCSR1B = (1<<RXEN1) | (1<<TXEN1) | (1<<RXCIE1);
}
void cdc_send(const char data){
	// Wait for transmitter to be ready for more data
	while (0x00 == (UCSR1A & (1<<UDRE1)));
	// Send the data
	UDR1 = data;
}
void send_message(const char* message){
	while (*message)
	cdc_send(*message++);
	// String markers requires Null-termination
	cdc_send(0);
}
int main(void){
	adc_init();
	spi_init();
	cdc_init();
	while (1){
		adc_value = adc_sample();
		// Send the ADC value over SPI to the host
		// Only the 8 lower bits contain useful data
		spi_send(adc_value & 0xFF);		
		//  higher adc value == less light
		if (adc_value > nightmode_threshold){
			if (0x00 == nightmode_active){
				// Changing from night mode inactive to active
				nightmode_active = 0x01;
				send_message(message_on);
			}
			} else {
			if (0x01 == nightmode_active){
				// Changing from night mode active to inactive
				nightmode_active = 0x00;
				send_message(message_off);
			}
		}
	}
}