print_time_uint64_t.c

#define BAUD (9600)
#define STRLEN (256)

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/setbaud.h>
#include <util/delay.h>
#include <stdbool.h>
#include <string.h>
#include <limits.h>
#include <stdio.h>
#include <time.h>

volatile uint8_t  countup10;
volatile uint64_t unix_time;

void USART_Init() {
	UBRR0H = (uint8_t)UBRRH_VALUE;
	UBRR0L = (uint8_t)UBRRL_VALUE;
	// Data frame : 8bits, Stop bits : 2bits
	UCSR0B = UCSR0B & (0b0 << UCSZ02);
	UCSR0C =  (0b11 << UCSZ00) | (1 << USBS0);
	// Enable Tx/Rx
	UCSR0B = (1 << RXEN0) | (1 << TXEN0);
}

void USART_Transmit(const unsigned char data) {
	// wait for free of buffer
	loop_until_bit_is_set(UCSR0A, UDRE0);
	UDR0 = data;
}

unsigned char USART_Receive(void) {
	loop_until_bit_is_set(UCSR0A, RXC0);
	return UDR0;
}

#define usart_putc(c) USART_Transmit(c);

void usart_print(const char* str) {
	for (uint8_t i = 0; str[i] != '\0'; i++) {
		USART_Transmit(str[i]);
	}
	return;
}

void usart_puts(const char* str) {
	usart_print(str);
	USART_Transmit('\n');
	return;
}

int usart_gets(char* str, const int len) {
	int i;
	for (i = 0; i < len; i++) {
		char c = USART_Receive();
		if (c == '\n' || c == '\0') {
			str[i] = '\0';
			break;
		} else {
			str[i] = c;
		}
	}
	return i;
}

char* uint64_str(const uint64_t unix_time, char* buf, size_t len) {
	char* start_ptr;
	char* end_ptr;
	uint64_t dived;

	// convert uint64 to string
	for (
		dived = unix_time, end_ptr = buf;
		dived != 0 && end_ptr < (buf + len);
		dived /= 10, end_ptr++
	) {
		*end_ptr = dived % 10 + '0';
	}

	if (end_ptr >= (buf + len)) { // when over size of buf
		return NULL;
	} else if (end_ptr > buf) { // when some numbers are written in buf
		*end_ptr = '\0';
		for (
			start_ptr = buf, end_ptr -= 1;
			start_ptr < end_ptr;
			start_ptr++, end_ptr--
		) {
			char tmp = *start_ptr;
			*start_ptr = *end_ptr;
			*end_ptr = tmp;
		}
	} else { // when buf has no numbers
		buf[0] = '0';
		buf[1] = '\0';
	}
	return buf;
}

void Timer1_Init() {
	countup10 = 0;
	unix_time = 1468377662ULL;
	set_system_time((time_t)unix_time - UNIX_OFFSET);

	// タイマ動作をCTCに設定, 64分周に設定
	TCCR1B = (1 << WGM12) | (0b011 << CS10);
	// タイマカウントを1秒周期に設定
	OCR1A = (F_CPU / 64 / 10) - 1;
	// 1Aの比較一致で割り込み発生
	TIMSK1 = (1 << OCIE1A);
	// 割り込み許可
	sei();
}

void print_time() {
	char output[STRLEN] = {'0'};
	uint64_t real_unix_time = unix_time - (uint64_t)UNIX_OFFSET;
	usart_print("Current Unix Time: ");
	usart_print(ctime((time_t*)&real_unix_time));
	usart_print(ctime((time_t*)time(NULL)));
	usart_putc('(');
	if (uint64_str(unix_time, output, STRLEN) != NULL) {
		usart_print(output);
	}
	usart_putc(')');
	usart_putc('\n');
}

ISR(TIMER1_COMPA_vect) {
	if (countup10 >= 10) {
		system_tick();

		countup10 = 0;
		unix_time++;

		print_time();
	}
	countup10++;
}

int main(void) {
	char str[] = "Hello. This is AVR ATmega88P-20PA.\nNice to meet you.\n";
	char input[STRLEN] = {};

	Timer1_Init();

	USART_Init();
	usart_puts(str);

	int i = 0;
	while (true) {
		sprintf(input, "[%d] > ", i++);
		usart_print(input);
		usart_gets(input, STRLEN);

		if (strcmp(input, "time") == 0) {
			//char* tm = ctime((time_t)unix_time);
			//sprintf(input, "Current Unix Time: %s\n", tm);
			//usart_puts(input);
		} else if (strcmp(input, "")){
			usart_puts("Unknown Command Error!");
		}
	}
	while (true) {
		_delay_ms(5000UL);
	}
}