diff --git a/main.c b/main.c
index 0c3a470..07ee024 100644
--- a/main.c
+++ b/main.c
@@ -29,6 +29,27 @@
 #include "uart.h" // basic UART functions
 #include "main.h" // main definitions
 
+/* help strings */
+const char help_00[] PROGMEM = "commands:\n";
+const char help_01[] PROGMEM = "\thelp                  display this help\n";
+const char help_02[] PROGMEM = "\tled on|off|toggle     change LED state\n";
+const char help_03[] PROGMEM = "\tread [B-D][0-7]       read value from pin\n";
+const char help_04[] PROGMEM = "\twrite [B-D][0-7] 0|1  write value on pin\n";
+PGM_P const help_table[] PROGMEM = {
+    help_00,
+    help_01,
+    help_02,
+    help_03,
+    help_04
+};
+
+/* global variables */
+char uart_in[32] = {0}; // user input from USART
+volatile uint8_t uart_in_i = 0; // how many UART characters have been input
+
+/* flags, set in the interrupts and handled in the main program */
+volatile bool uart_flag = false; // UART input data is available
+
 /* switch off LED */
 void led_off(void)
 {
@@ -47,6 +68,27 @@ void led_toggle(void)
 	LED_PIN |= (1<<LED_IO);
 }
 
+void help(void)
+{
+	char* str;
+	for (uint8_t i=0; i<sizeof(help_table)/sizeof(PGM_P); i++) { /* display all help lines */
+		str = malloc(strlen_PF((uint_farptr_t)pgm_read_word(&(help_table[i]))));
+		strcpy_PF(str, (uint_farptr_t)pgm_read_word(&(help_table[i])));
+		printf(str);
+		free(str);
+	}
+}
+
+/* save the UART input into a string */
+ISR(USART_RX_vect) { /* UART receive interrupt */
+	uart_in[uart_in_i] = getchar(); // save input
+	if (uart_in_i<sizeof(uart_in)-1) { // go to next if there is space left (else overwrite last)
+		uart_in_i++; // prepare for next character
+		uart_in[uart_in_i] = 0; // always end the string
+	}
+	uart_flag = true; // notify new character is there
+}
+
 /* disable watchdog when booting */
 void wdt_init(void) __attribute__((naked)) __attribute__((section(".init3")));
 void wdt_init(void)
@@ -73,14 +115,140 @@ void io_init(void)
 	sei(); /* enable interrupts */
 }
 
+/* process command */
+void command_action()
+{
+	/* split command */
+	const char* delimiter = " ";
+	char* word = strtok(uart_in,delimiter);
+	if (!word) {
+		goto error;
+	}
+	/* parse command */
+	if (0==strcmp(word,"help")) {
+		help();
+	} else if (0==strcmp(word,"led")) {
+		/* expecting state */
+		word = strtok(NULL,delimiter);
+		if (!word) {
+			goto error;
+		}
+		if (0==strcmp(word,"on")) {
+			led_on();
+		} else if (0==strcmp(word,"off")) {
+			led_off();
+		} else if (0==strcmp(word,"toggle")) {
+			led_toggle();
+		} else {
+			goto error;
+		}
+	} else if (0==strcmp(word,"read")) {
+		/* expecting pin */
+		word = strtok(NULL,delimiter);
+		if (!word) {
+			goto error;
+		}
+		if (2==strlen(word) && word[0]>='B' && word[0]<='D' && word[1]>='0' && word[1]<='7') {
+			volatile uint8_t * pin = NULL;
+			volatile uint8_t * ddr = NULL;
+			switch (word[0]) {
+				case 'B':
+					pin = &PINB;
+					ddr = &DDRB;
+					break;
+				case 'C':
+					pin = &PINC;
+					ddr = &DDRC;
+					break;
+				case 'D':
+					pin = &PIND;
+					ddr = &DDRD;
+					break;
+			}
+			*ddr &= ~(1<<(word[1]-'0')); // set as input
+			printf("%s: %u\n", word, (*pin>>(word[1]-'0'))&0x1); // print value
+		} else {
+			goto error;
+		}
+	} else if (0==strcmp(word,"write")) {
+		/* expecting pin */
+		word = strtok(NULL,delimiter);
+		if (!word) {
+			goto error;
+		}
+		if (2==strlen(word) && word[0]>='B' && word[0]<='D' && word[1]>='0' && word[1]<='7') {
+			uint8_t pin = word[1]-'0';
+			volatile uint8_t * port = NULL;
+			volatile uint8_t * ddr = NULL;
+			switch (word[0]) {
+				case 'B':
+					port = &PORTB;
+					ddr = &DDRB;
+					break;
+				case 'C':
+					port = &PORTC;
+					ddr = &DDRC;
+					break;
+				case 'D':
+					port = &PORTD;
+					ddr = &DDRD;
+					break;
+			}
+			*ddr |= (1<<pin); // set as output
+			/* expecting value */
+			word = strtok(NULL,delimiter);
+			if (!word || strlen(word)!=1) {
+				goto error;
+			}
+			/* write value */
+			if ('0'==word[0]) {
+				*port &= ~(1<<pin);
+			} else if ('1'==word[0]) {
+				*port |= (1<<pin);
+			} else {
+				goto error;
+			}
+		} else {
+			goto error;
+		}
+	} else {
+		goto error;
+	}
+
+	return;
+error:
+	printf(PSTR("command not recognized\n"));
+	return;
+}
+
 int main(void)
 {
-	io_init(); // initialize IOs
+	uint8_t uart_out_i = 0; // how many UART input characters have been processed
 
+	io_init(); // initialize IOs
+	
+	printf(PSTR("welcome to the demo code\n"));
 	while (true) { // endless loop for micro-controller
-		/* blinking LED example */
-		led_toggle();
-		_delay_ms(500);
+		while (uart_flag) { // new UART input
+			uart_flag = false;
+			while (uart_out_i<uart_in_i) {
+				putchar(uart_in[uart_out_i++]); // print current character
+			}
+			if ('\n'==uart_in[uart_in_i-1] || '\r'==uart_in[uart_in_i-1]) { // end of line detected
+				if ('\r'==uart_in[uart_in_i-1]) { // be sure to print a newline (for windows and mac EOL)
+					putchar('\n');
+				}
+				uart_in[uart_in_i-1] = 0; // remove end of line
+				if (uart_in_i>1) { // ignore empty lines
+					command_action(); // process command
+				}
+				uart_out_i = 0; // reset output
+				uart_in_i = 0; // reset input
+			}
+		}
+		/* go to sleep and wait for next interrupt */
+		set_sleep_mode(SLEEP_MODE_IDLE);
+		sleep_mode();
 	}
 	return 0;
 }