diff --git a/lib/usart.c b/lib/usart.c
new file mode 100644
index 0000000..0190b58
--- /dev/null
+++ b/lib/usart.c
@@ -0,0 +1,172 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see .
+ *
+ */
+/* Copyright (c) 2015 King Kévin */
+/* this library handles the USART
+ * it uses the TX and RX pins
+ * it can be used with or without interrupts
+ */
+
+#include // Standard Integer Types
+#include // Standard IO facilities
+#include // General utilities
+
+#include // AVR device-specific IO definitions
+#include // Interrupts
+#include // Power Management and Sleep Modes
+
+#include "usart.h" // USART header
+#include // Helper macros for baud rate calculations
+
+/* assign input and output streams */
+FILE usart_input = FDEV_SETUP_STREAM(NULL, (int (*)(struct __file *)) usart_getchar, _FDEV_SETUP_READ);
+#if defined(USART_INTERRUPT) && (USART_INTERRUPT==1)
+FILE usart_output = FDEV_SETUP_STREAM((int (*)(char, struct __file *)) usart_putchar_nonblocking, NULL, _FDEV_SETUP_WRITE);
+FILE usart_io = FDEV_SETUP_STREAM((int (*)(char, struct __file *)) usart_putchar_nonblocking, (int (*)(struct __file *)) usart_getchar, _FDEV_SETUP_RW);
+#else
+FILE usart_output = FDEV_SETUP_STREAM((int (*)(char, struct __file *)) usart_putchar_blocking, NULL, _FDEV_SETUP_WRITE);
+FILE usart_io = FDEV_SETUP_STREAM((int (*)(char, struct __file *)) usart_putchar_blocking, (int (*)(struct __file *)) usart_getchar, _FDEV_SETUP_RW);
+#endif
+
+#if defined(USART_INTERRUPT) && (USART_INTERRUPT==1)
+/* input and output ring buffer, indexes, and available memory */
+uint8_t rx_buffer[32] = {0};
+volatile uint8_t rx_i = 0;
+volatile uint8_t rx_used = 0;
+uint8_t tx_buffer[32] = {0};
+volatile uint8_t tx_i = 0;
+volatile uint8_t tx_used = 0;
+/* show the user how much received data is ready */
+volatile uint8_t usart_incoming = 0; // same as rx_used, but since the user can write this variable we don't rely on it
+#endif
+
+/* configure USART serial port */
+void usart_init(void)
+{
+ UBRR0H = UBRRH_VALUE; // set baurate (calculated from BAUD)
+ UBRR0L = UBRRL_VALUE; // set baurate (calculated from BAUD)
+#if USE_2X
+ UCSR0A |= (1<=sizeof(rx_buffer)) {
+ return;
+ }
+ cli();
+ rx_buffer[(rx_i+rx_used)%sizeof(rx_buffer)] = UDR0; // put character in buffer
+ rx_used++; // update used buffer
+ usart_incoming = rx_used; // update available data
+ sei();
+}
+
+/* put character on USART stream (non-blocking using a buffer) */
+void usart_putchar_nonblocking(char c, FILE *stream)
+{
+ if (c == '\n') { // add carrier return before line feed. this is recommended for most UART terminals
+ usart_putchar_nonblocking('\r', stream);
+ }
+ if (UCSR0A&(1<=sizeof(tx_buffer)) {
+ set_sleep_mode(SLEEP_MODE_IDLE);
+ sleep_mode();
+ }
+ cli();
+ tx_buffer[(tx_i+tx_used)%sizeof(tx_buffer)] = c; // put character in buffer
+ tx_used++; // update used buffer
+ sei();
+ }
+}
+
+ISR(USART_UDRE_vect) { // USART transmit register interrupt
+ if (!tx_used) { // no data in the buffer to transmit
+ return;
+ }
+ UDR0 = tx_buffer[tx_i]; // put data in transmit register
+ tx_i = (tx_i+1)%sizeof(rx_buffer); // update location on buffer
+ tx_used--; // update used size
+}
+#endif
+
+
diff --git a/lib/usart.h b/lib/usart.h
new file mode 100644
index 0000000..bbc4a49
--- /dev/null
+++ b/lib/usart.h
@@ -0,0 +1,44 @@
+/* This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see .
+ *
+ */
+/* Copyright (c) 2015 King Kévin */
+/* This library handles the USART configuration */
+
+/* use interrupts */
+#define USART_INTERRUPT 1
+/* set tolerance to 3% to allow 115200 baudrate with 16 MHz clock, else use 9600 for default <2% */
+#define BAUD_TOL 3
+/* serial baudrate, in bits per second (with 8N1 8 bits, no parity bit, 1 stop bit settings) */
+#define BAUD 9600
+
+/* input & output streams */
+FILE usart_output;
+FILE usart_input;
+FILE usart_io;
+
+/* show the user how much received data is ready */
+volatile uint8_t usart_incoming;
+
+/* configure serial port */
+void usart_init(void);
+/* put character on USART stream (blocking) */
+void usart_putchar_blocking(char c, FILE *stream);
+/* ensure all data has been transmitted */
+void usart_flush();
+/* get character from USART stream (blocking) */
+char usart_getchar(FILE *stream);
+#if defined(USART_INTERRUPT) && (USART_INTERRUPT==1)
+/* put character on USART stream (non-blocking using a buffer) */
+void usart_putchar_nonblocking(char c, FILE *stream);
+#endif