[examples] Add simple uart driver for examples

Author: chris-durand

examples/common/uart.cpp

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2018, Christopher Durand
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#define BAUD 9600
+
+#ifndef F_CPU
+#error "F_CPU must be defined!"
+#endif
+
+#include <util/setbaud.h>
+
+#include <avr/io.h>
+#include <cstdio>
+
+// naive unbuffered uart implementation for C standard IO
+
+static int
+uart_putc(char c, [[maybe_unused]] FILE* stream)
+{
+	loop_until_bit_is_set(UCSR0A, UDRE0);
+	UDR0 = c;
+	return 0;
+}
+
+static int
+uart_getc([[maybe_unused]] FILE* stream)
+{
+	loop_until_bit_is_set(UCSR0A, RXC0);
+	return UDR0;
+}
+
+// constructor attribute: call function before main
+static void uart_init() __attribute__ ((constructor, used));
+
+static void
+uart_init()
+{
+	UBRR0H = UBRRH_VALUE;
+	UBRR0L = UBRRL_VALUE;
+
+#if USE_2X
+	UCSR0A |= _BV(U2X0);
+#else
+	UCSR0A &= ~(_BV(U2X0));
+#endif
+
+	UCSR0C = _BV(UCSZ01) | _BV(UCSZ00);
+	UCSR0B = _BV(RXEN0) | _BV(TXEN0);
+
+	static FILE uart_output = {};
+	static FILE uart_input = {};
+
+	fdev_setup_stream(&uart_output, uart_putc, nullptr, _FDEV_SETUP_WRITE);
+	fdev_setup_stream(&uart_input, nullptr, uart_getc, _FDEV_SETUP_READ);
+
+	stdout = &uart_output;
+	stdin = &uart_input;
+}