Added a printf example that is using irq and a ring buffer to write to USART1 port. It is also reflecting the text you write to the device.

examples/stm32/stm32-h103/Makefile

@@ -24,7 +24,7 @@ Q := @
 MAKEFLAGS += --no-print-directory
 endif
 
-all: miniblink fancyblink usart usart_irq usart_printf usb_cdcacm usb_hid button exti_both
+all: miniblink fancyblink usart usart_irq usart_printf usart_irq_printf usb_cdcacm usb_hid button exti_both
 
 miniblink:
 	@printf "  BUILD   examples/stm32/stm32-h103/miniblink\n"
@@ -46,6 +46,10 @@ usart_printf:
 	@printf "  BUILD   examples/stm32/stm32-h103/usart_printf\n"
 	$(Q)$(MAKE) -C usart_printf
 
+usart_irq_printf:
+	@printf "  BUILD   examples/stm32/stm32-h103/usart_irq_printf\n"
+	$(Q)$(MAKE) -C usart_irq_printf
+
 spi:
 	@printf "  BUILD   examples/stm32/stm32-h103/spi\n"
 	$(Q)$(MAKE) -C spi
@@ -77,6 +81,8 @@ clean:
 	$(Q)$(MAKE) -C usart_irq clean
 	@printf "  CLEAN   examples/stm32/stm32-h103/usart_printf\n"
 	$(Q)$(MAKE) -C usart_printf clean
+	@printf "  CLEAN   examples/stm32/stm32-h103/usart_irq_printf\n"
+	$(Q)$(MAKE) -C usart_irq_printf clean
 	@printf "  CLEAN   examples/stm32/stm32-h103/spi\n"
 	$(Q)$(MAKE) -C spi clean
 	@printf "  CLEAN   examples/stm32/stm32-h103/usb_cdcacm\n"
@@ -88,5 +94,5 @@ clean:
 	@printf "  CLEAN   examples/stm32/stm32-h103/exti_both\n"
 	$(Q)$(MAKE) -C exti_both clean
 
-.PHONY: miniblink fancyblink usart usart_irq usart_printf spi usb_cdcacm usb_hid button exti_both clean
+.PHONY: miniblink fancyblink usart usart_irq usart_printf usart_irq_printf spi usb_cdcacm usb_hid button exti_both clean
 

examples/stm32/stm32-h103/usart_irq_printf/Makefile

@@ -0,0 +1,23 @@
+##
+## This file is part of the libopencm3 project.
+##
+## Copyright (C) 2009 Uwe Hermann <uwe@hermann-uwe.de>
+##
+## 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 <http://www.gnu.org/licenses/>.
+##
+
+BINARY = usart_irq_printf
+
+include ../../Makefile.include
+

examples/stm32/stm32-h103/usart_irq_printf/usart_irq_printf.c

@@ -0,0 +1,272 @@
+/*
+ * This file is part of the libopencm3 project.
+ *
+ * Copyright (C) 2009 Uwe Hermann <uwe@hermann-uwe.de>,
+ *               2011 Piotr Esden-Tempski <piotr@esden.net>
+ *
+ * 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 <http://www.gnu.org/licenses/>.
+ */
+
+#include <libopencm3/stm32/rcc.h>
+#include <libopencm3/stm32/gpio.h>
+#include <libopencm3/stm32/usart.h>
+#include <libopencm3/stm32/nvic.h>
+#include <libopencm3/stm32/systick.h>
+
+#include <stdio.h>
+#include <errno.h>
+
+/******************************************************************************
+ * Simple ringbuffer implementation from open-bldc's libgovernor that
+ * you can find at:
+ * https://github.com/open-bldc/open-bldc/tree/master/source/libgovernor
+ ******************************************************************************/
+
+typedef s32 ring_size_t;
+
+struct ring {
+        u8 *data;
+        ring_size_t size;
+        u32 begin;
+        u32 end;
+};
+
+#define RING_SIZE(RING) ((RING)->size - 1)
+#define RING_DATA(RING) (RING)->data
+#define RING_EMPTY(RING) ((RING)->begin == (RING)->end)
+
+void ring_init(struct ring *ring, u8 * buf, ring_size_t size)
+{
+        ring->data = buf;
+        ring->size = size;
+        ring->begin = 0;
+        ring->end = 0;
+}
+
+s32 ring_write_ch(struct ring *ring, u8 ch)
+{
+        if (((ring->end + 1) % ring->size) != ring->begin) {
+                ring->data[ring->end++] = ch;
+                ring->end %= ring->size;
+                return (u32) ch;
+        }
+
+        return -1;
+}
+
+s32 ring_write(struct ring * ring, u8 * data, ring_size_t size)
+{
+        s32 i;
+
+        for (i = 0; i < size; i++) {
+                if (ring_write_ch(ring, data[i]) < 0) {
+                        return -i;
+                }
+        }
+
+        return i;
+}
+
+s32 ring_read_ch(struct ring * ring, u8 * ch)
+{
+        s32 ret = -1;
+
+        if (ring->begin != ring->end) {
+                ret = ring->data[ring->begin++];
+                ring->begin %= ring->size;
+                if (ch)
+                        *ch = ret;
+        }
+
+        return ret;
+}
+
+s32 ring_read(struct ring * ring, u8 * data, ring_size_t size)
+{
+        s32 i;
+
+        for (i = 0; i < size; i++) {
+                if (ring_read_ch(ring, data + i) < 0) {
+                        return i;
+                }
+        }
+
+        return -i;
+}
+
+/******************************************************************************
+ * The example implementation
+ ******************************************************************************/
+
+#define BUFFER_SIZE 1024
+
+struct ring output_ring;
+u8 output_ring_buffer[BUFFER_SIZE];
+
+void clock_setup(void)
+{
+	rcc_clock_setup_in_hse_8mhz_out_72mhz();
+
+	/* Enable GPIOA clock (for LED GPIOs). */
+	rcc_peripheral_enable_clock(&RCC_APB2ENR, RCC_APB2ENR_IOPCEN);
+
+	/* Enable clocks for GPIO port A (for GPIO_USART1_TX) and USART1. */
+	rcc_peripheral_enable_clock(&RCC_APB2ENR, RCC_APB2ENR_IOPAEN |
+				RCC_APB2ENR_AFIOEN |
+				RCC_APB2ENR_USART1EN);
+}
+
+void usart_setup(void)
+{
+
+        /* Initialize output ring buffer */
+	ring_init(&output_ring, output_ring_buffer, BUFFER_SIZE);
+
+	/* Enable the USART1 interrupt. */
+	nvic_enable_irq(NVIC_USART1_IRQ);
+
+	/* Setup GPIO pin GPIO_USART1_RE_TX on GPIO port B for transmit. */
+	gpio_set_mode(GPIOA, GPIO_MODE_OUTPUT_50_MHZ,
+                      GPIO_CNF_OUTPUT_ALTFN_PUSHPULL, GPIO_USART1_TX);
+
+	/* Setup GPIO pin GPIO_USART1_RE_RX on GPIO port B for receive. */
+	gpio_set_mode(GPIOA, GPIO_MODE_INPUT,
+		      GPIO_CNF_INPUT_FLOAT, GPIO_USART1_RX);
+
+	/* Setup UART parameters. */
+	usart_set_baudrate(USART1, 230400);
+	usart_set_databits(USART1, 8);
+	usart_set_stopbits(USART1, USART_STOPBITS_1);
+	usart_set_parity(USART1, USART_PARITY_NONE);
+	usart_set_flow_control(USART1, USART_FLOWCONTROL_NONE);
+	usart_set_mode(USART1, USART_MODE_TX_RX);
+
+	/* Enable USART1 Receive interrupt. */
+	USART_CR1(USART1) |= USART_CR1_RXNEIE;
+
+	/* Finally enable the USART. */
+	usart_enable(USART1);
+}
+
+void gpio_setup(void)
+{
+	gpio_set(GPIOC, GPIO12);
+
+	/* Setup GPIO6 and 7 (in GPIO port A) for led use. */
+	gpio_set_mode(GPIOC, GPIO_MODE_OUTPUT_50_MHZ,
+		      GPIO_CNF_OUTPUT_PUSHPULL, GPIO12);
+}
+
+void usart1_isr(void)
+{
+	/* Check if we were called because of RXNE. */
+	if (((USART_CR1(USART1) & USART_CR1_RXNEIE) != 0) &&
+		((USART_SR(USART1) & USART_SR_RXNE) != 0)) {
+
+		/* Indicate that we got data. */
+		gpio_toggle(GPIOC, GPIO12);
+
+		/* Retrieve the data from the peripheral. */
+		ring_write_ch(&output_ring, usart_recv(USART1));
+
+		/* Enable transmit interrupt so it sends back the data. */
+		USART_CR1(USART1) |= USART_CR1_TXEIE;
+	}
+
+	/* Check if we were called because of TXE. */
+	if (((USART_CR1(USART1) & USART_CR1_TXEIE) != 0) &&
+		((USART_SR(USART1) & USART_SR_TXE) != 0)) {
+
+		s32 data;
+
+		data = ring_read_ch(&output_ring, NULL);
+
+		if (data == -1) {
+			/* Disable the TXE interrupt as we don't need it anymore. */
+			USART_CR1(USART1) &= ~USART_CR1_TXEIE;
+		} else {
+			/* Put data into the transmit register. */
+			usart_send(USART1, data);
+		}
+	}
+}
+
+int _write (int file, char *ptr, int len)
+{
+	int ret;
+
+	if (file == 1) {
+		ret = ring_write(&output_ring, (u8 *)ptr, len);
+
+		if (ret < 0) ret = -ret;
+
+		USART_CR1(USART1) |= USART_CR1_TXEIE;
+
+		return ret;
+	}
+
+        errno = EIO;
+        return -1;
+}
+
+void systick_setup(void) {
+
+	/* 72MHz / 8 => 9000000 counts per second */
+	systick_set_clocksource(STK_CTRL_CLKSOURCE_AHB_DIV8);
+
+	/* 9000000/9000 = 1000 overflows per second - every 1ms one interrupt */
+	systick_set_reload(9000);
+
+	systick_interrupt_enable();
+
+	/* Start counting. */
+	systick_counter_enable();
+
+}
+
+void sys_tick_handler(void)
+{
+	static int counter = 0;
+	static float fcounter = 0.0;
+	static double dcounter = 0.0;
+
+	static u32 temp32 = 0;
+
+	temp32++;
+
+	/* We call this handler every 1ms so we are sending hello world every 10ms / 100Hz. */
+	if (temp32 == 10) {
+		printf("Hello World! %i %f %f\r\n", counter, fcounter, dcounter);
+		counter++;
+		fcounter+=0.01;
+		dcounter+=0.01;
+
+		temp32 = 0;
+	}
+}
+
+int main(void)
+{
+
+	clock_setup();
+	gpio_setup();
+	usart_setup();
+	systick_setup();
+
+	while (1) {
+		__asm__("nop");
+	}
+
+	return 0;
+}

examples/stm32/stm32-h103/usart_irq_printf/usart_irq_printf.ld

@@ -0,0 +1,31 @@
+/*
+ * This file is part of the libopencm3 project.
+ *
+ * Copyright (C) 2009 Uwe Hermann <uwe@hermann-uwe.de>
+ *
+ * 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 <http://www.gnu.org/licenses/>.
+ */
+
+/* Linker script for Open-BLDC (STM32F103CBT6, 128K flash, 20K RAM). */
+
+/* Define memory regions. */
+MEMORY
+{
+	rom (rx) : ORIGIN = 0x08000000, LENGTH = 128K
+	ram (rwx) : ORIGIN = 0x20000000, LENGTH = 20K
+}
+
+/* Include the common ld script. */
+INCLUDE libopencm3_stm32.ld
+