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diff --git a/libmaple/stm32f2/usart.c b/libmaple/stm32f2/usart.c
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+/******************************************************************************
+ * The MIT License
+ *
+ * Copyright (c) 2012 LeafLabs, LLC.
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *****************************************************************************/
+
+/**
+ * @file libmaple/stm32f2/usart.c
+ * @author Marti Bolivar <mbolivar@leaflabs.com>
+ * @brief STM32F2 USART.
+ */
+
+#include <libmaple/usart.h>
+#include <libmaple/gpio.h>
+#include "usart_private.h"
+
+/*
+ * Devices
+ */
+
+static ring_buffer usart1_rb;
+static usart_dev usart1 = {
+ .regs = USART1_BASE,
+ .rb = &usart1_rb,
+ .max_baud = 4500000UL, /* TODO: are these correct? */
+ .clk_id = RCC_USART1,
+ .irq_num = NVIC_USART1,
+};
+/** USART1 device */
+usart_dev *USART1 = &usart1;
+
+static ring_buffer usart2_rb;
+static usart_dev usart2 = {
+ .regs = USART2_BASE,
+ .rb = &usart2_rb,
+ .max_baud = 2250000UL, /* TODO: are these correct? */
+ .clk_id = RCC_USART2,
+ .irq_num = NVIC_USART2,
+};
+/** USART2 device */
+usart_dev *USART2 = &usart2;
+
+static ring_buffer usart3_rb;
+static usart_dev usart3 = {
+ .regs = USART3_BASE,
+ .rb = &usart3_rb,
+ .max_baud = 2250000UL, /* TODO: are these correct? */
+ .clk_id = RCC_USART3,
+ .irq_num = NVIC_USART3,
+};
+/** USART3 device */
+usart_dev *USART3 = &usart3;
+
+static ring_buffer uart4_rb;
+static usart_dev uart4 = {
+ .regs = UART4_BASE,
+ .rb = &uart4_rb,
+ .max_baud = 2250000UL, /* TODO: are these correct? */
+ .clk_id = RCC_UART4,
+ .irq_num = NVIC_UART4,
+};
+/** UART4 device */
+usart_dev *UART4 = &uart4;
+
+static ring_buffer uart5_rb;
+static usart_dev uart5 = {
+ .regs = UART5_BASE,
+ .rb = &uart5_rb,
+ .max_baud = 2250000UL, /* TODO: are these correct? */
+ .clk_id = RCC_UART5,
+ .irq_num = NVIC_UART5,
+};
+/** UART5 device */
+usart_dev *UART5 = &uart5;
+
+static ring_buffer usart6_rb;
+static usart_dev usart6 = {
+ .regs = USART6_BASE,
+ .rb = &usart6_rb,
+ .max_baud = 4500000UL, /* TODO: are these correct? */
+ .clk_id = RCC_USART6,
+ .irq_num = NVIC_USART6,
+};
+usart_dev *USART6 = &usart6;
+
+/*
+ * Routines
+ */
+
+void usart_config_gpios_async(usart_dev *udev,
+ gpio_dev *rx_dev, uint8 rx,
+ gpio_dev *tx_dev, uint8 tx,
+ unsigned flags) {
+ gpio_af af = usart_get_af(udev);
+ gpio_set_modef(rx_dev, rx, GPIO_MODE_AF, 0);
+ gpio_set_modef(tx_dev, tx, GPIO_MODE_AF, 0);
+ gpio_set_af(rx_dev, rx, af);
+ gpio_set_af(tx_dev, tx, af);
+}
+
+void usart_set_baud_rate(usart_dev *dev, uint32 clock_speed, uint32 baud) {
+ uint32 integer_part;
+ uint32 fractional_part;
+ uint32 tmp;
+ uint32 over8 = !!(dev->regs->CR1 & USART_CR1_OVER8);
+
+ ASSERT(!over8); /* OVER8 is currently unsupported. */
+
+ /* Figure out the clock speed, if the user doesn't give one. */
+ if (clock_speed == 0) {
+ clock_speed = _usart_clock_freq(dev);
+ }
+ ASSERT(clock_speed);
+
+ /* Convert desired baud rate to baud rate register setting. */
+ integer_part = (25 * clock_speed) / (2 * (2 - over8) * baud);
+ tmp = (integer_part / 100) << 4;
+ fractional_part = integer_part - (100 * (tmp >> 4));
+ tmp |= ((fractional_part * 16 + 50) / 100) & (uint8)0x0F;
+
+ dev->regs->BRR = tmp;
+}
+
+/**
+ * @brief Call a function on each USART.
+ * @param fn Function to call.
+ */
+void usart_foreach(void (*fn)(usart_dev*)) {
+ fn(USART1);
+ fn(USART2);
+ fn(USART3);
+ fn(UART4);
+ fn(UART5);
+ fn(USART6);
+}
+
+/**
+ * @brief Get GPIO alternate function mode for a USART.
+ * @param dev USART whose gpio_af to get.
+ * @return gpio_af corresponding to dev.
+ */
+gpio_af usart_get_af(usart_dev *dev) {
+ switch (dev->clk_id) {
+ case RCC_USART1:
+ case RCC_USART2:
+ case RCC_USART3:
+ return GPIO_AF_USART_1_2_3;
+ case RCC_UART4:
+ case RCC_UART5:
+ case RCC_USART6:
+ return GPIO_AF_USART_4_5_6;
+ default:
+ ASSERT(0); /* Can't happen */
+ return (gpio_af)-1;
+ }
+}
+
+/*
+ * Interrupt handlers.
+ */
+
+void __irq_usart1(void) {
+ usart_irq(&usart1_rb, USART1_BASE);
+}
+
+void __irq_usart2(void) {
+ usart_irq(&usart2_rb, USART2_BASE);
+}
+
+void __irq_usart3(void) {
+ usart_irq(&usart3_rb, USART3_BASE);
+}
+
+void __irq_uart4(void) {
+ usart_irq(&uart4_rb, UART4_BASE);
+}
+
+void __irq_uart5(void) {
+ usart_irq(&uart5_rb, UART5_BASE);
+}
+
+void __irq_usart6(void) {
+ usart_irq(&usart6_rb, USART6_BASE);
+}