diff --git a/src/platforms/f4discovery/platform.h b/src/platforms/f4discovery/platform.h
index e50b6c7..5523b9c 100644
--- a/src/platforms/f4discovery/platform.h
+++ b/src/platforms/f4discovery/platform.h
@@ -112,6 +112,7 @@ extern usbd_device *usbdev;
  */
 #define IRQ_PRI_USB		(2 << 4)
 #define IRQ_PRI_USBUSART	(1 << 4)
+#define IRQ_PRI_USBUSART_TIM	(3 << 4)
 #define IRQ_PRI_TRACE		(0 << 4)
 
 #define USBUSART USART3
@@ -124,6 +125,10 @@ extern usbd_device *usbdev;
 #define USBUSART_RX_PORT GPIOD
 #define USBUSART_RX_PIN  GPIO9
 #define USBUSART_ISR usart3_isr
+#define USBUSART_TIM TIM4
+#define USBUSART_TIM_CLK_EN() rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM4EN)
+#define USBUSART_TIM_IRQ NVIC_TIM4_IRQ
+#define USBUSART_TIM_ISR tim4_isr
 
 #define UART_PIN_SETUP() do {                                           \
         gpio_mode_setup(USBUSART_TX_PORT, GPIO_MODE_AF, GPIO_PUPD_NONE, \
diff --git a/src/platforms/native/platform.c b/src/platforms/native/platform.c
index 640ba58..214625e 100644
--- a/src/platforms/native/platform.c
+++ b/src/platforms/native/platform.c
@@ -126,8 +126,6 @@ int platform_init(void)
 	systick_interrupt_enable();
 	systick_counter_enable();
 
-	usbuart_init();
-
 	if (platform_hwversion() > 0) {
 		adc_init();
 	} else {
@@ -139,6 +137,7 @@ int platform_init(void)
 	SCB_VTOR = 0x2000;	// Relocate interrupt vector table here
 
 	cdcacm_init();
+	usbuart_init();
 
 	jtag_scan(NULL);
 
diff --git a/src/platforms/native/platform.h b/src/platforms/native/platform.h
index 20d1510..62d565b 100644
--- a/src/platforms/native/platform.h
+++ b/src/platforms/native/platform.h
@@ -128,6 +128,7 @@ extern usbd_device *usbdev;
  */
 #define IRQ_PRI_USB		(2 << 4)
 #define IRQ_PRI_USBUSART	(1 << 4)
+#define IRQ_PRI_USBUSART_TIM	(3 << 4)
 #define IRQ_PRI_USB_VBUS	(14 << 4)
 #define IRQ_PRI_TRACE		(0 << 4)
 
@@ -139,6 +140,10 @@ extern usbd_device *usbdev;
 #define USBUSART_PORT GPIOA
 #define USBUSART_TX_PIN GPIO9
 #define USBUSART_ISR usart1_isr
+#define USBUSART_TIM TIM4
+#define USBUSART_TIM_CLK_EN() rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM4EN)
+#define USBUSART_TIM_IRQ NVIC_TIM4_IRQ
+#define USBUSART_TIM_ISR tim4_isr
 
 #define TRACE_TIM TIM3
 #define TRACE_TIM_CLK_EN() rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM3EN)
diff --git a/src/platforms/stlink/platform.h b/src/platforms/stlink/platform.h
index c79ea13..a9e405b 100644
--- a/src/platforms/stlink/platform.h
+++ b/src/platforms/stlink/platform.h
@@ -114,6 +114,7 @@ extern usbd_device *usbdev;
  */
 #define IRQ_PRI_USB		(2 << 4)
 #define IRQ_PRI_USBUSART	(1 << 4)
+#define IRQ_PRI_USBUSART_TIM	(3 << 4)
 #define IRQ_PRI_USB_VBUS	(14 << 4)
 #define IRQ_PRI_TIM3		(0 << 4)
 
@@ -125,6 +126,10 @@ extern usbd_device *usbdev;
 #define USBUSART_PORT GPIOA
 #define USBUSART_TX_PIN GPIO2
 #define USBUSART_ISR usart2_isr
+#define USBUSART_TIM TIM4
+#define USBUSART_TIM_CLK_EN() rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM4EN)
+#define USBUSART_TIM_IRQ NVIC_TIM4_IRQ
+#define USBUSART_TIM_ISR tim4_isr
 
 #define DEBUG(...)
 
diff --git a/src/platforms/stm32/usbuart.c b/src/platforms/stm32/usbuart.c
index 1c71105..7f5ebde 100644
--- a/src/platforms/stm32/usbuart.c
+++ b/src/platforms/stm32/usbuart.c
@@ -21,6 +21,7 @@
 #include <libopencm3/stm32/rcc.h>
 #include <libopencm3/stm32/gpio.h>
 #include <libopencm3/stm32/usart.h>
+#include <libopencm3/stm32/timer.h>
 #include <libopencm3/cm3/nvic.h>
 #include <libopencm3/cm3/scs.h>
 #include <libopencm3/usb/usbd.h>
@@ -28,6 +29,20 @@
 
 #include <platform.h>
 
+#define USBUART_TIMER_FREQ_HZ 1000000U /* 1us per tick */
+#define USBUART_RUN_FREQ_HZ 5000U /* 200us (or 100 characters at 2Mbps) */
+
+#define FIFO_SIZE 128
+
+/* RX Fifo buffer */
+static uint8_t buf_rx[FIFO_SIZE];
+/* Fifo in pointer, writes assumed to be atomic, should be only incremented within RX ISR */
+static uint8_t buf_rx_in;
+/* Fifo out pointer, writes assumed to be atomic, should be only incremented outside RX ISR */
+static uint8_t buf_rx_out;
+
+static void usbuart_run(void);
+
 void usbuart_init(void)
 {
 #if defined(BLACKMAGIC)
@@ -56,6 +71,68 @@ void usbuart_init(void)
 	USBUSART_CR1 |= USART_CR1_RXNEIE;
 	nvic_set_priority(USBUSART_IRQ, IRQ_PRI_USBUSART);
 	nvic_enable_irq(USBUSART_IRQ);
+
+	/* Setup timer for running deferred FIFO processing */
+	USBUSART_TIM_CLK_EN();
+	timer_reset(USBUSART_TIM);
+	timer_set_mode(USBUSART_TIM, TIM_CR1_CKD_CK_INT,
+			TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP);
+	timer_set_prescaler(USBUSART_TIM,
+			rcc_ppre2_frequency / USBUART_TIMER_FREQ_HZ * 2 - 1);
+	timer_set_period(USBUSART_TIM,
+			USBUART_TIMER_FREQ_HZ / USBUART_RUN_FREQ_HZ - 1);
+
+	/* Setup update interrupt in NVIC */
+	nvic_set_priority(USBUSART_TIM_IRQ, IRQ_PRI_USBUSART_TIM);
+	nvic_enable_irq(USBUSART_TIM_IRQ);
+
+	/* turn the timer on */
+	timer_enable_counter(USBUSART_TIM);
+}
+
+/*
+ * Runs deferred processing for usb uart rx, draining RX FIFO by sending
+ * characters to host PC via CDCACM.  Allowed to read from FIFO in pointer,
+ * but not write to it. Allowed to write to FIFO out pointer.
+ */
+static void usbuart_run(void)
+{
+	/* forcibly empty fifo if no USB endpoint */
+	if (cdcacm_get_config() != 1)
+	{
+		buf_rx_out = buf_rx_in;
+	}
+
+	/* if fifo empty, nothing further to do */
+	if (buf_rx_in == buf_rx_out) {
+		/* turn off LED, disable IRQ */
+		timer_disable_irq(USBUSART_TIM, TIM_DIER_UIE);
+		gpio_clear(LED_PORT_UART, LED_UART);
+	}
+	else
+	{
+		uint8_t packet_buf[CDCACM_PACKET_SIZE];
+		uint8_t packet_size = 0;
+		uint8_t buf_out = buf_rx_out;
+
+		/* copy from uart FIFO into local usb packet buffer */
+		while (buf_rx_in != buf_out && packet_size < CDCACM_PACKET_SIZE)
+		{
+			packet_buf[packet_size++] = buf_rx[buf_out++];
+
+			/* wrap out pointer */
+			if (buf_out >= FIFO_SIZE)
+			{
+				buf_out = 0;
+			}
+
+		}
+
+		/* advance fifo out pointer by amount written */
+		buf_rx_out += usbd_ep_write_packet(usbdev,
+				CDCACM_UART_ENDPOINT, packet_buf, packet_size);
+		buf_rx_out %= FIFO_SIZE;
+	}
 }
 
 void usbuart_set_line_coding(struct usb_cdc_line_coding *coding)
@@ -108,41 +185,50 @@ void usbuart_usb_out_cb(usbd_device *dev, uint8_t ep)
 	gpio_clear(LED_PORT_UART, LED_UART);
 }
 
-static uint8_t uart_usb_buf[CDCACM_PACKET_SIZE];
-static uint8_t uart_usb_buf_size;
 
 void usbuart_usb_in_cb(usbd_device *dev, uint8_t ep)
 {
-	if (!uart_usb_buf_size) {
-		gpio_clear(LED_PORT_UART, LED_UART);
-		return;
-	}
-
-	usbd_ep_write_packet(dev, ep, uart_usb_buf, uart_usb_buf_size);
-	uart_usb_buf_size = 0;
+	(void) dev;
+	(void) ep;
 }
 
+/*
+ * Read a character from the UART RX and stuff it in a software FIFO.
+ * Allowed to read from FIFO out pointer, but not write to it.
+ * Allowed to write to FIFO in pointer.
+ */
 void USBUSART_ISR(void)
 {
 	char c = usart_recv(USBUSART);
 
-	/* Don't try to write until we are configured.
-	 * Otherwise enumeration hanged in some cases.
-	 */
-	if (cdcacm_get_config() != 1)
-		return;
-
+	/* Turn on LED */
 	gpio_set(LED_PORT_UART, LED_UART);
 
-	/* Try to send now */
-	if (usbd_ep_write_packet(usbdev, CDCACM_UART_ENDPOINT, &c, 1) == 1)
-		return;
+	/* If the next increment of rx_in would put it at the same point
+	* as rx_out, the FIFO is considered full.
+	*/
+	if (((buf_rx_in + 1) % FIFO_SIZE) != buf_rx_out)
+	{
+		/* insert into FIFO */
+		buf_rx[buf_rx_in++] = c;
 
-	/* We failed, so queue for later */
-	if (uart_usb_buf_size == CDCACM_PACKET_SIZE) {
-		/* Drop if the buffer's full: we have no flow control */
-		return;
+		/* wrap out pointer */
+		if (buf_rx_in >= FIFO_SIZE)
+		{
+			buf_rx_in = 0;
+		}
+
+		/* enable deferred processing if we put data in the FIFO */
+		timer_enable_irq(USBUSART_TIM, TIM_DIER_UIE);
 	}
-
-	uart_usb_buf[uart_usb_buf_size++] = c;
 }
+
+void USBUSART_TIM_ISR(void)
+{
+	/* need to clear timer update event */
+	timer_clear_flag(USBUSART_TIM, TIM_SR_UIF);
+
+	/* process FIFO */
+	usbuart_run();
+}
+
diff --git a/src/platforms/swlink/platform.h b/src/platforms/swlink/platform.h
index 614a389..d0c9618 100644
--- a/src/platforms/swlink/platform.h
+++ b/src/platforms/swlink/platform.h
@@ -106,6 +106,7 @@ extern usbd_device *usbdev;
  */
 #define IRQ_PRI_USB		(2 << 4)
 #define IRQ_PRI_USBUSART	(1 << 4)
+#define IRQ_PRI_USBUSART_TIM	(3 << 4)
 #define IRQ_PRI_USB_VBUS	(14 << 4)
 #define IRQ_PRI_TRACE		(0 << 4)
 
@@ -117,6 +118,10 @@ extern usbd_device *usbdev;
 #define USBUSART_PORT GPIOB
 #define USBUSART_TX_PIN GPIO6
 #define USBUSART_ISR usart1_isr
+#define USBUSART_TIM TIM4
+#define USBUSART_TIM_CLK_EN() rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM4EN)
+#define USBUSART_TIM_IRQ NVIC_TIM4_IRQ
+#define USBUSART_TIM_ISR tim4_isr
 
 #define TRACE_TIM TIM2
 #define TRACE_TIM_CLK_EN() rcc_peripheral_enable_clock(&RCC_APB1ENR, RCC_APB1ENR_TIM2EN)