From 614c1fdba2aca393e6769221cf8d8b55ee3464fe Mon Sep 17 00:00:00 2001
From: Scott Shawcroft <scott@chickadee.tech>
Date: Tue, 25 Oct 2016 18:23:04 -0700
Subject: [PATCH] atmel-samd: Only output to USB after DTR and don't send
 anything larger than the room left in the USB TX buffer.

---
 atmel-samd/boards/arduino_zero/conf_usb.h |  6 ++++--
 atmel-samd/mphalport.c                    | 15 +++++++++++++--
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/atmel-samd/boards/arduino_zero/conf_usb.h b/atmel-samd/boards/arduino_zero/conf_usb.h
index d1ee1e8493..4702fe93b1 100644
--- a/atmel-samd/boards/arduino_zero/conf_usb.h
+++ b/atmel-samd/boards/arduino_zero/conf_usb.h
@@ -54,8 +54,10 @@ extern void mp_cdc_disable(uint8_t port);
 #define  UDI_CDC_RX_NOTIFY(port)           usb_rx_notify()
 void usb_rx_notify(void);
 #define  UDI_CDC_SET_CODING_EXT(port,cfg)
-#define  UDI_CDC_SET_DTR_EXT(port,set)
-#define  UDI_CDC_SET_RTS_EXT(port,set)
+#define  UDI_CDC_SET_DTR_EXT(port,set) usb_dtr_notify(port, set)
+void usb_dtr_notify(uint8_t port, bool set);
+#define  UDI_CDC_SET_RTS_EXT(port,set) usb_rts_notify(port, set)
+void usb_rts_notify(uint8_t port, bool set);
 
 /**
  * USB CDC low level configuration
diff --git a/atmel-samd/mphalport.c b/atmel-samd/mphalport.c
index 3b70b3d052..7102bf220f 100644
--- a/atmel-samd/mphalport.c
+++ b/atmel-samd/mphalport.c
@@ -50,7 +50,7 @@ void mp_msc_disable()
 
 bool mp_cdc_enable(uint8_t port)
 {
-	mp_cdc_enabled = true;
+	mp_cdc_enabled = false;
 	return true;
 }
 
@@ -59,6 +59,14 @@ void mp_cdc_disable(uint8_t port)
 	mp_cdc_enabled = false;
 }
 
+void usb_dtr_notify(uint8_t port, bool set) {
+	mp_cdc_enabled = set;
+}
+
+void usb_rts_notify(uint8_t port, bool set) {
+    return;
+}
+
 void usb_rx_notify(void)
 {
     irqflags_t flags;
@@ -177,7 +185,10 @@ void mp_hal_stdout_tx_strn(const char *str, size_t len) {
     #endif
 
     #ifdef USB_REPL
-    if (mp_cdc_enabled && udi_cdc_is_tx_ready()) {
+    // Always make sure there is enough room in the usb buffer for the outgoing
+    // string. If there isn't we risk getting caught in a loop within the usb
+    // code as it tries to send all the characters it can't buffer.
+    if (mp_cdc_enabled && udi_cdc_get_free_tx_buffer() >= len) {
         udi_cdc_write_buf(str, len);
     }
     #endif