diff --git a/supervisor/shared/cpu.c b/supervisor/shared/cpu.c
new file mode 100644
index 0000000000..510225d197
--- /dev/null
+++ b/supervisor/shared/cpu.c
@@ -0,0 +1,40 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Scott Shawcroft for Adafruit Industries
+ *
+ * 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.
+ */
+
+#include <stdint.h>
+
+#include "supervisor/shared/cpu.h"
+
+bool cpu_interrupt_active(void) {
+    #if defined(__ARM_ARCH) && (__ARM_ARCH >= 6)
+    // Check VECTACTIVE in ICSR. We don't need to disable interrupts because if
+    // one occurs after we read, we won't continue until it is resolved.
+    return (*((volatile uint32_t *)0xE000ED04) & 0x1ff) != 0;
+    #else
+    // We don't know.
+    return false;
+    #endif
+}
diff --git a/supervisor/shared/cpu.h b/supervisor/shared/cpu.h
new file mode 100644
index 0000000000..9e2bed1e55
--- /dev/null
+++ b/supervisor/shared/cpu.h
@@ -0,0 +1,36 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Scott Shawcroft for Adafruit Industries
+ *
+ * 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.
+ */
+
+#ifndef MICROPY_INCLUDED_SUPERVISOR_SHARED_CPU_H
+#define MICROPY_INCLUDED_SUPERVISOR_SHARED_CPU_H
+
+#include <stdbool.h>
+#include <stddef.h>
+
+// True when we're in an interrupt handler.
+bool cpu_interrupt_active(void);
+
+#endif  // MICROPY_INCLUDED_SUPERVISOR_SHARED_CPU_H
diff --git a/supervisor/shared/serial.c b/supervisor/shared/serial.c
index 268cebdbba..bd2bf4ba22 100644
--- a/supervisor/shared/serial.c
+++ b/supervisor/shared/serial.c
@@ -28,6 +28,7 @@
 
 #include "py/mpconfig.h"
 
+#include "supervisor/shared/cpu.h"
 #include "supervisor/shared/display.h"
 #include "shared-bindings/terminalio/Terminal.h"
 #include "supervisor/serial.h"
@@ -136,6 +137,10 @@ void serial_write_substring(const char *text, uint32_t length) {
     uint32_t count = 0;
     while (count < length && tud_cdc_connected()) {
         count += tud_cdc_write(text + count, length - count);
+        // If we're in an interrupt, then don't wait for more room. Queue up what we can.
+        if (cpu_interrupt_active()) {
+            break;
+        }
         usb_background();
     }
 
diff --git a/supervisor/supervisor.mk b/supervisor/supervisor.mk
index 1744c4f47f..c206ff96be 100644
--- a/supervisor/supervisor.mk
+++ b/supervisor/supervisor.mk
@@ -4,6 +4,7 @@ SRC_SUPERVISOR = \
 	supervisor/shared/autoreload.c \
 	supervisor/shared/background_callback.c \
 	supervisor/shared/board.c \
+	supervisor/shared/cpu.c \
 	supervisor/shared/filesystem.c \
 	supervisor/shared/flash.c \
 	supervisor/shared/micropython.c \