diff --git a/ports/arm/atomport-tests.h b/ports/arm/atomport-tests.h
index 5df864b..1c52be9 100644
--- a/ports/arm/atomport-tests.h
+++ b/ports/arm/atomport-tests.h
@@ -48,8 +48,8 @@ extern void dbg_format_msg (char *format, ...) ;
  */
 #define _STR(x)     x
 
-/* Default thread stack size (in bytes) */
-#define TEST_THREAD_STACK_SIZE      1024
+/* Default thread stack size (in bytes). Allow plenty for printf(). */
+#define TEST_THREAD_STACK_SIZE      4096
 
 /* Uncomment to enable logging of stack usage to UART */
 /* #define TESTS_LOG_STACK_USAGE */
diff --git a/ports/arm/tests-main.c b/ports/arm/tests-main.c
index f345321..a157d54 100644
--- a/ports/arm/tests-main.c
+++ b/ports/arm/tests-main.c
@@ -63,8 +63,13 @@
  * stack saves and context switch saves, but the application main thread
  * will generally be carrying out more nested function calls and require
  * stack for application code local variables etc.
+ *
+ * 1KB might be adequate but if using printf() then at least 2KB would be
+ * prudent otherwise the stdio functions otherwise stack overruns are
+ * likely. Nearly 2KB was seen to be used on the toolchain used for
+ * development.
  */
-#define MAIN_STACK_SIZE_BYTES       1024
+#define MAIN_STACK_SIZE_BYTES       4096
 
 
 /* Local data */