diff --git a/hw/bsp/board.c b/hw/bsp/board.c
index 383a02ef4..9dae8a04d 100644
--- a/hw/bsp/board.c
+++ b/hw/bsp/board.c
@@ -25,7 +25,7 @@
 
 #include "board.h"
 
-#if defined(__MSP430__)
+#if defined(__MSP430__) || defined(__RX__)
   #define sys_write   write
   #define sys_read    read
 #else
diff --git a/hw/bsp/rx/boards/gr_citrus/gr_citrus.c b/hw/bsp/rx/boards/gr_citrus/gr_citrus.c
index b773e0dd0..caf5fd6fa 100644
--- a/hw/bsp/rx/boards/gr_citrus/gr_citrus.c
+++ b/hw/bsp/rx/boards/gr_citrus/gr_citrus.c
@@ -36,24 +36,20 @@
  * 
  * Connect the pins between GR-CITRUS and JLink as follows.
  * 
- * | JTAG Function | GR-CITRUS pin name| JLink pin No.| note     |
- * |:-------------:|:-----------------:|:------------:|:--------:|
- * | VTref         |   3.3V            |   1          |          |
- * | TRST          |   5               |   3          |          |
- * | GND           |   GND             |   4          |          |
- * | TDI           |   3               |   5          |          |
- * | TMS           |   2               |   7          |          |
- * | TCK           |   14              |   9          | short J4 |
- * | TDO           |   9               |  13          | short J5 |
- * | nRES          |   RST             |  15          |          |
+ * | Function  | GR-CITRUS pin | JLink pin No.| note     |
+ * |:---------:|:-------------:|:------------:|:--------:|
+ * | VTref     |   3.3V        |   1          |          |
+ * | TRST      |   5           |   3          |          |
+ * | GND       |   GND         |   4          |          |
+ * | TDI       |   3           |   5          |          |
+ * | TMS       |   2           |   7          |          |
+ * | TCK/FINEC |   14          |   9          | short J4 |
+ * | TDO       |   9           |  13          | short J5 |
+ * | nRES      |   RST         |  15          |          |
  *
  * JLink firmware needs to update to V6.96 or newer version to avoid
  * [a bug](https://forum.segger.com/index.php/Thread/7758-SOLVED-Bug-in-JLink-from-V6-88b-regarding-RX65N)
  * regarding downloading.
- *
- * When using SEGGER RTT, `RX_NEWLIB=0` should be added to make command arguments.
- * The option is used to change the C runtime library to `optlib` from `newlib`.
- * RTT may not work with `newlib`.
  */
 
 #include "../board.h"
@@ -253,3 +249,27 @@ uint32_t board_millis(void)
 #else
 uint32_t SystemCoreClock = 96000000;
 #endif
+
+int close(int fd)
+{
+    (void)fd;
+    return -1;
+}
+int fstat(int fd, void *pstat)
+{
+    (void)fd;
+    (void)pstat;
+    return 0;
+}
+off_t lseek(int fd, off_t pos, int whence)
+{
+    (void)fd;
+    (void)pos;
+    (void)whence;
+    return 0;
+}
+int isatty(int fd)
+{
+    (void)fd;
+    return 1;
+}
diff --git a/hw/bsp/rx/boards/gr_citrus/r5f5631fd.ld b/hw/bsp/rx/boards/gr_citrus/r5f5631fd.ld
index fa8142936..bb9c297c7 100644
--- a/hw/bsp/rx/boards/gr_citrus/r5f5631fd.ld
+++ b/hw/bsp/rx/boards/gr_citrus/r5f5631fd.ld
@@ -1,5 +1,5 @@
-__USTACK_SIZE = 0x00000200;
-__ISTACK_SIZE = 0x00000200;
+__USTACK_SIZE = 0x00000400;
+__ISTACK_SIZE = 0x00000400;
 
 MEMORY
 {
diff --git a/hw/bsp/rx/boards/rx65n_target/r5f565ne.ld b/hw/bsp/rx/boards/rx65n_target/r5f565ne.ld
index a02f0c02f..8e5617f23 100644
--- a/hw/bsp/rx/boards/rx65n_target/r5f565ne.ld
+++ b/hw/bsp/rx/boards/rx65n_target/r5f565ne.ld
@@ -1,5 +1,5 @@
-__USTACK_SIZE = 0x00000200;
-__ISTACK_SIZE = 0x00000200;
+__USTACK_SIZE = 0x00000400;
+__ISTACK_SIZE = 0x00000400;
 
 MEMORY
 {
diff --git a/hw/bsp/rx/boards/rx65n_target/rx65n_target.c b/hw/bsp/rx/boards/rx65n_target/rx65n_target.c
index 96d8f36f0..20867455e 100644
--- a/hw/bsp/rx/boards/rx65n_target/rx65n_target.c
+++ b/hw/bsp/rx/boards/rx65n_target/rx65n_target.c
@@ -48,11 +48,6 @@
  * JLink firmware needs to update to V6.96 or newer version to avoid
  * [a bug](https://forum.segger.com/index.php/Thread/7758-SOLVED-Bug-in-JLink-from-V6-88b-regarding-RX65N)
  * regarding downloading.
- *
- * When using SEGGER RTT, `RX_NEWLIB=0` should be added to make command arguments.
- * The option is used to change the C runtime library to `optlib` from `newlib`.
- * RTT may not work with `newlib`.
- *
  */
 
 #include "bsp/board.h"
@@ -299,3 +294,27 @@ uint32_t board_millis(void)
 #else
 uint32_t SystemCoreClock = 120000000;
 #endif
+
+int close(int fd)
+{
+    (void)fd;
+    return -1;
+}
+int fstat(int fd, void *pstat)
+{
+    (void)fd;
+    (void)pstat;
+    return 0;
+}
+off_t lseek(int fd, off_t pos, int whence)
+{
+    (void)fd;
+    (void)pos;
+    (void)whence;
+    return 0;
+}
+int isatty(int fd)
+{
+    (void)fd;
+    return 1;
+}
diff --git a/hw/bsp/rx/family.mk b/hw/bsp/rx/family.mk
index 7d7585b4a..5a8281718 100644
--- a/hw/bsp/rx/family.mk
+++ b/hw/bsp/rx/family.mk
@@ -11,25 +11,7 @@ CFLAGS += \
   -fdata-sections \
   -fshort-enums \
   -mlittle-endian-data \
-
-RX_NEWLIB ?= 1
-
-ifeq ($(CMDEXE),1)
-  OPTLIBINC="$(shell for /F "usebackq delims=" %%i in (`where rx-elf-gcc`) do echo %%~dpi..\rx-elf\optlibinc)"
-else
-  OPTLIBINC=$(shell dirname `which rx-elf-gcc`)../rx-elf/optlibinc
-endif
-
-ifeq ($(RX_NEWLIB),1)
-  CFLAGS += -DSSIZE_MAX=__INT_MAX__
-else
-  # setup for optlib
-  CFLAGS += -nostdinc \
-    -isystem $(OPTLIBINC) \
-    -DLWIP_NO_INTTYPES_H
-
-  LIBS += -loptc -loptm
-endif
+  -DSSIZE_MAX=__INT_MAX__
 
 SRC_C += \
 	src/portable/renesas/usba/dcd_usba.c \