diff --git a/FAQ-Build.md b/FAQ-Build.md
index c3f168a719..c793891260 100644
--- a/FAQ-Build.md
+++ b/FAQ-Build.md
@@ -101,4 +101,19 @@ https://launchpad.net/~terry.guo/+archive/ubuntu/gcc-arm-embedded
 
 https://github.com/tmk/tmk_keyboard/issues/212
 https://github.com/tmk/tmk_keyboard/wiki/mbed-cortex-porting#compile-error-cstddef
-https://developer.mbed.org/forum/mbed/topic/5205/
\ No newline at end of file
+https://developer.mbed.org/forum/mbed/topic/5205/
+
+
+## 'clock_prescale_set' and 'clock_div_1' not available
+Your toolchain is too old to support the MCU. For example WinAVR 20100110 doesn't support ATMega32u2.
+
+```
+Compiling C: ../../tmk_core/protocol/lufa/lufa.c
+avr-gcc -c -mmcu=atmega32u2 -gdwarf-2 -DF_CPU=16000000UL -DINTERRUPT_CONTROL_ENDPOINT -DBOOTLOADER_SIZE=4096 -DF_USB=16000000UL -DARCH=ARCH_AVR8 -DUSB_DEVICE_ONLY -DUSE_FLASH_DESCRIPTORS -DUSE_STATIC_OPTIONS="(USB_DEVICE_OPT_FULLSPEED | USB_OPT_REG_ENABLED | USB_OPT_AUTO_PLL)" -DFIXED_CONTROL_ENDPOINT_SIZE=8  -DFIXED_NUM_CONFIGURATIONS=1 -DPROTOCOL_LUFA -DEXTRAKEY_ENABLE -DCONSOLE_ENABLE -DCOMMAND_ENABLE -DVERSION=unknown -Os -funsigned-char -funsigned-bitfields -ffunction-sections -fdata-sections -fno-inline-small-functions -fpack-struct -fshort-enums -fno-strict-aliasing -Wall -Wstrict-prototypes -Wa,-adhlns=obj_alps64/protocol/lufa/lufa.lst -I. -I../../tmk_core -I../../tmk_core/protocol/lufa -I../../tmk_core/protocol/lufa/LUFA-git -I../../tmk_core/common -std=gnu99 -include config.h -MMD -MP -MF .dep/obj_alps64_protocol_lufa_lufa.o.d  ../../tmk_core/protocol/lufa/lufa.c -o obj_alps64/protocol/lufa/lufa.o
+../../tmk_core/protocol/lufa/lufa.c: In function 'setup_mcu':
+../../tmk_core/protocol/lufa/lufa.c:575: warning: implicit declaration of function 'clock_prescale_set'
+../../tmk_core/protocol/lufa/lufa.c:575: error: 'clock_div_1' undeclared (first use in this function)
+../../tmk_core/protocol/lufa/lufa.c:575: error: (Each undeclared identifier is reported only once
+../../tmk_core/protocol/lufa/lufa.c:575: error: for each function it appears in.)
+make: *** [obj_alps64/protocol/lufa/lufa.o] Error 1
+```
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