+- http://www.mcselec.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=92&option=com_phpshop&Itemid=1
+
+
+## Linux udev rules
+On Linux you need proper privilege to access device file of MCU, you'll have to use `sudo` when flashing firmware. You can circumvent this with placing these files in `/etc/udev/rules.d/`.
+
+**/etc/udev/rules.d/50-atmel-dfu.rules:**
+```
+# Atmel ATMega32U4
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2ff4", MODE:="0666"
+# Atmel USBKEY AT90USB1287
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2ffb", MODE:="0666"
+# Atmel ATMega32U2
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2ff0", MODE:="0666"
+```
+
+**/etc/udev/rules.d/52-tmk-keyboard.rules:**
+```
+# tmk keyboard products https://github.com/tmk/tmk_keyboard
+SUBSYSTEMS=="usb", ATTRS{idVendor}=="feed", MODE:="0666"
+```
+
+
+
+## Cortex: cstddef: No such file or directory
+GCC 4.8 of Ubuntu 14.04 had this problem and had to update to 4.9 with this PPA.
+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/
+
+
+## '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
+```
+
+
+## BOOTLOADER_SIZE for AVR
+Note that Teensy2.0++ bootloader size is 2048byte. Some Makefiles may have wrong comment.
+
+```
+# Boot Section Size in *bytes*
+# Teensy halfKay 512
+# Teensy++ halfKay 2048
+# Atmel DFU loader 4096 (TMK Alt Controller)
+# LUFA bootloader 4096
+# USBaspLoader 2048
+OPT_DEFS += -DBOOTLOADER_SIZE=2048
+```
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