Use `clock_prescale_set()` from avr-libc (#7540)

* Use `clock_prescale_set()` from avr-libc

* Remove outdated FAQ info

4 files changed, 2 insertions, 50 deletions

diff --git a/docs/faq_build.md b/docs/faq_build.md
index 023eab01fa..4c91cb28b1 100644
--- a/docs/faq_build.md
+++ b/docs/faq_build.md
@@ -111,21 +111,6 @@ 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.
 
diff --git a/docs/fr-fr/faq_build.md b/docs/fr-fr/faq_build.md
index 774bf6880f..6fba1e4304 100644
--- a/docs/fr-fr/faq_build.md
+++ b/docs/fr-fr/faq_build.md
@@ -112,21 +112,6 @@ 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
-
-Votre chaîne d'outils (Toolchain) est trop vieille pour supporter le MCU. Par exemple, WinAVR 20100110 ne supporte pas 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 pour AVR
 
 Notez que la taille du bootloader pour les Teensy2.0++ est de 2048bytes. Quelques Makefiles peuvent contenir une erreur et avoir le mauvais commentaire.
diff --git a/docs/zh-cn/faq_build.md b/docs/zh-cn/faq_build.md
index 8fb449db30..2f33fbca97 100644
--- a/docs/zh-cn/faq_build.md
+++ b/docs/zh-cn/faq_build.md
@@ -81,21 +81,6 @@ 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
-你的工具链太旧了不支持MCU。比如WinAVR 20100110就不支持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
-```
-
-
 ## AVR的BOOTLOADER_SIZE
 注意Teensy2.0++ bootloader的大小是2048字节。有些Makefile注释错了。
 
diff --git a/tmk_core/protocol/lufa/lufa.c b/tmk_core/protocol/lufa/lufa.c
index eb166c828e..8319b34e94 100644
--- a/tmk_core/protocol/lufa/lufa.c
+++ b/tmk_core/protocol/lufa/lufa.c
@@ -914,14 +914,11 @@ void virtser_send(const uint8_t byte) {
  */
 static void setup_mcu(void) {
     /* Disable watchdog if enabled by bootloader/fuses */
-    MCUSR &= ~(1 << WDRF);
+    MCUSR &= ~_BV(WDRF);
     wdt_disable();
 
     /* Disable clock division */
-    // clock_prescale_set(clock_div_1);
-
-    CLKPR = (1 << CLKPCE);
-    CLKPR = (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0);
+    clock_prescale_set(clock_div_1);
 }
 
 /** \brief Setup USB