Updated FAQ (markdown)

FAQ.md

@@ -179,3 +179,17 @@ Pressing any key during sleep should wake host.
 - http://arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf
 
 Arduino leonardo and micro have **ATMega32U4** and can be used for TMK, though Arduino bootloader may be a problem.
+
+
+## Using PF4-7 pins of USB AVR(ATMega32U* or AT90USB*)?
+You need to set JTD bit of MCUCR yourself to use PF4-7 as GPIO. Those pins are configured to serve JTAG function by default. If you are using Teensy this isn't needed. Teensy is shipped with JTAGEN fuse bit unprogrammed to disable the function.
+
+See this code.
+```
+    // JTAG disable for PORT F. write JTD bit twice within four cycles.
+    MCUCR |= (1<<JTD);
+    MCUCR |= (1<<JTD);
+```
+https://github.com/tmk/tmk_keyboard/blob/master/keyboard/hbkb/matrix.c#L67
+
+And read **26.5.1 MCU Control Register – MCUCR** of ATMega32U4 datasheet.