From 3c446334e9ab2761af3c76855250183585b61aed Mon Sep 17 00:00:00 2001
From: tmk <tmk@users.noreply.github.com>
Date: Thu, 19 Jun 2014 21:12:04 -0700
Subject: Updated FAQ (markdown)

---
 FAQ.md | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

(limited to 'FAQ.md')

diff --git a/FAQ.md b/FAQ.md
index 9bb74c7276..a411c7c6f1 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -178,4 +178,18 @@ Pressing any key during sleep should wake host.
 - http://arduino.cc/en/uploads/Main/arduino-leonardo-schematic_3b.pdf
 - 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.
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+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.
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