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@@ -178,4 +178,18 @@ Pressing any key during sleep should wake host.
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- http://arduino.cc/en/uploads/Main/arduino-leonardo-schematic_3b.pdf
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- http://arduino.cc/en/uploads/Main/arduino-micro-schematic.pdf
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-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.
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+
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+
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+## Using PF4-7 pins of USB AVR(ATMega32U* or AT90USB*)?
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+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.
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+
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+See this code.
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+```
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+ // JTAG disable for PORT F. write JTD bit twice within four cycles.
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+ MCUCR |= (1<<JTD);
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+ MCUCR |= (1<<JTD);
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+```
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+https://github.com/tmk/tmk_keyboard/blob/master/keyboard/hbkb/matrix.c#L67
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+
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+And read **26.5.1 MCU Control Register – MCUCR** of ATMega32U4 datasheet.
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tmk