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Pull‐Up Resistor
https://github.com/tmk/tmk_keyboard/wiki/FAQ#pull-up-resistor
Keyboard converter needs pull up resistor on signal lines in most cases. Without the pull-up resistors it may not work correctly.
Place the resistor between VCC and signal line like diagram below. In general 1K-4.7K Ohm is recommended for keyboard converters. Consult with instructions of target converter.
You can use 1/10W or higher power rating of resistor. See this for resistor power rating.
Keyboard MCU(AVR)
-----. ,-----.
5V|------------------------------+--+----|VCC |
| | | | |
| R R | |
| | | | |
DATA|------------------------------|--+----|PD0 |
CLOCK|------------------------------+-------|PD1 |
| | |
GND|--------------------------------------|GND |
-----' `-----'
R: 1K Ohm resistor
https://geekhack.org/index.php?topic=103648.msg2840078#msg2840078
You must have an external pull-up resistor between VCC(5V) and DATA(PD0) line. 1k ohm is strongly recommended.(470 ohm is more legitimate? see this.)
4.7k-10k won't work in some cases like longer curled cable or daisy-chained multiple devices. See Troubles below.
Keyboard MCU(AVR)
-----. ,-----.
5V|---------------------------------+----|VCC |
| | | |
| R | |
| | | |
DATA|---------------------------------+----|PD0 |
| | |
GND|--------------------------------------|GND |
-----' `-----'
R: 1K Ohm resistor
https://geekhack.org/index.php?topic=14290.0
https://github.com/tmk/tmk_keyboard/wiki/Apple-Desktop-Bus#pull-up-resistor
People have had many troubles without pull-up.
- 4.7k didn't work with daisy-chained multiple devices: https://github.com/tmk/tmk_keyboard/issues/683#issuecomment-850190574
- Some mouses required 4.7k ohm and didn't work with 10k pull-up. https://geekhack.org/index.php?topic=14290.msg3117619#msg3117619
- 10k Ohm didn't work for Turbo Mouse 5 with vintage coiled ADB cable, which is expected to have quite a bit capacitance. 4.7k did work. Note that 4.7k won't work when multiple devices are connected. See above.
- Low value resistor like 100 Ohm doesn't work also: https://geekhack.org/index.php?topic=14290.msg2903590#msg2903590
- Rise time: http://geekhack.org/index.php?topic=14290.msg1070139#msg1070139
- Signal form: http://geekhack.org/index.php?topic=14290.msg296052#msg296052