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I found many schematics out there for a 4x4 keyboard/switch matrix, both with and without diodes.

Yet I don't get it. If one doesn't add pull-down resistors, rows 1 to 4 will all be floating.

Can someone explain why pull down resistors they are not required in this case?

I plan to use an atmega32u4 microcontroller.

enter image description here

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    \$\begingroup\$ The direction flag on the column names should show inputs (point the other way), then it would probably be a little clearer. The rows need to be scanned one at a time to determine which switch is active in each row (set output, read each column, clear output). \$\endgroup\$ – Ron Beyer Apr 27 at 20:09
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    \$\begingroup\$ thx Ron, but why active low instead of active high ? any reason for this ? \$\endgroup\$ – user7082181 Apr 27 at 20:17
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    \$\begingroup\$ @user7082181 - Hi, Please don't be offended at polite suggestions. Your site profile doesn't indicate that you're not a native English speaker. The comment you replied to, linked to part of the site help center, in case you could follow those recommendations. Also there is a site Code of Conduct requiring everyone to be kind & respectful. Your comment was (understandably) flagged as unkind, so I have deleted it - as well as the comment you responded to, as it has served its purpose to inform you of that recommendation. So it's now a clean slate. \$\endgroup\$ – SamGibson Apr 27 at 21:03
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The way that you perform one scan of a keyboard like that goes like this:

  • Pull row 1 low, all other rows high. Read the column nibble.
  • Pull row 2 low, all other rows high. Read the column nibble.
  • Pull row 3 low, all other rows high. Read the column nibble.
  • Pull row 4 low, all other rows high. Read the column nibble.
  • Combine the four nibbles into 16 key presses (a low bit = key pressed).

The rows are pulled low by the hardware external to the keyboard (in this case, by the microcontroller's GPIO).

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    \$\begingroup\$ And use some method to debounce. :) \$\endgroup\$ – jonk Apr 27 at 20:28
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    \$\begingroup\$ WWWWhy wwwould I wwwwwant to do that? \$\endgroup\$ – TimWescott Apr 27 at 20:41
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    \$\begingroup\$ You might leave all four row drivers idling at logic "low". This allows the four column inputs to generate an interrupt-on-change when you hit any key. Then you proceed with Tim's scanning procedure, only allowing one-row-low at a time. \$\endgroup\$ – glen_geek Apr 27 at 21:39
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Because the rows are outputs, resistors are not needed.

In fact, since AVR microcontrollers have inputs with internal pull-ups, the resistors on the columns are not needed either.

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  • \$\begingroup\$ ok so we're talking open drains right ? \$\endgroup\$ – user7082181 Apr 27 at 20:19
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    \$\begingroup\$ No, I wasn't, but now that you mentioned it, it does not matter if it is an open drain output or push pull output. \$\endgroup\$ – Justme Apr 27 at 20:26
  • \$\begingroup\$ To be clear, the MCU row driver outputs can be push-pull because the keypad has key diodes. Without key diodes, you'd use open-drain drivers as multiple presses in a column will connect drivers together. \$\endgroup\$ – TonyM Apr 27 at 22:24

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