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@@ -1,13 +1,8 @@
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----
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-title: Keyboard Scanning
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-author: Pat Beirne <patb@pbeirne.com>
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-date: Dec, 2024
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-version: 1.0
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----
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+# Keypad Scanning
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There's a dozen ways to connect a keyboard to a microcontroller, and this paper attempts to enumerate many of them.
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There are similar articles on the internet [[see below](#ext_ref)],
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-but this paper introduces some new techniques: [row-grounded](#3x4gnd),
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+but this paper introduces some new techniques: [row-grounded](#3x4-plus-ground-[new-design]),
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[row-grounded-with-diodes](#3x4diodes),
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[row-grounded-with-4-diodes](#3x4double_diodes)
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@@ -27,7 +22,8 @@ Sometimes, this is appropriate if the keypad is small or in an awkward location,
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In this document, *n* refers to the number of keys that are in the keyboard. The letter *k* refers to a specific key
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that is being pressed during an explanation.
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-## One per Pin <a name="one_per_pin" />
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+<a name="one_per_pin" />
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+## One per Pin
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The simplest way to connect keys to a microcontroller is one key to one pin.
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Connect the other side of the key to ground.
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@@ -60,7 +56,8 @@ microcontrollers have an A/D converter, and that can be used to determine which
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A naive approach would be to simply use equal-valued resistors, and then each tap would be *k*/(*n*+1) of the supply voltage.
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-### 3x4 Chain <a name="3x4chain" />
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+ <a name="3x4chain" />
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+### 3x4 Chain
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Sometimes a keyboard will be pre-wired as a grid, so you don't have access to the individual keys. You can still use
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the resistor ladder to determine the key-press.
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@@ -164,7 +161,8 @@ The resulting 4 groups of 3 bits indicate the state of the keyboard; 1=open, 0=k
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Again, the same logic will allow you to manage a 4x4 keyboard with 7 GPIO pins.
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-### 3x4 plus Ground and 2 Diodes [new design] <a name="3x4diodes" />
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+<a name="3x4diodes" />
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+### 3x4 plus Ground and 2 Diodes [new design]
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Again, we can scrape off one more GPIO if we can add a pair of diodes to the row pins.
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@@ -217,7 +215,8 @@ look for the symbol **FIVE_PIN**.
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This configuration is also interrupt-capable, by setting the row drivers to output-low, and the columns as
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input+pullup+interrupt.
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-### 3x4 plus Ground and 4 Diodes [new design] <a name="3x4double_diodes" />
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+<a name="3x4double_diodes" />
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+### 3x4 plus Ground and 4 Diodes [new design]
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As an extreme, you can decode a 3x4 keypad with only 4 GPIO pins,
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using 4 external diodes.
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