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+title = "Matrix scanning"
+author = "Yann Herklotz"
+tags = []
+categories = []
+backlinks = ["5a1"]
+forwardlinks = ["5a1b"]
+zettelid = "5a1a"
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+
+The main idea of how the keyboard circuit works together with the
+embedded processor, is that instead of requiring an IO pin for each of
+the keys on the keyboard, it instead really only needs one IO pin for
+all the keys in the matrix. The keys are placed in a matrix circuit such
+that when a key is pressed, the columns of the matrix can be scanned
+sequentially to figure out where the key is.
+
+This can be done, for example, by connecting all the keys, which are
+also switches, into on column with the other keys, and then also
+connecting the keys in the same row together. To then identify if a key
+has been pressed, the micro-controller has to scan all the rows and
+columns. This is done by setting all the columns to high sequentially,
+and then scanning which rows receive that signal. This means that one
+can then get a matrix representation with a bit set for each key that
+has been set.
+
+However, there are situations when keys can be ghosted, where pressing
+three keys connects all the lines in the matrix, which means that it
+then is not possible to distinguish a key from the others. The larger
+the matrix, the more often keys can be ghosted. To solve this issue, we
+therefore have to stop current from flowing backwards in the matrix,
+which is the original cause of the ghosted keys. This can be done by
+adding diodes after each key, before the key connects to the row. This
+means that ghosted keys are therefore not possible, as the only way to
+go from one row to the other is if the switch in the current row is set.