The keypad on the Smart Remote 3 DK add-on board (PCA63519) board has 39 buttons.
The matrix has six rows and eight columns that gives room for 48 buttons in firmware, of which 39 are used by the keypad, and one row is used for two push buttons that function as the left and right mouse buttons. A total of 41 of 48 locations are in use.
Figure 1. Keypad matrix
The matrix is connected to an I/O expander that is controlled by the nRF device using I2C. See Keypad matrix I/O expander.
The I/O expander circuit is added on the DK add-on so that it strictly uses the I/O set available in the Arduino interface system. On the product example, which does not have an Arduino interface, the I/O expander circuit is not needed.
Figure 2. Keypad matrix I/O expander
