The keypad matrix is located on a UI module PCB,intefacing is acomplished by the board to board connector X101. The power-on key is also connected to the PSCLD to switch power on. Due to the internal pull up inside PSCLD, N300 to a higher battery voltage, a rectifier, V418 on the UI board is required in the keypad matrix for the power on keypad to prevent the higher voltage to interfere with the keypad matrix.
Series resistors, R261-R264 are implemented in the Column output to reduce the EMI radiation to the UI PCB. Capacitors C257-C260 reduces the EMC radiation and absorbs any ESD produced over an air gap to the keymat.
As the serial display driver interface uses ROW5 for data transmission series resistors are needed to prevent keypad or double keypad pressing from interfering with the display communication. In a similar way R265-R269 in the ROW lines reduces the EMI to the UI board. Capacitors C251-C256 implements a LP-filter together with each resistor in the ROW line. The capacitors also absorbs ESD pulses over an air gap to the keymat.
During idle mode when no keyboard activity is present the MCU sets the column outputs to "0" and enables the keyboard interrupt. An interrupt is generated when a ROW input is pulled low. Each ROW input on the ASIC, D151 has an internal pull-up. The keyboard interrupt starts up the MCU, which begins the scanning procedure. As there are keypads to be detected outside the matrix the MCU sets all columns to "1" and reads the ROW inputs. If a logic "0" is read on any ROW this means that one of the 6 possible non matrix keypads has been pressed. If the result was a "1" on each ROW the MCU writes a "0" on each column consecutively while the rest of the column outputs are kept in tri-state to allow dual keypad activation to be detected.
After that the keyboard scanning is completed and no activity is found the MCU writes "0" to all columns, enables the keyboard interrupt and enters sleep mode where the clock to the MCU is stopped. A key press will again wake up the MCU.
Technical Documentation System Module
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