STMPE1801 datasheet(36/60 Pages) STMICROELECTRONICS

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Keypad controller

STMPE1801

36/60

Doc ID 17884 Rev 3

Figure 7.

Keypad controller

The keypad rows enabled by the KPC_ROW register are normally 'high', with the

corresponding input pins pulled up by resistors internally. After reset, all the keypad columns

enabled by the KPC_COL register are driven 'low' via weak-pull down resistors. The pull-

down resistors on the column are weaker than the pull-up resistors on the rows. If a key is

pressed, the stronger pull-up drive on the corresponding row overwrites the weaker pull-

down drive on the selected column thus allowing the keyscan controller to sense a "high"

input on the selected column.

Once the keyscan controller senses a "high" on the selected column, the output buffer for

the selected column drives the line low overwriting the pull-up resistor on the corresponding

row. The row that senses the "low" signal enables the key scan controller to decode the key

coordinates (its corresponding row number and column number), save the key data into a

de-bounce buffer if available, confirm if it is a valid key press after de-bouncing, and update

the key data into output data FIFO if valid.

The key press/release detection mechanism is listed below:

When the GPIO is configured as keypad, the ROWS have internal "strong" pull-up and

COLUMNS have internal "weak" pull-down. The initial states of the ROWS are Logic

High and the COLUMNS are Logic Low.

When a keypad is pressed, the corresponding Row and Column form a Resistor

Voltage Divider Network. Since the pull-up resistance of the ROW is stronger than the

pull-down resistance of the COLUMN, the COLUMN is pulled to Logic High.

Once the COLUMN's state changes to Logic High, the state machine initiates a key-

scan cycle and drives the selected COLUMN to Logic Low. A low is detected on the

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