I2C keypad How To - ska-la/kPad_i2c GitHub Wiki

HARDWARE DETAILS.

What we have on a board? On the schematic diagram (Pic 1) it's included into a purple rectangle. It's no exactly same what we have, but main features are present here. Pins 9(En), 10(R/W), 11(Rs) and 12 (PCF8574 chip) are connected to keypad's inputs (look at plain keypad explanation). Pins 4-7 (D4-D7) are connected to a keypad's outputs and pulled up (to +5V) with 1 kOhm resistors (4 pieces) each.

Pic 1

Pins 4-7 of PCF8574 correspond to a LCD I2C interface board's pins into a green rectangle (Pic 2). And pins 9-12 correspond to pins into a domain with red border. Red and green rectangles on Pic 2 are corresponding to the same colors rectangles on Pic 1.

Pic 2

CAUTION!

• Don't remove a cap into a purple oval (Pic 2). It's responsible for the signal on board's pin 16 is correct.
• Don't forget to change i2c address the board if you have device (like LCD) with the same I2C/TWI adapter. Simply solder together one of the address pairs (A0-A2 into a domain of I2C address selector Pic 2).

SOFTWARE DETAILS.

File kPad_i2c.h has such definition:

#define BOARD_MASK 0xF7

Why? Look at pictures 3 and 4. Colors green, emerald and yellow denote the same things on both pictures: transistor, two-pin connector with a cap and 16th pin of the I2C interface board. 16th pin of the board provides a signal from a point into a yellow ring that's between transistor T1 and resistor R1. As result, we have an inverted signal value of 12th pin of PCF8574 chip on 16th board's pin.

Pic 3	Pic 4

Board's pins 4,5,6 and 16 form a low nibble of a control byte which we use to select a keypad row. And pins 11,12,13 and 14 form a high nibble which we use to detect which key is pressed. If you will use an own realization of i2c (on bare "remote I/O extender" like PCF8574) for a keypad without a I2C interface adapter, you must change BOARD_MASK definition:

#define BOARD_MASK 0xFF

Test an included example kPad_i2c.ino .