LCD Keypad : How to use an lcd and keypad on one 8 bit microcontroller port.

This LCD keypad project saves you lots of valuable port pins by giving you twelve input keys and one LCD display on a single 8-bit port. Normally you need a single port pin for each input key but this method multiplexes the keys onto 7 pins and shares the port with the LCD.

Bonus 2:

Here are the download links.(One for the schematic files and one for the schematic tool).

Schematic project files click to download.

ProSchematic tool click to download.

The LCD is a standard HD44780 type and the keypad is a 12 key keypad arranged as four rows by three columns.

It works if you keep the LCD strobe line (E) low while controling the keypad. This stops the LCD from reacting to the other port pins (the strobe line when strobed high starts a read or write action).

Since you only need seven port lines to access the keypad the eighth is used for the LCD strobe line (RB2).

Note: If you use LVP and the 16F877A (PGM on RB3) then you can still use this system - the LCD is fully controllable but you lose a column of keys - but you still have 8 useable keys left - (note untested).

LCD Keypad project details.

LCD Keypad Project
Compiler	Mikroelectronika C compiler V5.0.0.3 Free!
Target	16F877A (retargetable to other PICs that have an 8 bit port!).
Software level	Easy.
Software notes	Switching between keypad and LCD.
Hardware level	Easy.
Hardware notes	Use internal pullups on portB (you must add pullups to row inputs when using a different port).
Project version 1.00	Initial Release
Project version 1.01	Mod to operate fully at 20Mhz (0,# lost). Allow settling time after set col outputs.
Project files	Download here

You can program the PIC in circuit through the ICSP connector.

LCD Keypad Port Directions

When using the LCD you need to set all LCD pins as outputs and for the keypad you need to set rows as inputs and columns as outputs.

To make it work all row inputs must be pulled up to 5V (either by internal port pullups or by external 10k resistors).

Note : During keypad access the LCD strobe signal (E) must be kept at zero so that the LCD is not activated.

How the lcd keypad works

Reading the keypad

The software generates a walking 0 through the columns. Whenever you press a key the keypad shorts column to the row so the zero from the column output appears at the row input pulling the voltage to ground. When there are no keys pressed the pullup resistors pull the row inputs to 5V.

The routine exits with the first key detected. This key is derived from the returned row value and the current mask value and you can decode it using the definitions found in lcd_keypad.h e.g. KEY_2 has the value 226 which is the value returned by read_KEYPAD when key 2 is hit.

Column shorts

Pressing two keys in different columns and rows shorts together two columns that are at different voltages (one at 0V and one at 5V). The column drivers are isolated by the three 10k resistors to prevent damage. They also allow the LCD to operate correctly even when keys are pressed.

Kepad wiring

The following diagram shows the keypad wiring which is a little awkward - probably to allow a simple pcb routing path.

Software operation

The software consists of three files:

16F887A_lcd_keypad.c
lcd_keypad.c
lcd_keypad.h

The most important routines are KEYPAD_access() and LCD_access() - which set the port up to let it control the keypad and lcd respectively. The interesting point about the code is that the standard LCD routines (provided by compiler manufacturer) are used after lcd_access() is executed.

Pressing a key displays the result on the lower right of the 2 row LCD.

Note that key debouncing has not been done as it does not affect the demonstration but you must use it for a 'real' application.

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