1. Peripherals – Keypad
The Keypad provides a simple means of numerical data or control input.
The keys can be attributed whatever data or control values are needed in a particular application (e.g. setting clock values, entering an alarm code, etc.).
The Keypad has 12 buttons arranged in a
4 Row * 3 Column Matrix:
C0 C1 C2 R R R
R3 * 0 #
Interface connections
Port bit:
Keypad signal: N/C R0 R1 R2 R3 C0 C1 C2
Connector
pin 0
Column
Row 1 2 3 1
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

2. Peripherals – Keypad (continued)
Operation:
When a key is pressed, the corresponding row and column signals are connected (according to the matrix).
Reading (scanning) the keypad requires:
Set the port so that row pins are outputs and column pins are inputs.
Use ‘pullup resistors’ to make each column read as a ‘1’ when no key is
pressed (i.e. set the pullup resistors ‘on’ on the column bits of the port).
(Without the ‘pullup resistors’ the signal lines ‘float’ between 0 and 5 volts
and thus can read as either a ‘1’ or ‘0’, i.e. they are unpredictable).
For one row at a time, set the output bit to ‘0’, while setting the other row
bits to ‘1’.
Read all column inputs and check to see if there are any with a ‘0’ value.
This indicates that the key where the column crosses the current ‘0’ value
row (in step 3) is pressed (otherwise they are pulled high and read as ‘1’).
Cycle through steps 3 and 4 for all rows.
After a key-press is detected, wait a short time to allow for mechanical
bouncing of the key contacts before reading the keypad again (this is
called the ‘debounce delay’ – typically between 20 and 100 milliseconds). 2
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

3. indicate if a ‘special’ key
Peripherals – Keypad – A simple demonstration program
A simple keyboard demonstration program is available on the course website.
The program shows how to set the port for mixed input and output, how to set the pullup resistors, and how to do the row / column scanning.
The connection of the keypad, and the output when the ‘5’ key is pressed is shown below.
These bits
show which
key was
pressed
indicate if a ‘special’ key
was pressed
(‘*’ ‘0’ ‘#’) 3
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

4. indicate if a ‘special’ key
Peripherals – Keypad – Simple demonstration program (continued)
The output when the ‘0’ key is pressed is shown below.
(The keys * 0 # are implemented as special keys for illustration of the concept of control keys which have specific handler routines).
The ‘special key’ indicator for ‘0’ is illuminated, as well as the key-code value (1011 binary = B hex = 11 decimal).
These bits
indicate if a ‘special’ key
was pressed
(‘*’ ‘0’ ‘#’)
show which
key was
pressed 4
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

5. Peripherals – Liquid Crystal Display (LCD)
The LCD provides a versatile means of displaying alphanumeric data and messages (such as error codes, status messages, and user instructions).
The LCD can display 2 rows of 16 characters.
The cursor can be turned on / off and positioned wherever required.
The system clock must be set to 1MHz (set via ‘fuses’) because of timing constraints of the LCD.
Fit the LCD via the dedicated connector on the edge of the STK300 board and make sure it is the same way around as shown.
LCD display panel 5
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

6. Peripherals – Liquid Crystal Display (LCD)
The LCD is a quite complex peripheral, with its own on-board controller chip
(With registers, memory, special command modes etc. Retains data in between data / control inputs).
In addition to 8-bit data (Port A, output);
there are also various control signals that must be set:
‘Register Select’ Port C bit 6, Low to access command register
High to access data register
‘Device Enable’ Port C bit 7, Low to disable LCD, High to enable LCD
‘Write’ Port G bit 0, for write (command or data)
‘Read’ Port G bit 1, for read (command or data)
‘Busy Flag’ Port A bit 7, (from the control register, not data)
To access, this bit must be set for input in DDRA
and Register Select must be ‘low’ 6
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

7. Peripherals – Liquid Crystal Display (LCD) - Continued
To simplify using the LCD, a library of commonly needed subroutines has been provided (see teachmat).
The subroutines provide functions such as:
Check if the LCD device is busy, if so wait.
Output a function command to the LCD.
Clear the LCD display and set the cursor to the 'home' position.
Set the cursor to a specific display position.
Write a data character to the LCD at the current cursor position.
Display a 2-bar bar graph on the LCD. 7
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

8. Peripherals – Liquid Crystal Display (LCD) - Continued
The LCD has a 256 symbol character set:
Place this code
in the upper 4 bits
in the lower 4 bits
For example
‘£’ has the
code:0xA5 8
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

9. Peripherals – Liquid Crystal Display (LCD) - Continued
The second half of the character set: 9
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich

10. Peripherals – LCD
A simple demonstration program has been provided, showing how to display a message (stored in program memory) on the LCD.
The use of the ‘include’ file, and the library subroutines with in it is illustrated.
The demonstration program also shows how to use special characters, in addition to standard ASCII alpha-numeric values. 10
Embedded Systems Programming II Richard Anthony, Computer Science, The University of Greenwich