Embedded Systems Design with the Atmel AVR Microcontroller Part II

7.3. INPUT DEVICES 189
7.3.4 KEYPADS
A keypad is simply an extension of the simple switch configuration. A typical keypad configuration
and interface are shown in Figure 7.4. As you can see, the keypad is simply multiple switches in the
same package. A hexadecimal keypad is provided in the figure. A single row of keypad switches are
asserted by the microcontroller and then the host keypad port is immediately read. If a switch has
been depressed, the keypad pin corresponding to the column the switch is in will also be asserted.
The combination of a row and a column assertion can be decoded to determine which key has
been pressed as illustrated in the table. Keypad rows are continually asserted one after the other in
sequence. Since the keypad is a collection of switches, debounce techniques must also be employed.
The keypad may be used to introduce user requests to a microcontroller. A standard keypad
with alphanumeric characters may be used to provide alphanumeric values to the microcontroller
such as providing your personal identification number (PIN) for a financial transaction. However,
some keypads are equipped with removable switch covers such that any activity can be associated
with a key press.
In Figure 7.5, we have connected the ATmega164 to a hexadecimal keypad via PORTC.
PORTC[3:0] is configured as output to selectively assert each row. PORTC[7:4] is configured as
input. Each row is sequentially asserted low. Each column is then read via PORTC[7:4] to see if
any switch in that row has been depressed. If no switches have been depressed in the row, an “F”
will be read from PORTC[7:4]. If a switch has been depressed, some other value than “F” will be
read. The read value is then passed into a switch statement to determine the ASCII equivalent of
the depressed switch. The function is not exited until the switch is released. This prevents a switch
“double hit.”
//*************************************************************************
unsigned char get_keypad_value(void)
{
unsigned char PORTC_value, PORTC_value_masked;
unsigned char ascii_value;
DDRC = 0x0F;
//set PORTC[7:4] to input,
//PORTC[3:0] to output
//switch depressed in row 0?
PORTC = 0xFE; //assert row 0 via PORTC[0]
PORTC_value = PINC; //read PORTC
PORTC_value_masked = (PORTC_value & 0xf0); //mask PORTC[3:0]
//switch depressed in row 1?