Keypad Interfacing
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
Keypad
• Typical keypads have 12 or 16
buttons
• A mechanical keypad simply
consists of a set of vertical wires
(one for each column) and a set of
horizontal wires (one for each row)
• When you press a button, you press
the row wire onto the column wire,
shorting the two together, thus
making an electrical contact
Microcomputer Architecture and Interfacing
Colorado School of Mines
There are 7 pins,
corresponding to the
3 columns and 4 rows
Professor William Hoff
2
Keypad
• To test if a key is
pressed, we simply
test to see if there
is a connection
between a row
and a column
• For example, if pin
2 is electrically
connected to pin
3, then we know
key “1” is pushed
Microcomputer Architecture and Interfacing
3
1
5
1
2
1
4
5
6
7
8
9
*
0
#
pin numbers
on keypad
2
7
6
4
Colorado School of Mines
Professor William Hoff
3
Approach
• Scanning
– We scan through the rows and columns (very quickly) to see if any key
is being pressed
• Issue: debouncing
– Mechanical switches “bounce” slightly before coming to rest; we have
to ignore the bounces
Switch output signal
+5V
switch
output
Microcomputer Architecture and Interfacing
5 to 20 ms
Colorado School of Mines
Professor William Hoff
4
•
•
•
•
•
Set up 3 pins
for output; 4
pins for input
Then scan thru
columns,
outputting a
low on each in
turn
For each
column, scan
thru rows
If no key is
pressed, input
is high;
otherwise is
low
If low, key code
(1..12) is
determined by
(row,col)
Scanning
PT6
Need
pullup
resistors any value is
ok if it
limits the
current to <
25 mA
PT5
PT4
1
2
1
10K
4
5
6
10K
7
8
9
10K
*
0
#
+5V
PT3
+5V
PT2
+5V
PT1
PT0
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
+5V
10K
5
Debouncing
VDD
Set
Q
R
• Debouncing can be
handled in hardware
or in software
• A software solution is
“wait and see”;
namely, if you detect
a key press, wait 10
ms and see if it is still
pressed
Q
Reset
Set
Reset
R
(a) Set-reset latch
VDD
R
4050
Vout
(b) CMOS gate debouncer
VDD
R
Vout
C
H
Threshold level
L
Switch closed
(c) Integrating RC circuit debouncer
Figure 7.42 Hardware debouncing techniques
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
6
Pseudo code
function getkey()
for each column (0,1,2)
output a low to that column’s pin
for each row (0,1,2,3)
read pin for that row
if pin is low
wait 10 ms and check again
if pin is low
return the code corresponding to
the key at (row, col)
end
end
end
end
return 0 to indicate no key is pressed
end
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
7
• It’s not a
good idea to
have a circuit
that can be
damaged if
you have a
bug in the
software
• For example,
what if you
accidentally
made PT3 an
output?
• Even if you
don’t have a
bug, what if
a person
presses two
keys at the
same time?
Safety
issue
PT6
PT5
PT4
1
2
1
10K
4
5
6
10K
7
8
9
10K
*
0
#
+5V
PT3
+5V
PT2
+5V
PT1
PT0
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
+5V
10K
8
Current limiting resistors
• Let’s add small current
limiting resistors on each
pin in case we make a
mistake in the software
and accidentally make a
pin an output; or a person
presses two keys
simultaneously
• The resistor values should
be high enough so that
current is less than 25 mA
Microcomputer Architecture and Interfacing
R1
PTx
R2
PTy
• (5V)/(R1+R2) < 25 mA
– or
• R1+R2 > 200 ohms
Colorado School of Mines
Professor William Hoff
9
PT6
PT5
• Will
R1,R2
affect
logic
levels?
R1
R1
R1
PT4
R2
1
2
1
R2
4
5
6
10K
R2
7
8
9
10K
R2
*
0
#
10K
+5V
PT3
+5V
PT2
+5V
PT1
PT0
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
+5V
10K
10
Logic Levels
• Standard CMOS logic levels are:
– Input range is 0V to 1.5V for logic low
– Input range is 3.5V to 5.0V for logic high
• So we need to make sure that our inputs are in this range
• We don’t want voltage drops across R1 and R2 to cause invalid
logic levels
• Consider resistor R2
– It is on the input to the MCU
– The input is high impedance; very little current flows through R2
– So very little voltage drop across R2; its value can be large or small
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
11
Logic Levels (continued)
• R1 may affect the logic level
R1
PTx
• Consider what happens with
switch is closed and pin PTx is
outputting low (V=0)
• We have a voltage divider
with R1 and the 10K pullup
• We want the output voltage
to be a valid logic low (i.e.,
0V to 1.5 V)
10K
+5V
+5V
10K
Vout
• So
R1
(5V)(R1/(R1+10K)) < 1.5V
R1 < 4.3K
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
12
Consider physical layout
• The keypad can plug right
into the protoboard
You will
also need
the pullup
resistors
(10Kohm
DIP pack)
• Let’s plan on using a resistor
DIP pack (1.5 Kohm) to
connect the Port T pins on H1
to the keypad pins
• Instead of using PT0:PT3 for
input and PT4:PT6 for
output, we might want to
pick different pins to simplify
the wiring
1.5 Kohm
DIP pack
Header H1
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
13
• Use these
connections
to simplify
wiring
•
1.5K
pin numbers
on H1 header
19 20 21 2223 24 25
PT0
Microcomputer Architecture and Interfacing
Colorado School of Mines
PT6
Professor William Hoff
Pin
numbers on
header (H1)
and keypad
(KP)
Connections
H1 KP
PT0 19 1
PT1 20 2
PT2 21 3
PT3 22 4
PT4 23 5
PT5 24 6
PT6 25 7
14
PT4
PT0
PT2
23
1.5K
19
1.5K
21
1.5K
3
PT0
PT1
PT2
PT3
PT4
PT5
PT6
H1
19
20
21
22
23
24
25
KP
1
2
3
4
5
6
7
PT1
PT6
PT5
PT3
20
25
24
22
1.5K
1.5K
1.5K
1.5K
1
1
2
1
7
4
5
6
6
7
8
9
4
*
0
#
2
pin numbers
on keypad
5
2
10K
7
10K
6
10K
+5V
+5V
+5V
+5V
4
10K
pin numbers on H1 header
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
15
One more issue
• If the user presses and holds a key down, this must be
interpreted as a single keystroke
– Must not only look for key depressing but also for key releasing
– We can do this by keeping track of the last key pressed
• The function “getKey” can save the last key pressed in a static
variable called “lastKey”
– If the same key is being pressed, then don’t treat as a new key; just
return zero
– If a new key is being pressed, then return that keycode and save it in
“lastKey”
– If no key is being pressed, then save a zero in “lastKey”
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
16
C Code for function
• Function header and comments
// Get a keypress (if any) from the keypad.
// If a key is pressed, it returns its code, which is an integer
// from 1 (top left) to 12 (lower right).
// If no key is pressed, it returns 0.
// If the same key is still pressed from last time, return 0.
char getkey(void);
• We will also need these (global variables)
// Define the Port T bits corresponding to columns
char columnBits[] = {
0x04,
// column 0 (left) is pin PT2
0x01,
// column 1 (center) is pin PT0
0x10
// column 2 (right) is pin PT4
};
// Define the Port
char rowBits[] = {
0x02,
// row
0x40,
// row
0x20,
// row
0x08
// row
};
T bits corresponding to rows
0
1
2
3
(top) is
(2nd) is
(3rd) is
(bottom)
pin PT1
pin PT6
pin PT5
is pin PT3
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
17
char getkey(void)
{
char col, cbit, row, rbit, key;
static char lastKey = 0;
// Scan across columns 0,1,2
for (col=0; col<3; col++)
{
// Write a 0 to the bit corresponding to the column;
// write 1 to all other bits in Port T
cbit = columnBits[col];
PTT = ~cbit;
// Scan across rows 0,1,2,3
for (row=0; row<4; row++)
{
// Get the mask corresponding to this row
rbit = rowBits[row];
// Read Port T and check only that bit; a key is
// pressed if the bit is 0.
if (!(PTT & rbit)) {
// A key is pressed
DelayuSec(10000);
if (!(PTT & rbit)) { // check same key again
key = keycodes[row][col];
// get keycode
// If this is still the same key, return 0
if (key==lastKey)
return(0);
// It is a new key being pressed
lastKey = key;
return(key);
}
}
}
}
// We got to here, so no key is being pressed
lastKey = 0;
return(0);
}
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
Notes
• “Static” means
that the variable
retains its value
after the
function exits
• It is similar to a
global variable,
except that it
only visible
within this
function
•
•
keycodes[r][c] is
an array that
contains the
code for each
key, at row r and
column c
Could just be the
numbers 1..12,
or anything you
want
18
Summary / Questions
• A mechanical keypad is a very simple device, and thus low
cost and rugged. We make up for the simplicity using
software, to create a sophisticated input device.
• Keypads can also use capacitive switches or Hall-effect
switches (with similar issues in interfacing)
• If you only needed a few of the keys on the keypad, can you
design a simpler interface?
Microcomputer Architecture and Interfacing
Colorado School of Mines
Professor William Hoff
19