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Keys and switches on an ADC => keypad
||AVR-Single-Chip-Processors AT90S, ATtiny, ATmega
of ATMEL in practical examples.
Connecting a keypad to an AVR
This page shows how to connect a common 12-key-keypad to an
AVR and read it by assembler software. The chapters are:
Keypads are switches, that are connected to rows and columns.
If key "1" is pressed, the column 1 is electrically connected
to row 1, if key "2" is pressed column 2 with row 1, and so on
- How a keypad works
- AVR: I/O-connection of the matrix
- AVR: Connecting to an ADC with a resistor network
out, if any one of the 12 keys is pressed, it would only be
necessary to tie the three column lines to ground,
connect all four row lines and, via a resistor of
10 kΩ, pull them to the operating voltage. If no
key is pressed, the row lines are at plus. Any pressed key
pulls down the row lines to zero volts.
To detect, which one of the 12 keys is pressed, pull down the
three column lines one by one to ground (the other two column
lines to plus) and read the resulting four row-lines. If one
of the four row-lines is low, stop further reading and identify
the key code from the column and row info. Like that:
Reading such a keypad using digital logic components, you need at least:
Or a complete IC doing all that (probably you won't get such an IC at your
local electronic parts dealer). Or you better use a micro.
- an oscillator, a shift register and a start/stop gate for generating
the column signals,
- detection if one of the four row signals is zero,
- a recoder for conversion of the seven signals to the keycode.
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A keypad matrix can be connected directly to an I/O port of an AVR,
with no additional components.
The example shows a connection to the lower seven I/O pins of Port B.
Other ports can be used similiarly.
The port pins PB4..PB6 are defined as outputs, they provide the column
signals. The port pins PB0..PB3 are used to read in the row results.
Pull-up resistors on these inputs are enabled by software, external resistors
are not necessary.
The following software example first demonstrates initialization of the
ports. This software part has to be excecuted only once at the AVR's program
; Init keypad-I/O
.DEF rmp = R16 ; define a multipurpose register
; define ports
.EQU pKeyOut = PORTB ; Output and Pull-Up-Port
.EQU pKeyInp = PINB ; read keypad input
.EQU pKeyDdr = DDRB ; data direction register of the port
ldi rmp,0b01110000 ; data direction register column lines output
out pKeyDdr,rmp ; set direction register
ldi rmp,0b00001111 ; Pull-Up-Resistors to lower four port pins
out pKeyOut,rmp ; to output port
Check for any key pressed
The following routine detects if any one of the 12 keys is pressed. This
routine is called in intervals, e.g. in a delay loop or by use of a timer.
; Check any key pressed
ldi rmp,0b00001111 ; PB4..PB6=Null, pull-Up-resistors to input lines
out pKeyOut,rmp ; of port pins PB0..PB3
in rmp,pKeyInp ; read key results
ori rmp,0b11110000 ; mask all upper bits with a one
cpi rmp,0b11111111 ; all bits = One?
breq NoKey ; yes, no key is pressed
Identify the key pressed
Now the keypad is read out. One after the other the port bits PB6, PB5 and
PB4 are set to low, and PB0..PB3 are checked for zeros. The register
pair Z (ZH:ZL) points to a table with the key codes. When leaving the
routine, this pair points to the key code of the pressed key. By using the LPM
instruction, the key code is read from the table in the flash memory
to the register R0.
; Identify the key pressed
ldi ZH,HIGH(2*KeyTable) ; Z is pointer to key code table
; read column 1
ldi rmp,0b00111111 ; PB6 = 0
in rmp,pKeyInp ; read input line
ori rmp,0b11110000 ; mask upper bits
cpi rmp,0b11111111 ; a key in this column pressed?
brne KeyRowFound ; key found
adiw ZL,4 ; column not found, point Z one row down
; read column 2
ldi rmp,0b01011111 ; PB5 = 0
in rmp,pKeyInp ; read again input line
ori rmp,0b11110000 ; mask upper bits
cpi rmp,0b11111111 ; a key in this column?
brne KeyRowFound ; column found
adiw ZL,4 ; column not found, another four keys down
; read column 3
ldi rmp,0b01101111 ; PB4 = 0
in rmp,pKeyInp ; read last line
ori rmp,0b11110000 ; mask upper bits
cpi rmp,0b11111111 ; a key in this column?
breq NoKey ; unexpected: no key in this column pressed
KeyRowFound: ; column identified, now identify row
lsr rmp ; shift a logic 0 in left, bit 0 to carry
brcc KeyFound ; a zero rolled out, key is found
adiw ZL,1 ; point to next key code of that column
rjmp KeyRowFound ; repeat shift
KeyFound: ; pressed key is found
lpm ; read key code to R0
rjmp KeyProc ; countinue key processing
rjmp NoKeyPressed ; no key pressed
; Table for code conversion
.DB 0x0A,0x07,0x04,0x01 ; First column, keys *, 7, 4 und 1
.DB 0x00,0x08,0x05,0x02 ; second column, keys 0, 8, 5 und 2
.DB 0x0B,0x09,0x06,0x03 ; third column, keys #, 9, 6 und 3
The routines KeyProc and NoKeyPressed have to debounce the pressed
key. For example by counting a counter up whenever the same key is
identified. Repeat this for e.g. 50 milliseconds. The NoKeyPressed
routine clears the counter and the key pressed. Because the timing
depends on other necessary timing requirements of the AVR program,
it is not shown here.
The routines shown above do not leave more time between the setting of
the column address and the reading of the row information. At high
clock frequencies and/or longer connections between keypad and
processor it is necessary to leave more time between write and read
(e.g. by inserting NOP instructions).
The internal pull-ups resistors have values around 50 kΩ.
Long lines or a noisy environment might interfere and produce
glitches. If you like it less sensitive, add four external pull-ups.
A disadvantage of the circuit is that it requires seven port lines
exclusively. The modification with an AD converter and a resistor
network (see chapter 3) is more economic and saves port lines.
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Most of the Tiny and Mega-AVR devices nowadays have an AD converter on
board. Without additional external hardware these are capable of measuring
analog voltages and resolve these with 10 bit resolution. Those who want
to save I/O ports only have to get the keypad to produce an analog
voltage. That's the task for a resistor matrix.
Hint: An improved version of this text with more examples, a commandline
software tool etc. is available here!
A graphical software tool for studying different scheme versions and
sizes can be found here.
such a resistor matrix. The columns are connected to ground, in between
the column connections are three stacked resistors. The rows are
connected via four such stacked resistors to the operating voltage
(e.g. 5 V). The AD converter input is blocked by a condensator of
1 nF because the ADC doesn't like high frequencies, that could
be caught by the keys, the resistors and the more or less long lines
in between all this.
If the key "5" is pressed, a voltage divider gets active:
* 1 k + 820 Ω = 1,82k to ground,
* 3,3 k + 680 Ω + 180 Ω = 4,16k to plus.
At an operating voltage of 5 Volt a divided voltage of
5 * 1,82 / (1,82 + 4,16) = 1,522 Volt
is seen on the AD converter input. If we consider 5% tolerance of the resistors,
the resulting voltage is somewhere between 1,468 and 1,627 Volts.
The 10-bit AD converter converts this (at 5 V reference voltage)
to a value between 300 and 333. If we ignore the lower two bits of the
result (e.g. divide the AD result by four or left-adjusting the result -
if the ADC provides that function) this yields an 8-bit-value between
74 and 78.
Each key pressed produces a typical voltage range, to be converted to
the key code.
Key encoding table
The matrix uses resistors of the E12 row, which are available at every
electronic store. That is why the resistor values (e.g. 820 or 3300)
are a little bit funny numbers.
The different resistors are necessary to achieve a smooth curve of analog
voltages from those 12 different keys. If you think that it is more practical
to just use one value for all, try to calculate the resulting voltages.
You will soon learn that all these resistors have to have different values.
The reason for that is in part that they add up in the two rows, another
reason is the voltage divider formula, which is not linear. If you are
able to solve that in an optimal way with a few formulas, try that. I gave
up very soon.
The following is a table for the above displayed matrix that calculates
the resulting resistors for each key pressed. Of course, if you assume
that two keys can be pressed at once, you have to move to a PC keyboard
instead. But not many people press two keys at once.
The second row of the table displays the resistor values towards ground,
the third towards the operating voltage. The voltage that results from
that voltage dividers is calculated with the formula
U = Uref * RGND / (RGND + RVCC)
Because available resistors have a tolerance of +/-5% the minimum and
maximum has to be calculated. To calculate the minimum, all resistors
to ground are assumed to have -5%, all towards VCC are assumed to have
+5%. That is not very realistic, but we do not want to hand-select
resistors and test them in summer and winter.
The voltages that are associated with the keys, and their variation through
tolerance is displayed in the following graph.
|Resistor divider||Voltages||ADC values|
As can be seen, the tolerance influences only the middle keys.
In the table, the values N are added that would result from an AD conversion
with 8-Bit. This resolution is fine as all the values have enough distance
from the previous and the next value.
Voltages and key recognition
As can be seen from the table, there is no overlapping of the different
detection values for the keys, taking 5% tolerance of the resistors into
Those who like to play around with other resistor combinations, can
download the calculation sheet
Hints for the AD converter hardware
ATtiny devices in the most cases provide only the opportunity to use
an internally generated voltage or the supply voltage of the AVR
as reference for the AD converter. For the keypad conversion only
the supply voltage is suitable as reference. This option has to be
selected when initiating the AD converter hardware at program start.
Many ATmega types can connect the reference voltage to an external
pin, AREF. This pin can either be input or output. It is an output if
either the supply voltage or the internal reference are selected as
AD converter reference. In this case the AREF pin should have a
condensator to ground to further reduce noise on the reference voltage.
The AREF pin is an input, if an external reference source is selected
as option. In this case an external source provides the reference
voltage. If an external source provides the reference voltage, the
keypad matrix should also be supplied by this source. Note in that
case that the keypad consumes up to 10 mA, to improve noise
ATmega devices allow to supply the AD converter from an extra pin
(AVCC) to further reduce noise. If only the keypad uses AD conversion
the low necessary resolution of 8 bits does not require a separate
supply for the AVCC pin, it can be tied to the normal supply voltage.
If other measuring tasks have to be performed on other channels, the
AVCC pin should be connected to the supply voltage via a coil of
22 µH and should be blocked by a condensator of 100 nF
Initiation and reading the AD converter result
For reading the keypad matrix voltage one AD converter channel is
required. The AD converter is initiated once during program start.
The two example codes show a start sequence for single conversion,
here for an ATmega8, and one for an interrupt driven automatic start
of the ADC, here for an ATtiny13.
ATmega8: manual start of the ADC
The first example shows a routine for an ATmega8, without interrupts,
with a manual start and stop of the the AD converter. The keypad
signal is connected to AD channel ADC0.
Please note that this single conversion requires 25 * 128 clock
cycles, at 1 Mcs/s clock 3.2 milli-seconds. Only do this that way,
running around in circles, if you don't need to care for other things in
between this time delay (except if these other things are done inside interrupts).
.DEF rKey = R15 ; Register for AD value
.DEF rmp = R16 ; Multi purpose register
; set MUX to channel 0, left adjust the result, AREF taken from AVCC
ldi rmp,(1<<REFS0)|(1<<ADLAR) ; ADMUX channel 0, AREF from AVCC
; switch AD conversion on, start conversion, divider rate = 128
; wait until the AD conversion is complete
; check ADSC bit, conversion complete if bit is zero
sbic ADCSRA,ADSC ; conversion ready?
rjmp WaitAdc1 ; not yet
; read MSB of the AD conversion result
; switch AD converter off
ldi rmp,(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0) ; switch ADC off
ATtiny13: Autostart AD conversion, interrupt-driven
Yes, an ATtiny13 with its 8 pins can read our keypad matrix (we cannot
connect the keypad matrix itself due to the limited number of I/O pins).
A typical routine for this task would be the following sequence, that
converts the voltage on ADC3 (pin 2 of the ATtiny13) continously (after
conversion complete, the next conversion is started automatically).
Running in interrupt mode requires defining the respective int jump vector,
; Start AD converter
; PB3=ADC3 is used for the conversion
ldi rmp,0b00001000 ; disconnect the PB3 digital driver, saves supply current
; Reference = supply voltage, Left-adjust the result
; ADMUX to ADC3
ldi rmp,0b00100011 ; reference voltage = supply voltage, chose ADC3
; select autostart option
ldi rmp,0b00000000 ; free-running conversion (autostart)
; start ADC, allow interrupt, select clock divider
ldi rmp,0b11101111 ; start ADC, autostart,
out ADCSRA,rmp ; Int Enable, clock divider to 128
; initiation complete
Of course, the stack must be initiated to use interrupts, and the
interrupt status flag must be set (SEI).
; Reset and int vectors, ATtiny13
.CSEG ; assemble into code segment
.ORG $0000 ; at the beginning of the code segment
rjmp main ; Reset vector
reti ; Int0 interrupt vector
reti ; PCINT0 vector
reti ; TC0 overflow vector
reti ; Eeprom ready vector
reti ; Analog comparator int vector
reti ; TC0 CompA vector
reti ; TC0 CompB vector
reti ; WDT vector
rjmp intadc ; ADC conversion complete vector
The service routine intadc reads the AD conversion result. Because
Left-Adjust has been selected, it is sufficient to read the MSB of
The register rKey continously provides the current value of the
; Interrupt Service Routine AD conversion
.DEF rKey = R15 ; result register for the conversion result
in rKey,ADCH ; read AD converter MSB
reti ; return from interrupt
Converting the AD result to the key code
The conversion result is, as such, not very useful. The voltages and
the conversion result do not follow easy mathematical laws (the
resistor values 4.7 - 5.6 - 6.8 - 8.2 must have been designed by a
drunken math professor, the formula
V = R1 / (R1 + R2) is not very easy
to handle), so that we better
use a table to resolve our key codes. The table cannot be a primitive
look-up table, because we have 256 different possible results of the
conversion, and we like more slim tables.
Like a monkey, we climb the matrix tree by going step by step through
the following table:
The first byte is the compare value for our conversion result, the
second byte is the key code, if this compare value is greater than our
result. If the result is between 0 and <7: no key is pressed (key
code is 255), if it is between 7 and <18 the key code is 1, etc.
.DB 7, 255, 18, 1, 28, 2, 42, 3, 64, 4, 91, 5
.DB 121, 6, 156, 7, 185, 8, 207, 9, 225, 10, 237, 0, 255, 11
Or, if you prefer ASCII for the key codes:
The code for key translation goes like this:
.DB 7, 0 , 18, '1', 28, '2', 42, '3', 64, '4', 91, '5'
.DB 121, '6', 156, '7', 185, '8', 207, '9', 225, '*', 237, '0', 255, '#'
Of course we have to check, if no key is pressed (R0 = 0xFF resp. if ASCII:
R0 = 0) and we have to check for glitches (if the same key code comes
20 or more times, I take it for serious ...).
; Converting a AD result to a key code
; if the AD result can change in between, the result must be copied first
mov R1,rKey ; copy AD result to R1
ldi ZH,HIGH(2*KeyTable) ; Z points to conversion table
lpm ; read one table value
cp R1,R0 ; compare AD result with table value
brcs GetKeyCode2 ; less than table value, key identified
inc R0 ; test, if table end is reached
breq GetKeyCode2 ; reached end of table
adiw ZL,2 ; point to next table entry
rjmp GetKeyCode1 ; go on comparing next entry
adiw ZL,1 ; point to MSB = key code
lpm ; read key code to R0
The hard- and software work very reliable. In the first version the
resistor values of the matrix were ten times higher. This version was
more vulnerable to HF noise, e.g. when transmitting with a 2 W
VHF transmitter nearby.
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