View detail for Switch Matrix Module Design Using LIN SiPs ATA6612/13

38
Atmel Automotive Compilation, Vol. 6
Standard
Products
Networking
LIN
Switch Matrix Module Design Using
LIN SiPs ATA6612/13
Markus Schmid
Introduction
ing years can be explained by the
out so that the customers can use
continued requirement for comfort
the well-established AVR tools. It
The LIN bus is well established
functions in next-generation auto-
also provides the same flexibility for
in automotives where it is used for
motives. As almost all comfort func-
the applications as when using the
low-cost and low-speed applica-
tions can be controlled by the pas-
discrete parts.
tions as an alternative to the CAN
sengers via keypads, it is expected
bus. The implementation of the LIN
that the amount of the keypads will
bus is still increasing: in 2011 there
also increase. This article covers
are approximately 600 million LIN
efficient keypad evaluation using a
transceivers expected in all cars
switch matrix module.
The popularity of LIN devices can
The easiest way to evaluate a push
button is to connect it directly to an
worldwide, and in 2014 this number
is set to increase to 900 million.
Handling Keypads
LIN SiPs ATA6612
and ATA66613
I/O pin of a microcontroller. This is
the preferred solution, when only
a couple of push buttons need to
be evaluated by a single microcon-
in part be attributed to their flexibility: they can be used in many
The ATA6612/13 consists of two
troller. But with an increasing num-
different
Typical
chips in a single package; the first
ber of switches, the number of I/O
LIN applications either evaluate
one is the LIN system basis chip
pins increases accordingly. In many
the data of sensors or switches
(LIN SBC) ATA6624, which has
cases this leads to a need for larger
or drive actuators like motors or a
an integrated LIN transceiver, a
and
heating system. However, those LIN
5V voltage regulator, and a win-
microcontrollers. Using a switch
applications must not have high-
dow watchdog. The second chip
matrix the number of I/O pins
speed or even real-time require-
is an AVR
8-bit RISC automo-
required can be reduced. Using a
ments. Therefore, LIN devices can
tive microcontroller. The ATA6612
3 x 2 matrix up to six push but-
be found in window lifts, mirrors, or
consists of the LIN SBC ATA6624
tons can be monitored using only
keypads in car doors. They are also
and the ATmega88 with 8-kBytes
five I/O pins. With the conventional
used in seat heating or adjustment,
flash, while the ATA6613 includes
solution where a single push but-
in climate control, in the dashboard
the ATA6624 and the ATmega168
ton is connected to one I/O pin,
or for wiper control, where LIN is
with 16-kBytes flash. The only dif-
only five push buttons can be
currently the unchallenged stan-
ference between the two micro-
evaluated. With more push but-
dard.
controllers is the size of the Flash
tons, a switch matrix will lead to
applications.
®
therefore
more
expensive
memory. All pins of the LIN System
even greater savings in the number
the
Basis Chip as well as all pins of the
of I/O pins required. For example,
amount of LIN devices in the com-
AVR microcontroller are bonded
a 3 x 3 matrix can handle up to nine
The
expected
increase
in
www.atmel.com
PD7
VCC
VS
VCC
S1
S2
S3
S4
S5
S6
100 nF
S21
S16
S22
S17
S23
S18
S24
PB6
VDD2
PB7
PD5
PD6
PB0
P7D
PB1
PB2
PD3
GND4
LIN
ADC7
GND
ATA6612/13
PC0
NTRIG
PC2
EN
PC3
VS
PC4
VCC
PC5
PC6
S26
S27
S28
S29
100 nF
36
1 kΩ
220 pF
S30
LIN
WAKE
33 kΩ
10 kΩ
VS
PVCC
KL_15
24
S25
LIN
Master
pull-up
WAKE
PC1
MODE
S20
S15
PD4
AREF
TM
S19
S14
ADC6
WD_OSC
S13
GND1
NRES
100 nF
GND2
VDD1
AVDD
TXD
S12
INH
S11
RXD
S10
PD2
S9
PB4
PB5
PD1
S8
1
PD0
S7
100 nF
PB3
48
+
+
100 nF 10 µF
S31
S32
S33
S34
S35
S36
51 kΩ
10 kΩ
GND
100 nF 22 µF
10 kΩ
PD7
VBAT
100 nF
47 kΩ
KL15
Figure 1. Switch Matrix Application
push buttons with six I/O pins lead-
umns or rows. In general, pressing
tially, reading the column result for
ing to a saving of three I/O pins
a key connects the key’s row and
each row, thus getting all pressed
compared to the single push button
column lines. When pressing the
keys.
per I/O pin solution.
top left key, the left-most column
Switch Matrix Using
LIN SiPs ATA6612/13
line is connected to the top-most
The illustrated pull-up resistors in
row line. The microcontroller can
the light blue area are very impor-
detect this connection by scan-
tant for the scan algorithm to moni-
ning the columns and rows. When
tor the push buttons. These do
only single keys are pressed, a
not necessarily have to be external
Figure 1 illustrates an application
quick method is to first select
resistors; however, as internal resis-
that can be commonly seen in cli-
(drive low) all row lines and read
tors are very high ohmic and many
mate control panels. It shows a
the column result. Then all column
applications need to have a higher
switch matrix serving some addi-
lines are selected, and the row
current through a closed push
tional requirements.
result is read. Returned column
button; it is common to add more
and row values are combined into
low-ohmic, external resistors allow-
The keyboard matrix is orga-
a unique scan code for the specific
ing a higher current to flow. Using
nized in 6 x 6 push buttons (light
key pressed. When the detection
only the internal pull-up resistors
green area), which enables the
of simultaneous key presses is
of the AVR will lead to a current of
ATA6612/13 to handle up to 36
required, this method has to be
approximately 160 µA through the
push buttons. That is a very large
slightly changed. The rows must
pressed keys.
number of push buttons, but it can
be scanned separately. The row
be easily reduced by removing col-
lines must be driven low sequen-
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39
Atmel Automotive Compilation, Vol. 6
Almost all keypads in a contempo-
of the keypads themselves. Shown
the current consumption does not
rary car have the ability to switch on
in figure 1 as two variable resis-
increase since the INH pin will be
a backlight in order to illuminate the
tors in the gray area, which can be
switched off in this mode.
push buttons. This enables the pas-
evaluated using two of the available
sengers to find the desired key even
eight ADC channels. As the resis-
The other external circuitry in the
in a dark environment. The back-
tors are connected to the supply
dark green box in the schematic are
light illumination consists of several
voltage in this example, it is best to
blocking capacitors for the micro-
LEDs, whose brightness can be
also use the supply voltage as ref-
controller or, as the INH pin, relevant
varied when they are controlled by
erence voltage for the conversion.
for the features of the LIN part. The
a PWM pin of the microcontroller.
It is also possible to choose a fixed
resistor connected to the WD_OSC
In the schematic, the four LEDs in
internal 1.1V or an external refer-
pin, for example, is important as it
the yellow area are connected to
ence voltage between 1.0V and the
adjusts the watchdog frequency of
the PWM pins PB1, PB2, PD5 and
voltage connected to the AVCC pin
the internal window watchdog. The
PD6. In general, the ATA6612/13
of the microcontroller. In addition,
circuitry connected to the WAKE,
has up to six PWM pins.
pin PD7 serves as an input for an
KL_15, and the LIN pin is optional
analog-to digital conversion. It is
and can be adjusted to the needs
In addition to the backlight illumina-
connected via a voltage divider to
of the application.
tion in an automotive keypad, there
the INH pin. Therefore, the module’s
are very often rotary encoders to
battery voltage in standard opera-
be evaluated by the microcontroller
tion mode can be monitored very
which also controls the evaluation
efficiently. In current save mode
S5
S6
S4
S5
S6
48
S10
S11
S12
100 nF
1
PB4
PB5
48
AVDD
S10
S11
S12
100 nF
GND4
PB4
PB5
AVDD
AREF
100 nF
1
AREF
100 nF
GND4
PC0
PC0
PC1
PC1
PC2
PC2
Figure 2. Current Flow During Simultaneous Key Presses
PB3
S4
PB3
40
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Detection of
Simultaneous Key
Presses
Current Consumption
ing mode. The ATmega88/168 offers
five different current saving modes
Current consumption is, as in all
with different active components
electronic modules in cars, a key
and wake-up sources. To continue
Some additional hardware precau-
parameter for the switch matrix.
to scan the keys, the user should
tions must be taken when detec-
A current consumption limit of
choose a power-saving mode with
tion of simultaneous key presses is
100 µA in silent mode is the most
a cyclic wake-up. After recognizing
required. The schematic as shown
common requirement, which is met
a pressed key, the microcontroller
in figure 1 may lead to erroneous
by the ATA6612/13. The ATA6624
can then wake-up the ATA6624 as
results, which is explained in the
has two different current saving
well as the complete LIN network,
following.
modes. The first is the sleep mode,
and start the transmission of this
which shuts down the watchdog, the
event via the LIN bus after a master
If the keys S5, S11 and S12 are
LIN transceiver as well as the voltage
request. With this set-up, no event
pressed at the same time, the
regulator. Thus, the microcontroller
will be missed even in power-saving
microcontroller will determine that
remains currentless in sleep mode
mode.
key S6 is also pressed, although
and it is not possible to continue to
this is not the case. The current,
scan the keys. The ATA6612/13’s
which flows when the three keys
typical current consumption in sleep
S5, S11 and S12 are pressed,
mode is 10 µA. The second current
The ATA6612/13 is a perfect choice
is shown in figure 2 in red. The
saving mode is the silent mode. In
for applications where many keys
microcontroller can not distinguish
this mode the watchdog and the
have to be monitored and the key
whether the three pins are pressed
LIN transceiver are switched off,
press events have to be transmit-
or if the key S6 is pressed. In order
but the internal voltage regulator
ted via the LIN bus. Figure 1 illus-
to avoid this behavior an additional
remains active, thus allowing the
trates how such a module can be
diode has to be included at every
microcontroller to continue to scan
built. However, in many cases it is
key which stops the undesired cur-
the keys. The ATA6624’s typical
necessary to adapt to the special
rent flow and enables to achieve the
current consumption in this mode is
needs of a particular matrix. The
correct result. Using these diodes
57 µA. The ATA6624 can wake-up
ATmega88/168 included in the
prevents undesired result. Shottky
via a message on the LIN bus or
ATA6612/13 is a standard micro-
diodes are preferable as they have
via an event at the pins WAKE or
controller, therefore, the extensive
a lower forward voltage compared
KL_15. In order to achieve the
application notes related to the
to normal diodes.
100 µA limit, the microcontroller
ATmega88/168 can also be used for
must switch itself into a current sav-
the ATA6612/13.
Conclusion
Application notes Atmega88/168 see
http://www.atmel.com/dyn/products/product_card.asp?family_id=607&family_name=AVR%26reg%3B+8%2DBit+RISC+&part_
id=3757 and
http://www.atmel.com/dyn/products/product_card.asp?family_id=607&family_name=AVR%26reg%3B+8%2DBit+RISC+&part_
id=3758
www.atmel.com
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