AN2600 Freescale Semiconductor / Motorola, AN2600 Datasheet - Page 3

• Manufacturer Part Number: AN2600
• Description: A Simple Keypad Using LIN with the MC68HC908QT/QY MCU
• Manufacturer: Freescale Semiconductor / Motorola

Keypad Construction
Hardware Concept
MOTOROLA
A Simple Keypad Using LIN with the MC68HC908QT/QY MCU
The described implementation of the keypad is as a LIN slave, hence it cannot
initiate communication with the other nodes of the network, because this is the
responsibility of the LIN master. The master periodically requests the frame
from the slave by sending a master task. When this master task with the
defined identifier (ID) is received on the slave, the slave sends back a slave
task, in which data describing the status of the keypad is attached.
In the other direction of communication, the master is able to send both master
and slave tasks. Using this approach, it can send information to slaves. This
message is used to control the keypad LED.
Information to the bus is sent in fixed format messages of selectable length.
The maximum number of data bytes in every message frame is eight, but this
application uses 2-byte frames for both directions.
The keypad requirements may vary among applications; however, the number
of the inputs (keys) and outputs (e.g., indicators) is limited only by the number
of available GPIO pins on the MCU and the input/output interface designs.
There are many approaches to connecting the keys, and the actual solution
presented here uses direct interfacing to the switches. When a different key
configuration is used, such as a matrix or connection using A/D inputs, it is
possible to achieve a higher number of switches with the same number of pins.
In that case, the software handler for the keypad would have to be modified.
The target MCU for the keypad module is the MC68HC908QY/QT. This MCU
includes an internal oscillator, a power-on reset module, and selectable pullups
on all ports. Therefore, a power supply with a bypass capacitor is the only thing
necessary to connect to the MCU. The circuit diagram of this keyboard
application is shown in
Other than the MCU itself, two chips are required to implement a simple LIN
node. These are the LIN interface (in this case the Motorola MC33399) and a
5-V regulator. These chips could possibly be replaced by a single chip, for
example the Motorola MC33689 LIN SBC (system basis chip). As a regulator,
a 3-pin 7805 or an 8-pin LT1121 chip is used. This configuration has the
capability of forcing the LIN device into a low-power sleep mode under the
control of the MCU. This option is not used in this application. The MC33399
includes a 30-kΩ LIN pullup, so this does not need to be included on the PCB.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
Figure
1.
Keypad Construction
AN2600/D
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