Page 52 - The Mechatronics Handbook

of RAM designated for this purpose, in a process known as a push. After a push, the microprocessor can
then load the address of the Interrupt Service Routine and complete the input/output. When that portion
of code is complete, the contents of the stack are reloaded to the registers in an operation known as a
Pop (or Pull) and normal processing resumes.

Input and Output Transmission
Once the input or output is ready for transmission, there are several modes that can be used. First, data
can be moved in either parallel or serial mode. Parallel mode means that multiple bits (e.g., 16 bits) move
in parallel down a multiple pathway or bus from source to destination. Serial mode means that the bits
move one at a time, in a series, down a single pathway. Parallel mode traffic is faster in that multiple bits
are moving together, but the number of pathways is a limiting factor. For this reason parallel mode is usually
used for components located close to one another while serial transmission is used if any distance is involved.
Serial data transmission can also be differentiated by being asynchronous or synchronous. Asynchro-
nous data transmission uses separate clocks between the sender and receiver of data. Since these clocks
are not synchronized, additional bits called start and stop bits are required to designate the boundaries
of the bytes being sent. Synchronous data transmission uses a common or synchronized timing source.
Start and stop bits are thus not needed, and overall throughput is increased.
A third way of differentiating data transmission is by direction. A simplex line is a one direction only
pathway. Data from a sensor to the microcontroller may use simplex mode. Half-duplex mode allows
two-way traffic, but only one direction at a time. This requires a form of flow control to avoid data
transmission errors. Full-duplex mode allows two-way simultaneous transmission of data.
The agreement between sending and receiving units regarding the parameters of data transmission
(including transmission speed) is known as handshaking.

HC12 Microcontroller Input–Output Subsystems
There are four input–output subsystems on the Motorola HC12 microcontroller that can be used to
exemplify the data transmission section above.
The serial communications interface (SCI) is an asynchronous serial device available on the HC12. It can
be either polled or interrupt driven and is intended for communication between remote devices. Related to
SCI is the serial peripheral interface (SPI). SPI is a synchronous serial interface. It is intended for commu-
nication between units that support SPI like a network of multiple microcontrollers. Because of the synchro-
nization of timing that is required, SPI uses a system of master/slave relationships between microcontrollers.
The pulse width modulation (PWM) subsystem is often used for motor and solenoid control. Using
registers that are mapped to both the PWM unit and the microprocessor, a PWM output can be com-
manded by setting values for the period and duty cycle in the proper registers. This will result in a
particular on-time and off-time voltage command.
Last, the serial in-circuit debugger (SDI) allows the microcontroller to connect to a PC for checking
and modifying embedded software.

Microcontroller Network Systems

There is one last topic that should be mentioned in this section on inputs and outputs. Mechatronic
systems often work with other systems in a network. Data and commands are thus transmitted from
one system to another. While there are many different protocols, both open and proprietary, that could
be mentioned about this networking, two will serve our purposes. The first is the manufacturing auto-
mation protocol (MAP) that was developed by General Motors Corporation. This system is based on
the ISO Open Systems Interconnection (OSI) model and is especially designed for computer integrated
manufacturing (CIM) and multiple PLCs. The second is the controller area network (CAN). This
standard for serial communications was developed by Robert Bosch GmbH for use among embedded
systems in a car.

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