It’s essential that the
chosen MCU offer system
protection, including
such options as watchdog
computer-operating-
properly (COP) reset
with an alternative to run
from a dedicated 1-kHz
internal clock source or
bus. Other “must have”
system protection features
include low-voltage
detect with reset or
interrupt, illegal opcode
detection with reset,
illegal address detection
with reset, and flash
block protection.
A variety of embedded
peripherals will ease
the implementation of
the designer’s application.
An eight-channel,
10-bit analog-to-digital
converter (ADC) is recommended
for accurate successive approximation. Consider an
ADC that’s hardware-triggerable using
the RTI counter and features automatic
compare, asynchronous clock source,
temperature sensor, and internal bandgap
reference channel.
Other essential peripherals for sensing
and control applications include an
analog comparator module (ACMP) with
an option to compare internal reference;
serial communications interface (SCI)
module; SPI module; inter-integrated circuit
(IIC) bus module; two-channel timer/
pulse-width modulator for input capture,
output compare, buffered edge-aligned
PWM, or buffered center-aligned PWM;
8-bit modulo timer module with prescaler;
and 8-pin keyboard interrupt module
with software-selectable polarity on
edge or edge/level modes.
There are multiple small-footprint
MCU packaging options that satisfy
sensing and control design requirements.
These help optimize limited
board space, particularly in end node,
battery-operated functions. A few of
the microcontroller packages that meet
these considerations include low-pincount
plastic dual-in-line (PDIP), quad
flat no-lead (QFN), thin shrink small-outline
(TSSOP), dual flat no-lead (DFN),
and narrow-body, small-outline IC (NB
SOIC) packages.
It’s also prudent to consider, as part of
the MCU selection, hardware and software
design tool ease of use, documentation
clarity, reference designs, available
application code, and other design
support offerings.
Similarly, for the RF or modem side of
the design, an effective integrated development
environment (IDE) for MCUs
should include GUI-driven tools with
built-in features and utilities that simplify
coding and project file management
to expedite the design process. Expert
tools that abstract the hardware layer
and generate optimized, MCU-specific
C code tailored to the application allow
the designer to concentrate on application
concepts. Fast and easy debug
as well as flash programming capability
need to be considered. It also helps to
have access to features so designers
can create reusable software components
for reuse between projects.
SUBSYSTEM REFERENCE DESIGN
A reference design for sensing, monitoring,
and control subsystems can
prove valuable as an application baseline
from which to evolve design-specific
requirements. The Wireless UART reference
design, for example, uses a SiP
solution—the MC13211 RF transceiver
from Freescale Semiconductor. Schematic
files for the Freescale 1321X-SRB
sensor reference board, bill of materials,
Gerber files, software project files
(.mcp), and other design support materials
are provided at www.Freescale.com/zigbee under “Reference Designs.” The 1321X-SRB (Fig. 4) includes the Freescale
MMA7260Q tri-axis acceleration
sensor as part of the reference design.
The reference design thus contains all
components necessary to set up working
networks in a matter of minutes for
proof of concept. It can be developed
using the unlimited use license for the
SMAC code base. Using Freescale’s
BeeKit wireless connectivity toolkit and
CodeWarior IDE (a free 32-kbyte version
download), you can begin your application
software development from the
.mcp file (Wireless_Uart.mcp) provided
with the reference design.
To set up your new project, simply
download the complimentary BeeKit GUI
radio setup software tool from http://www.freescale.com/webapp/sps/site/prod_summary.jsp?
code=BEEKIT_WIRELESS_CONNECTIVITY_TOOLKIT&nodeId=01J4Fs25657103&fpsp=1&t
ab=Design_Tools_Tab. Select BEEKITDOWNLOADPACKAGE.
ZIP (last item)
and install the BeeKit from the easy-touse
instructions included with the tool
download package. Once BeeKit starts,
you will see the step by step instructions.
A solution explorer and wizard allow
for quick configuration of parameters
before creating the project, reducing the
need to manually configure parameters
and sort through individual files. A comprehensive
code base provides wireless
networking libraries, application templates,
and sample applications.
Once you’ve created the project,
you can customize (if desired) and the
BeeKit will validate any customized project
selections to ensure that none are
conflicting. Once the radio solution is set
up, export the project in an .xml file and
import into CodeWarrior to start your
application software development.
If you’re working from different reference
designs from other providers, supporting
files are typically available from
the supplier through Web downloads.
All downloads should include necessary
schematics, bills of materials, Gerber
files, software, and other documentation
for complete reference-design
implementation.
Through organized research and
analysis, a clear choice emerges for the
wireless networking application solution.
Embedded systems designers can generate
the information to make a logicbased
decision on how to incorporate
value-added wireless networking features
into their end product. The proactive
effort invested in developing a matrix
analysis will save significant design time
and expense by reducing false starts
and the chance for error.