be expected, when the radio and MCU are integrated into a single package or chip, the transceiver communicates to the MCU through the onboard or internal SPI command channel.
Also, integrated solutions which include low noise amplifiers (LNA), power amplifiers (PA) with internal voltage controlled oscillator (VCO), integrated transmit/receive switch, on-board power supply regulation and full spread-spectrum encoding and decoding reduce the need for external components in the system and lower overall system cost.
A wide array of system clock configurations gives you flexibility in end system design. Options which allow either an external clock source or crystal oscillator for CPU timing are most suitable. A 16 MHz external crystal is typically required for the modem clocking. The ability to trim the modem crystal oscillator frequency helps to maintain the tight standards required by the IEEE® 802.15.4 specification.
Depending on the complexity and requirements of the end design, you are best served by vendors who offer multiple network software topology alternatives. These may include
a simple media access controller (MAC) configuration which utilizes MCU flash memory sizes from 4 KB and up and supports point-to-point or simple star networks. Fully 802.15.4 compliant MAC and full ZigBee compatible topologies, while requiring more memory, provide the added support of mesh and cluster tree networks.
Ease the design process by using vendor provided reference designs, hardware development tools and software development tools. For hardware development tools, simple getting started guides, essential boards with incorporated LED and LCD for a visual monitor plus cables and batteries provide an easy out-of-the-box experience. These tools help you set up a network within minutes and actually evaluate network
and solution performance. In the past some software design tools, specifically those which support fully ZigBee compliant networks, have been extremely difficult to use. To reduce the complexity of RF modem preparation, look for vendors that offer graphical users interface (GUI)-based software design tools that walk the designer through a step-by-step transceiver set-up.
Antenna design can be a complex issue, particularly for digital designers who have limited to no experience in RF design. Typically, designers will take into account such factors as selecting the correct antenna, antenna tuning, matching, gain/loss and knowing the required radiation pattern. It is advisable to gain a basic knowledge of antenna factors through application notes provided by the transceiver vendor. However,
most digital engineers prefer to consider working with a vendor solution where antenna design is provided. This allows them to focus on the application design. Look for antenna solutions where the antenna design is offered in completed Gerber files, which can be provided directly to the printed circuit board manufacturer for implementation. A vendor who provides such antenna design solutions eliminates the issues associated with
good antenna design, good range and stable throughputs in wireless applications.
The quad flat no-lead package (QFN) is the optimum small footprint packaging solution for the transceiver portion of a low cost wireless networking subsystem. The packaging takes into consideration the board space limitations often driven by sensing and control solutions. Size is particularly important in the case of end nodes that are often battery operated with limited implementation space.
Microcontroller
Multiple alternatives exist in selecting a sensing and control implementation scheme. Some designers select a system in package (SiP) or platform in package™ (PiP) which includes transceiver and MCU functionality in a single package.
However, should you opt for a stand-alone transceiver and microcontroller configuration, you gain the flexibility to choose from a variety of MCUs to mix and match for multiple end product configurations.
When choosing the latter implementation scheme, appropriate MCU selection requires thorough research. MCU selection depends upon matching the complexity of the sensing and control application with suitable performance factors, memory
configurations and peripheral modules. Often for low cost wireless sensing systems, 8-bit MCUs in the 20 MHz CPU operating frequency (10 MHz bus clock) range offer an
easy-to-implement, low-cost alternative which best suits these applications. Background debugging and breakpoint capability to support single breakpoint (tag and force options) setting during in-circuit debug (plus two breakpoints in on-chip debug module) offer the preferred debugging environment. Many MCU solutions provide support for up to 32 interrupt/reset sources.
Memory requirements for sensing and control applications are typically 8 KB of flash with 512B of RAM or as low as 4 KB of flash with 256B of RAM. Flash read, program or rase over the full operating voltage and temperature are essential.
A variety of operation modes provides precise control over power consumption, a key feature for extending battery operated solutions. Look for MCUs that support normal
operating (run) mode, active background mode for on-chip debug, a variety of stop modes (bus and CPU clocks are halted) and wait mode alternatives.
Consider a microcontroller with an internal clock source module containing a frequency-locked-loop (FLL) controlled by an internal or external reference with precision trimming of internal reference that allows 0.2 percent resolution and 2 percent deviation over temperature and voltage. The internal clock source module should support bus frequencies from 1 MHz to 10 MHz. MCUs with selectable clock inputs for key modules
provide control over the clock to drive the module function. As well, look for MCUs with a low-power oscillator module with software selectable crystal or ceramic resonator in the range of 31.25 KHz to 38.4 KHz or 1 MHz to 16 MHz that supports external clock source input up to 20 MHz.
It is 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 detection 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 your application. An 8-channel, 10-bit analog-to-digital (ADC) converter is recommended for accurate successive approximation. Specific functions should include automatic compare, asynchronous clock source, temperature sensor, internal bandgap reference channel and an ADC that is hardware triggerable using the real-time interrupt (RTI) counter.
Other essential peripherals for sensing and control applications include: an analog comparator module (ACMP) with an option to compare internal reference; serial communications interface module (SCI); serial peripheral interface module (SPI); interintegrated circuit (I2C) bus module; 2-channel timer/pulse-width modulator for input capture; output compare; buffered edgealigned 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 foot print 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 MCU packages that meet these considerations are low pin-count 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
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