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Software Overview
4.7.5
High-Frequency Crystal
The board has a footprint for a high-frequency (4 MHz) microprocessor crystal (Y3) as well as the
low-frequency crystal. The factory board configuration uses the 32-kHz watch crystal (Y4). An internal
frequency multiplier generates the 2.4576-MHz system clock.
4.7.6
Discrete LC Filter or SAW Filter
A SAW filter is typically used to band-limit the received RF signal and reduce RF interference outside the
ISM band. The SAW filter from Murata (SAFC915MA70N-TC11) gives a pass band typical of 902 MHz to
928 MHz, while providing about 30-dB attenuation in the stop band. Insertion loss in the pass band is
about 4.5 dB. This causes a corresponding loss in operational range due to signal attenuation. The SAW
filter is not normally installed at the factory in order to maximize range rather than noise immunity.
There are other hardware configurations for attenuating transmitter harmonics and filtering out-of-band
received noise. It is recommended that discrete LC filters be used for this purpose. See the TRF6903
Design Guide (SWRU009) for a detailed discussion on the design of discrete LC filters.
4.7.7
Disabling the MSP430
In normal operation, the MSP430 microprocessor controls the TRF4903 transmitter and the game user
interface. The RF section can also be controlled directly with external computer software. To disable the
microcontroller, either short the MSP430 reset pin (RST) to ground at the JTAG connector (preferred
method), or remove R42, which disables the voltage supply to the MSP430. The TRF4903 can then be
controlled directly through J5, the 14-pin surface-mount header.
4.7.8
Game and RF Operational Modes (Jumper JP2)
The TRF4903 circuit board comes with jumper JP2 set in the position for the game (demonstration link)
mode, where two circuit boards communicate with each other in game mode to demonstrate a wireless
link. In the other jumper position (RF evaluation mode), the TRF4903 is controlled from external software
run on a PC and the commands are passed through the MSP430. This software can be downloaded from
the http://www.ti.com/ismrf website.
5
Software Overview
This chapter describes the link demonstration firmware that has been flashed to the microcontroller and
external control software for RF test and evaluation.
5.1
Software Description
The demonstration software implements a simple one-way game application utilizing a half-duplex
unidirectional wireless link. The transmit board and receive board have different software. The 4903-based
transmit side software routines are discussed in this section. The software was developed using IAR's
Embedded Workbench, version 3.20a (www.iar.com). The software files are:
• trf4903_demo_us.ewp – Project file
• trf4903_main.c – Main body of the program written in C languge. All subroutines are called from this
program
• rf_reg.c – C file to configure the TRF4903 registers
• api_4903.c – C file for UART interface
• trf4903.s43 – Assembly subroutine to program the TRF4903 registers
• radio49.s43 – Assembly subroutines that execute wireless transmission commands
• setdco.s43 – Assembly routine that sets the baseband clock.
14
TRF4903 With MSP430 Demonstration and Development Evaluation Kit
SWRU010A – October 2004 – Revised May 2005
www.ti.com
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