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PDF AN1252 Data sheet ( Hoja de datos )
| Número de pieza | AN1252 | |
| Descripción | Interfacing the MRF49XA Transceiver to PIC Microcontrollers | |
| Fabricantes | Microchip | |
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AN1252
Interfacing the MRF49XA Transceiver to PIC® Microcontrollers
Author: Cristian Toma
Microchip Technology Inc.
INTRODUCTION
Microchip Technology’s MRF49XA is a highly
integrated RF transceiver, used in the 433, 868 and
915 MHz frequency bands. The transceiver uses FSK
modulation internally.
A transceiver is a device that can both transmit and
receive. Thus, the word ‘transceiver’. A system that can
send and receive data at the same time is called a full-
duplex system. On the other hand, a system that can
only send or receive at a time is called a half-duplex
system. Thus, half-duplex systems use only one
frequency carrier and the two ends share the same
frequency. Full-duplex systems use two carrier
frequencies, known as uplink frequency and downlink
frequency.
This document discusses what is required to
successfully develop a half-duplex radio application
using the Microchip Technology MRF49XA transceiver.
For more information on this transceiver, please refer to
the MRF49XA data sheet (DS70590).
FSK SHORT THEORY
The most common radio modulation used in Remote
Keyless Entry (RKE) systems is the Amplitude Shift
Keying (ASK). Data is transmitted by varying the
amplitude of a fixed-frequency carrier. When data is
encoded as maximum amplitude for a ‘1’ or mark, and
zero amplitude – the power amplifier (PA) is switched
off – for a ‘0’ or space, this type of modulation is also
named On-Off Keying, or OOK. This modulation format
allows very simple and low-cost transmitter designs.
Another type of modulation is Frequency Shift Keying
(FSK). This is done by shifting the carrier’s frequency
on either side of an average (or carrier) frequency. The
amount by which the carrier shifts on either side of the
carrier’s frequency is known as deviation. The FSK
modulation has several advantages over the ASK
modulation. While the AM modulation is very sensitive
to variations of amplitude and noise, the FSK encoded
transmissions are more immune to signal attenuation
or other amplitude-based disturbance. Although the
apparent bandwidth is from f0 – ∆f to f0 + ∆f, in reality, the
bandwidth spreads larger than the span between f0 – ∆f
to f0 + ∆f, because the speed of transition between the
two frequencies generates additional spectral content.
In short, think of FSK modulation as a more reliable
transmission medium having much less noise. In order
to achieve a successful design, you will need a deeper
understanding of the requirements of an FSK
modulated radio link.
FIGURE 1:
THE COMBINED SPECTRUM
GENERATED BY A
'01010...' PATTERN
To see an example of what FSK looks like, take a look
at Figures 1 through 3. These plots are taken from a
spectrum analyzer, a tool that plots amplitude (in dB)
versus frequency (linearly, in Hz). Each of the plots has
about an 80 dB range, with a frequency range or “span”
of 320 kHz (since there are 10 divisions, this is 32 kHz
per division).
The plot is “centered” at 915 MHz. This means perfectly
aligned between the left and right side of the plot, at
915 MHz. Left of this point is the lower frequency and
to the right is the higher frequency (at 32 kHz per
division).
Figure 1 shows what the frequency plot looks like for
our example design when its transmitter generates a
continually alternating stream of ones and zeros (a
01010101… pattern). The green line is from the
spectrum analyzer and shows two peaks. Since FSK
means shifting the frequency based on the symbol sent
(a ‘1’ or a ‘0’), there are two peaks.
© 2009 Microchip Technology Inc.
DS01252A-page 1
Free Datasheet http://www.Datasheet4U.com
1 page  
The actual frequency can be calculated using the
formula below:
EQUATION 4:
For 915 MHz:
Fref = (Frequency[Mhz]) * 91.5
For 868 MHz:
Fref = (Frequency[Mhz]) * 86.8
For 433 MHz:
Fref = (Frequency[Mhz]) * 43.3
A 30 ppm, 10 MHz crystal will generate a maximum
error of:
EQUATION 5:
Δf0
=
C-----r---y---s---t--a---l--A-----c---c---u---r---a----c---y---[--p----p---m-----]
106
*
XtalFrequency[MHz]
* 106
=
--3---0---
*
10
6
* 10 =
300 H z
106
Thus, a maximum frequency error of:
EQUATION 6:
F0 = F-----r--e----q---u----e--1-n--0-c---y---[---M-----h----z--]-* ⎝⎛XtalFrequency[MHz] * 106 ± Δf0⎠⎞
= 915 MHz±27.45KHz
Where the TransmissionBand is 868 or 915, depending on the
frequency setting.
Adjusting the RX-TX frequency offset can be done
using the General Configuration register and changing
the crystal load capacitance. This allows small changes
in the reference frequency. Adjust the crystal load
capacitance in order to get the same frequency on both
devices – as close as possible.
AN1252
FIGURE 6:
EXAMPLE OF BADLY
TUNED TRANSMITTER
In Figure 6, we have the example of a badly tuned
transmitter. Here we see that the center frequency is
misaligned. The red lines represent the receiver
baseband filter response. As you can clearly see, a
radio link cannot be established in this case, since the
receiver cannot interpret the ‘1’ and the ‘0’ symbols.
The amplitudes of the signals are not of interest here
and are shown only for illustration purposes.
CONCLUSION
MRF49XA is a highly integrated RF transceiver. It
requires only a few external components and can be
controlled via an SPI interface.
Thus, MRF49XA is ideal for low-power, short-range
radio communications, where the host system is a
microcontroller, such as Microchip’s PIC®
microcontrollers.
© 2009 Microchip Technology Inc.
DS01252A-page 5
Free Datasheet http://www.Datasheet4U.com
5 Page | ||
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| PDF Descargar | [ Datasheet AN1252.PDF ] | |
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