http://www.www.datasheet4u.com

900,000+ Datasheet PDF Search and Download

Datasheet4U offers most rated semiconductors datasheets pdf




  Microchip Technology Semiconductor Electronic Components Datasheet  

AN1252 Datasheet

Interfacing the MRF49XA Transceiver to PIC Microcontrollers

No Preview Available !

AN1252 pdf
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


  Microchip Technology Semiconductor Electronic Components Datasheet  

AN1252 Datasheet

Interfacing the MRF49XA Transceiver to PIC Microcontrollers

No Preview Available !

AN1252 pdf
AN1252
The red line represents the baseband filter response of
the receiver, discussed later in this document. In this
design, the receiver portion needs the green line to fit
inside of each area between the red lines. As you can
see, each green peak is reasonably centered under
each area, showing that the transceiver performance
should function correctly. We will show mismatch
examples later.
FIGURE 2:
THE SPECTRUM GENERATED
BY A ‘0’ SYMBOL
Figure 2 shows what the output looks like when only a
0’ is transmitted. Note that only one peak is shown, the
lower frequency peak exists only in this example.
FIGURE 3:
THE SPECTRUM
GENERATED BY A ‘1
SYMBOL
Figure 3 shows the result of just transmitting a ‘1
symbol, which has only the higher frequency peak as
expected.
DS01252A-page 2
CONTROL INTERFACE
MRF49XA uses a 4-line SPI interface to communicate
with the host microcontroller/system. These lines are
SDO, SDI, CLK and CS. The SPI port is used for the
control interface and for sending data to and from the
16-bit data TX register/RX FIFO (if the TXDEN/FIFOEN
bit is enabled in the General Configuration register).
In order to use a MRF49XA radio device, it has to be
initialized first. Initializing the device is done by writing
commands to the internal register through the control
interface. There are 16 control (commands) and one
Status Read register. The explanation of these
registers can be read from the MRF49XA data sheet.
Commands to the transceiver are sent serially. Data
bits on pin SDI are shifted into the device upon the
rising edge of the clock on pin SCK whenever the Chip
Select pin, CS, is low. When the CS signal is high, it
initializes the serial interface. All registers consist of a
command code, followed by a varying number of
parameters or data bits. All data are sent, MSB first
(e.g., bit 15 for a 16-bit register). On a Power-on-Reset,
the circuit sets the default values for all the registers.
The transceiver will generate an interrupt request to the
host microcontroller by pulling the IRO line low if one of
the following events takes place:
• TX register is ready to receive the next byte
• RX FIFO has received the pre-programmed
amount of bits
• FIFO overflow/TX register underrun (TXUROW
overflow in Receive mode and underrun in
Transmit mode)
• Negative pulse on interrupt input pin, INT
• Wake-up timer time-out
• Supply voltage below the pre-programmed value
is detected
• Power-on Reset
After receiving an interrupt request, the host
microcontroller identifies the source of the interrupt by
reading the Status bits.
DEVICE INITIALIZATION
The device features a Power-on-Reset circuit, which
has a time-out of 100 ms. During this time, the oscillator
should have enough time to start the oscillations and
reach a point of stability.
In Figure 4, signal 1 (yellow) is the CLK output (1 MHz),
signal 2 (green) is the waveform at the crystal output
and signal 3 (violet) is VDD. This oscilloscope print
indicates that, after applying the VDD voltage to the
device, it takes 31.1 ms for the crystal to start
oscillating and stabilize and for the digital circuitry to
begin operation. SPI commands sent before the POR
© 2009 Microchip Technology Inc.
Free Datasheet http://www.Datasheet4U.com


Part Number AN1252
Description Interfacing the MRF49XA Transceiver to PIC Microcontrollers
Maker Microchip
Total Page 10 Pages
PDF Download
AN1252 pdf
Download PDF File


Buy Electronic Components




Related Datasheet

1 AN1250 Microchip CTMU Microchip
Microchip
AN1250 pdf
2 AN1252 Interfacing the MRF49XA Transceiver to PIC Microcontrollers Microchip
Microchip
AN1252 pdf






Part Number Start With

0    1    2    3    4    5    6    7    8    9    A    B    C    D    E    F    G    H    I    J    K    L    M    N    O    P    Q    R    S    T    U    V    W    X    Y    Z

site map

webmaste! click here

contact us

Buy Components