Rainbow Electronics AT91CAP9S250A - Manual (Page 541)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
field in the US_RHR register and the RXSYNH is updated. RXCHR is set to 1 when the received
character is a command, and it is set to 0 if the received character is a data. This mechanism
alleviates and simplifies the direct memory access as the character contains its own sync field in
the same register.
As the decoder is setup to be used in unipolar mode, the first bit of the frame has to be a zero-to-
one transition.
35.6.3.5 Radio Interface: Manchester Encoded USART Application
This section describes low data rate RF transmission systems and their integration with a
Manchester encoded USART. These systems are based on transmitter and receiver ICs that
support ASK and FSK modulation schemes.
The goal is to perform full duplex radio transmission of characters using two different frequency
carriers. See the configuration in
Figure 35-17 .
Figure 35-17. Manchester Encoded Characters RF Transmission
The USART module is configured as a Manchester encoder/decoder. Looking at the down-
stream communication channel, Manchester encoded characters are serially sent to the RF
emitter. This may also include a user defined preamble and a start frame delimiter. Mostly, pre-
amble is used in the RF receiver to distinguish between a valid data from a transmitter and
signals due to noise. The Manchester stream is then modulated. See
Figure 35-18 for an exam-
ple of ASK modulation scheme. When a logic one is sent to the ASK modulator, the power
amplifier, referred to as PA, is enabled and transmits an RF signal at downstream frequency.
When a logic zero is transmitted, the RF signal is turned off. If the FSK modulator is activated,
two different frequencies are used to transmit data. When a logic 1 is sent, the modulator out-
puts an RF signal at frequency F0 and switches to F1 if the data sent is a 0. See
Figure 35-19 .
From the receiver side, another carrier frequency is used. The RF receiver performs a bit check
operation examining demodulated data stream. If a valid pattern is detected, the receiver
LNA
VCO
RF filter
Demod
control
bi-dir
line
PA
RF filter
Mod
VCO
control
Manchester
decoder
Manchester
encoder
USART
Receiver
USART
Emitter
ASK/FSK
Upstream Receiver
ASK/FSK
downstream transmitter
Upstream
Emitter
Downstream
Receiver
Serial
Configuration
Interface
Fup frequency Carrier
Fdown frequency Carrier