Datasheet

SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
788
The USART module is configured as a Manchester encoder/decoder. Looking at the downstream 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, preamble 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 example 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 outputs 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 switches to receiving mode. The
demodulated stream is sent to the Manchester decoder. Because of bit checking inside RF IC, the data transferred
to the microcontroller is reduced by a user-defined number of bits. The Manchester preamble length is to be
defined in accordance with the RF IC configuration.
Figure 35-18. ASK Modulator Output
Figure 35-19. FSK Modulator Output
35.7.3.6 Synchronous Receiver
In synchronous mode (SYNC = 1), the receiver samples the RXD signal on each rising edge of the Baud Rate
Clock. If a low level is detected, it is considered as a start. All data bits, the parity bit and the stop bits are sampled
and the receiver waits for the next start bit. Synchronous mode operations provide a high speed transfer capability.
Configuration fields and bits are the same as in asynchronous mode.
Figure 35-20 illustrates a character reception in synchronous mode.
Manchester
encoded
data
default polarity
unipolar output
Txd
ASK Modulator
Output
Uptstream Frequency F0
NRZ stream
10 0 1
Manchester
encoded
data
default polarity
unipolar output
Txd
FSK Modulator
Output
Uptstream Frequencies
[F0, F0+offset]
NRZ stream
10 0 1