Datasheet

SAM9G20
DS60001516A-page 458  2017 Microchip Technology Inc.
31.6.3.6 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 31-17.
Figure 31-17: Manchester Encoded Characters RF Transmission
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 31-18 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 mod-
ulator outputs an RF signal at frequency F0 and switches to F1 if the data sent is a 0. See Figure 31-19.
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 Man-
chester preamble length is to be defined in accordance with the RF IC configuration.
Figure 31-18: ASK Modulator Output
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
Manchester
encoded
data
default polarity
unipolar output
Txd
ASK Modulator
Output
Uptstream Frequency F0
NRZ stream
10 0 1