AN2707 Freescale Semiconductor / Motorola, AN2707 Datasheet - Page 8

• Manufacturer Part Number: AN2707
• Description: Software Drivers for Tango3 RF Transmitter and Romeo2 RF Receiver ICs
• Manufacturer: Freescale Semiconductor / Motorola

AN2707
Manchester
Encoding
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Software Drivers for Tango3 RF Transmitter and Romeo2 RF Receiver ICs
(FSK) modulation or On/Off Keying (OOK) modulation. Romeo2 receives the
FSK/OOK signal, removes the Manchester encoding and passes the message
to the software driver via the SPI interface. The software driver writes the
message to a RAM buffer where the CPU can read the message. The Romeo2
driver must be correctly configured to match the message format, data rate and
RF carrier frequency used by the Tango3 transmitter.
For Tango3 to transmit RF data, it must be supplied with a bitstream containing
the data in Manchester encoded format. A Manchester encoded bit is
represented by a sequence of two opposite logic levels. A '0' bit of data is
encoded as sequence '01', a '1' bit of data is encoded as sequence '10'.
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sequence '11001' using Manchester encoding. Note that there is always a level
transition in the middle of a bit, but not always a transition on a bit boundary.
On the MCU, a timer I/O pin with an output compare function is used to
generate each bit. The timer modulus (or timebase) is set to match the
timebase of the Manchester encoded data. The output compare function is set
to half the timebase. By controlling the level of the I/O pin when output compare
occurs or the timer 'rolls over' to zero, the driver can generate the correct
sequence.
and the generated output.
Manchester encoding is performed by the Tango3 software driver.
Figure 6. Data Flow and Message Encoding Steps in Message Transfer
shows what will be seen on Tango3's DATA input when transmitting the data
Freescale Semiconductor, Inc.
HC08 CPU
HC08 CPU
buffer
buffer
Ram
Ram
Msg from buffer
Msg from buffer
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CPU writes msg
CPU writes msg
Driver reads
Driver reads
to ram buffer
to ram buffer
Figure 7
Tango3
Tango3
driver
driver
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transmits to
transmits to
Machester
Machester
encodes
encodes
Tango3
Tango3
Driver
Driver
msg,
msg,
shows the relationship between the timer counter value
Tango3
Tango3
modulated data
modulated data
Tango3 sends
Tango3 sends
FSK or OOK
FSK or OOK
and removes
and removes
Manchester
Manchester
FSK/OOK
FSK/OOK
encoding
encoding
Romeo2
Romeo2
decodes
decodes
Romeo2
Romeo2
To mcu via SPI
To mcu via SPI
Decoded msg
Decoded msg
interface
interface
Romeo2
Romeo2
driver
driver
from ram buffer
from ram buffer
CPU reads msg
CPU reads msg
msg to buffer
msg to buffer
Driver writes
Driver writes
MOTOROLA
HC08 CPU
HC08 CPU
buffer
buffer
Ram
Ram
Figure