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ST7580

Physical layer

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9.2.2

FSK physical frame

Figure 13

shows the physical frame for FSK modulations.

Figure 13. FSK physical frame structure (length in bytes)

The meaning of each field is as follows:



Preamble: a sequence of alternating 1 and 0 symbols (0xAA bytes) required by the

receiver PLL to achieve bit synchronization. Its length is programmable from 2 to 5

bytes.



Unique word: a programmable sequence used to mark the start of a physical frame. Its

length and values are programmable.



Physical SDU (service data unit): payload of the physical layer. Its length is specified in

its first byte, which is always present.

9.2.3

FSK settings

In the FSK physical frame there is no automatic modulation recognition. In order to

communicate using FSK modulation, the transmitter and the receiver must use the same

settings in terms of baud rate, deviation and unique word. ST7580 uses a single set of FSK

settings at a time, both for transmitting and receiving.

9.3

Channel and modulation selection

The ST7580 supports a range of carrier frequencies for modulation, offering a dual channel

configuration for both transmission and reception. The two frequency channels f1, f2 have to

be set with the following constraints:



Minimum frequency carrier value: 9 kHz



Maximum frequency carrier for transmission: 250 kHz



Maximum central frequency (f1 + f2) /2: 249.999 kHz



Maximum frequency difference (f1 - f2): 38.461 kHz.

The transmission is always performed on one channel at a time, modulating the output

signal around either the high channel or the low channel. Any modulation type can be used.

The reception can be configured in single channel mode or in dual channel mode.



In single channel mode the receiver listens to the high channel (the carrier with a higher

frequency value) only, while it neglects any communication on the low channel (the

carrier with a lower frequency value).



In dual channel mode the receiver always listens to both channels. As soon as it

detects a valid frame on a channel, it stops listening to the other channel.