Datasheet

ST7540 Functional description
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Figure 11. Data transmission ➨ control register read ➨ data reception timing diagram
Figure 12. Data transmission
➨ control register write ➨ data reception timing diagram
6.6 Receiving mode
The receive section is active when RxTx Pin =”1” and REG_DATA=0.
The input signal is read on RX_IN Pin using SV
SS
as ground reference and then pre-filtered
by a Band pass Filter (62kHz max bandwidth at -3dB). The Pre-Filter can be inserted setting
one bit in the Control Register. The Input Stage features a wide dynamic range to receive
Signal with a Very Low Signal to Noise Ratio. The Amplitude of the applied waveform is
automatically adapted by an Automatic Gain Control block (AGC) and then filtered by a
Narrow Band Band-Pass Filter centered around the Selected Channel Frequency (14kHz
max at -3dB). The resulting signal is down-converted by a mixer using a sinewave generated
by the FSK Modulator. Finally an Intermediate Frequency Band Pass-Filter (IF Filter)
improves the Signal to Noise ration before sending the signal to the FSK demodulator. The
FSK demodulator then send the signal to the RX Logic for final digital filtering. Digital
filtering Removes Noise spikes far from the BAUD rate frequency and Reduces the Signal
Jitter. RxD Line is forced to “0” or “1” (according the UART/SPI pin level) when neither mark
or space frequencies are detected on RX_IN Pin.
Mark and Space Frequency in Receiving Mode must be distant at least BaudRate/2 to have
a correct demodulation.
While ST7540 is in Receiving Mode (RxTx pin =”1”), the transmit circuitry, Power Line
Interface included, is turned off. This allows the device to achieve a very low current
consumption (5mA typ).
T
CC
T
DS
T
DH
T
CR
T
CR
T
CR
T
B
T
DS
T
DH
T
CC
CLR_T
RxD
TxD
REG_DATA
RxTx
D03IN1405
BIT23 BIT22
T
S
T
H
T
CC
T
DS
T
CR
T
CR
T
CR
T
DH
T
B
T
CC
CLR_T
TxD
RxD
REG_DATA
RxTx
D03IN1401
BIT23 BIT22
T
S
T
H
T
S
T
H