ST7540 datasheet(23/44 Pages) STMICROELECTRONICS

23 / 44 page

ST7540

Functional description

23/44

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 SVSS 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).

TCC

TDS

TDH

TCR

TDS

TDH

TCC

CLR_T

RxD

TxD

REG_DATA

RxTx

D03IN1405

BIT23

BIT22

TSTH

TCC

TDS

TCR

TDH

TCC

CLR_T

TxD

RxD

REG_DATA

RxTx

D03IN1401

BIT23

BIT22

TSTH

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