Auxiliary analog and digital functions ST7540
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7 Auxiliary analog and digital functions
7.1 Band in use
The Band in Use Block has a Carrier Detection like function but with a different Input
Sensitivity (83.5 dBµV Typ.) and with a different BandPass filter Selectivity (40dB/Dec).
BU/THERM line is forced High when a signal in band is detected.
To prevent BU/THERM line false transition, Band in Use signal is conditioned to Carrier
Detection Internal Signal. This function is enabled only in Receiving mode (in Transmission
mode the BU/THERM pin is used for Thermal shutdown signaling, see Section 7.8: Thermal
shutdown).
7.2 Time out
Time Out Function is a protection against a too long data transmission. When Time Out
function is enabled after 1 or 3 second of continuos transmission the transceiver is forced in
receiving mode. This function allows ST7540 to automatically manage the CENELEC
Medium Access specification. When a time-out event occur, the transmission section is
disabled for at least 125 ms. To Unlock the Time Out condition RxTx should be forced High.
During the time out period only register access or reception mode are enabled.
During Reset sequence if RxTx line =”0” & REG_DATA line =”0”, Time Out protection is
suddenly enabled and ST7540 must be configured in data reception after the reset event
before starting a new data transmission.
Time Out time is programmable using Control Register bits 7 and 8 (Ta bl e 1 2 ).
Figure 22. Time-out timing and unlock sequence
T
OUT
T
OFF
T
OFFD
RxTx
Time Out function
D03IN1409
