Datasheet

Auxiliary analog and digital functions ST7538Q
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6 Auxiliary analog and digital functions
6.1 Band in use
The Band in Use Block has a Carrier Detection like function but with a different Input
Sensibility (77dBµV Typ.)
and with a different BandPass filter Selectivity (40dB/Dec).
BU line is forced High when a signal in band is detected.
To prevent BU line false transition, BU signal is conditioned to Carrier Detection Internal
Signal.
6.2 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 ST7538Q to automatically manage the CENELEC
Medium Access specification. When a time-out event occur, TOUT is forced high, and is
held high 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”, TIMEOUT protection is
suddendly enabled and ST7538Q 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 1).
Figure 20. Time-out timing and unlock sequence
6.3 Reset & watchdog
RSTO Output is a reset generator for the application circuitry. During the ST7538Q startup
sequence is forced low. RSTO becomes high after a T
RSTO
delay from the end of oscillator
startup sequence.
Inside ST7538Q is also embedded a watchdog function. The watchdog function is used to
detect the occurrence of a software fault of the Host Controller. The watchdog circuitry
generates an internal and external reset (RSTO low for T
RSTO
time) on expiry of the internal
watchdog timer. The watchdog timer reset can be achieved applying a negative pulse on
WD pin (Figure 21).
T
OUT
T
OFF
T
OFFD
RxTx
Time Out function
D03IN1409