ST7538
AUXILIARY ANALOG AND DIGITAL FUNCTIONS
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.
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 ST7538 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 en-
abled.
During Reset sequence if RxTx line =”0” & REG_DATA line =”1”, TIMEOUT protection is suddendly en-
abled and ST7538 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 (table 6).
Figure 17. Time-out Timing and Unlock Sequence
RxTx
T
T
T
OFFD
OUT
OFF
TOUT
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Reset & Watchdog
RSTO Output is a reset generator for the application circuitry. During the ST7538 startup sequence is
forced low. RSTO becomes high after a T delay from the end of oscillator startup sequence.
RSTO
Inside ST7538 is also embedded a watchdog function. The watchdog function is used to detect the occur-
rence of a software fault of the Host Controller. The watchdog circuitry generates an internal and external
reset (RSTO low for T
time) on expiry of the internal watchdog timer. The watchdog timer reset can
RSTO
be achieved applying a negative pulse on WD pin Fig 18.
Figure 18. Reset and Watchdog Timing
T
T
T
RSTO
RSTO
WO
RSTO
WD
T
WM
T
WD
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STMICROELECTRONICS [ ST ]
