ST7538Q datasheet(36/44 Pages) STMICROELECTRONICS

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Auxiliary analog and digital functions

ST7538Q

36/44

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 (77dB

µV 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 (Table 11).

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 TRSTO 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 TRSTO time) on expiry of the internal

watchdog timer. The watchdog timer reset can be achieved applying a negative pulse on

WD pin (Figure 21).

TOUT

TOFF TOFFD

RxTx

Time Out function

D03IN1409

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