ADDITIONAL DIGITAL AND ANALOG FUNC-

TIONS

Time base

A time base section delivers all the internal clocks

from a crystal oscillator (11.0592MHz). The crystal

is connected between XTAL1 and XTAL2 pins and

needs two external capacitors C3 and C4 (22pF

±10%, 10V) for proper operation.

Reset and watch-dog

The reset output (RSTO) is driven high when the

supply voltage is lower than Vrh (typically 7.6V) with

an hysteresis Vrh-Vrl (typically 300mV) or when no

negative transition occurs on the watch-dog input

(WD) for more than 1.5 second (see the timing

chart on Figure 2). When a reset occurs RSTO is

held high for at least 50ms.

Signal detection

The Carrier Detect output (CD) is driven low when

the input signal amplitude on RAI is greater than

chart on Figure 3). When the input signal desap-

is the carrier detection threshold voltage which is

External power amplifier bias control

Two dedicated digital output (PABC and PABC)

delivering a signal between 0V and 10V are driven

low respectively high, when the circuit is set in the

receive mode (Rx/Tx=1) or when the transmit mode

time out (1 second) is exceeded; in the same time

the output ATO is put in a high impedance state.

TESTING FEATURES

- An additionnal amplifier allows the observation of

the Rx band-pass filter output on pin RxFO.

- A direct input to the Tx band-pass filter and to the

IF filter (TxIFI) is selected when TEST3 = 1.

- The 1 second normal duration of the Tx to Rx

mode automatic switching is reduced to 488

µs

and the 1.5 second watch-dog time out is reduced

to 46.3

µs when TEST2 = 1.

- When TEST1 = 1 the Tx to Rx mode automatic

switching is desactivated and the functional mode

of the circuit is fully controlled by Rx/Tx.

- TEST4 is a reset input which allows an undelayed

control of RSTO and of the internal state of the

circuit.

POWER SUPPLIES WIRING PRECAUTIONS

The ST7537HS1 has two positive power supply

and digital supplies. The analog and digital termi-

nals of each supply pair must be connected to-

gether externally for proper operation.

proper operation.

Rx/Tx

CD

TxD

RxD

DATA

DATA

Figure 1 : Data Timing Chart

ST7537HS1

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