ST7538Q datasheet(37/44 Pages) STMICROELECTRONICS

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ST7538Q

Auxiliary analog and digital functions

37/44

Figure 21. Reset and watchdog timing

6.4

Zero crossing detection

The Mains Voltage Zero Crossing can be detected, through a proper connection of ZCIN to

the Mains. ZCIN comparator has a threshold fixed at SGND. ZCOUT is a TTL Output forced

High after a positive zero-crossing transition, and low after a negative one.

Setting the Bit 11 inside the Control Register to “1” the transmission is automatically

synchronized to the mains positive zero-crossing transition. This function is achieved turning

on the PLI when RX/TX is low and delaying the CLR/T first transition until the first zero-

crossing event. The automatic synchronization procedure can work only if the synchronous

interface is programmed. If asynchronous interface is in use the Zero Crossing

synchronization can be achieved managing the ZCOUT line.

Figure 22. Synchronous zero-crossing transmission

TWO

TRSTO

TWD

TWM

TRSTO

RSTO

D03IN1410

ZCIN

RxTx

CLR/T

TxD

ZCOUT

ZCDEL

D03IN1423

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