Datasheet

ST7538Q Auxiliary analog and digital functions
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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
T
WO
T
RSTO
T
WD
T
WM
T
RSTO
RSTO
WD
D03IN1410
t
ZCIN
RxTx
CLR/T
TxD
ZCOUT
ZC
DEL
D03IN1423