STR720 datasheet(371/401 Pages) STMICROELECTRONICS

371 / 401 page

STR720 - SYSTEM RESET

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26.3 JTRST Input Pin

This pin is used to initialize on-chip emulation and debug logic independently from the rest of

the system. To improve the noise immunity of this critical input pin, JTRST has a Schmitt

trigger input circuit with hysteresis and an on-chip RC filter is implemented, similar to the one

described for RESET input pin. The filter is sized to filter out all spikes shorter than 50 ns and

minimum JTRST pulse duration is recommended to be longer than 500 ns. This means that

all the pulses whose length is between 50 ns and 500 ns can either be filtered or recognized

as valid, depending on the operating conditions and process variations.

26.4 Power-on/off and Stand-by entry/exit

At power-on or Stand-by exit, it is recommended to held low RSTIN pin to guarantee a proper

initialization of STR720 system and avoid malfunctions of the RTC logic. In particular, when

VDD and VDD3 are still too low to allow the internal logic to work properly, the system must be

mantained under reset status, forcing a HW Reset through the RSTIN pin. In this way the

system sees a HW Reset and the flag register (CLK_FLAG in RCCU module) assumes the

corresponding configuration (all Reset flags cleared).

To avoid unpredictable current injection and power consumption, it is recommended to have

the VDD3 value always greater than VDD during the power-on phase, as illustrated in next

Figure 69 on page 371.

The distinction between power-on reset and hardware system reset consists in the fact that

typical hardware reset sequences do not affect Real-Time-Clock section, leaving RTC running

while rest of the system is restarted. This happens also when system enters and exit stand-by

mode. On the contrary a power-on reset will see both system and RTC section being

initialized. In order to distinguish between a power-on reset and a hardware system reset (or

Figure 69. Power-on supply sequence

VDD3

VDD

RSTIN

Reset pin driven low

STR720 Datasheet(PDF) 371 Page - STMicroelectronics