ST52X420 datasheet(35/84 Pages) STMICROELECTRONICS

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ST52T410/ST52T420/E420

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and the watchdog remain active. During WAIT

mode, Interrupts are enabled. The MCU will

remain in Wait mode until an Interrupt or a RESET

occurs, whereupon the Program Counter jumps to

the interrupt service routine or, if a RESET occurs,

at the beginning of the user program.

REMARK: In Wait mode the CPU clock does not

stop.

5.3.2 Halt Mode

Halt mode is MCU’s lowest power consumption

mode, which is entered by executing the HALT

instruction. The internal oscillator is turned off,

causing all internal processing to stop, including

the operations of the on-chip peripherals.

Figure 5.2 Reset Block Diagram

Figure 5.3 Simple Reset Circuit

Halt mode cannot be used when the watchdog

is enabled.

If the HALT instruction is executed while the

watchdog system is enabled, it will be skipped

without modifying the normal CPU operations.

The ICU can exit Halt mode after an external inter-

rupt or reset. The oscillator is then turned on and

stabilization time is provided before restarting

CPU operations. Stabilization time is 4096 CPU

clock cycles after the interrupt and 1.000.000 after

the Reset.

After the start up delay, the CPU restarts opera-

tions by serving the external interrupt routine.

Reset makes the ICU exit from HALT mode and

restart, after the delay, from the beginning of the

user program after the delay.

Warning: if the External Interrupt is disabled, the

ICU exits from the Halt mode and jumps to the

lower priority interrupt routine.

Figure 5.4 WAIT Flow Chart

WATCHDOG RESET

RESET

INTERNAL

RESET

Vcc

100

10k

2.2k

1 F

RESET

ST52X420 Datasheet(PDF) 35 Page - STMicroelectronics

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