ST62T35B datasheet(23/82 Pages) STMICROELECTRONICS

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ST62T35B/E35B

RESETS (Cont’d)

3.2.3 Watchdog Reset

The MCU provides a Watchdog timer function in

order to ensure graceful recovery from software

upsets. If the Watchdog register is not refreshed

before an end-of-count condition is reached, the

internal reset will be activated. This, amongst oth-

er things, resets the watchdog counter.

The MCU restarts just as though the Reset had

been generated by the RESET pin, including the

built-in stabilisation delay period.

3.2.4 Application Notes

No external resistor is required between VDD and

the Reset pin, thanks to the built-in pull-up device.

The POR circuit operates dynamically, in that it

triggers MCU initialization on detecting the rising

edge of VDD. The typical threshold is in the region

of 2 volts, but the actual value of the detected

threshold depends on the way in which VDD rises.

The POR circuit is

NOT designed to supervise

static, or slowly rising or falling VDD.

3.2.5 MCU Initialization Sequence

When a reset occurs the stack is reset, the PC is

loaded with the address of the Reset Vector (lo-

cated in program ROM starting at address

0FFEh). A jump to the beginning of the user pro-

gram must be coded at this address. Following a

Reset, the Interrupt flag is automatically set, so

that the CPU is in Non Maskable Interrupt mode;

this prevents the initialisation routine from being

interrupted. The initialisation routine should there-

fore be terminated by a RETI instruction, in order

to revert to normal mode and enable interrupts. If

no pending interrupt is present at the end of the in-

itialisation routine, the MCU will continue by

processing the instruction immediately following

the RETI instruction. If, however, a pending inter-

rupt is present, it will be serviced.

Figure 14. Reset and Interrupt Processing

Figure 15. Reset Block Diagram

RESET

VECTOR

JP:2 BYTES/4 CYCLES

RETI

RETI: 1 BYTE/2 CYCLES

INITIALIZAT ION

ROUTINE

VA00181

VDD

RESET

300k

Ω

2.8k

Ω

POWER

WATC HDOG RESET

COUNTE R

RESET

ST6

INTERNAL

RESET

fOSC

RESET

ON RESET

VA0200B

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