ST62T85B datasheet(21/76 Pages) STMICROELECTRONICS

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ST62T85B/E85B

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 (locat-

ed in program ROM starting at address 0FFEh). A

jump to the beginning of the user program must be

coded at this address. Following a Reset, the In-

terrupt flag is automatically set, so that the CPU is

in Non Maskable Interrupt mode; this prevents the

initialisation routine from being interrupted. The in-

itialisation routine should therefore 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 initialisation routine, the

MCU will continue by processing the instruction

immediately following the RETI instruction. If, how-

ever, a pending interrupt is present, it will be serv-

iced.

Figure 11. Reset and Interrupt Processing

Figure 12. Reset Block Diagram

RESET

VECTOR

JP:2 BYTES/4 CYCLES

RETI

RETI: 1 BYTE/2 CYCLES

INITIAL IZATION

ROUTINE

VA00181

VDD

RESE T

300k

Ω

2.8k

Ω

POWER

WATCHDOG RESET

COUNTER

RESET

ST6

INTERNA L

RESET

fOSC

RESET

ON RESET

VA0200B

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