ST7260 datasheet(36/117 Pages) STMICROELECTRONICS

36 / 117 page

ST7260

36/117

11 ON-CHIP PERIPHERALS

11.1 WATCHDOG TIMER (WDG)

11.1.1 Introduction

The Watchdog timer is used to detect the occur-

rence of a software fault, usually generated by ex-

ternal interference or by unforeseen logical condi-

tions, which causes the application program to

abandon its normal sequence. The Watchdog cir-

cuit generates an MCU reset on expiry of a pro-

grammed time period, unless the program refresh-

es the counter’s contents before the T6 bit be-

comes cleared.

11.1.2 Main Features

■ Programmable

free-running

counter

(64

increments of 49,152 CPU cycles)

■ Programmable reset

■ Reset (if watchdog activated) when the T6 bit

reaches zero

■ Optional

reset

HALT

instruction

(configurable by option byte)

■ Hardware Watchdog selectable by option byte.

11.1.3 Functional Description

The counter value stored in the CR register (bits

T6:T0), is decremented every 49,152 machine cy-

cles, and the length of the timeout period can be

programmed by the user in 64 increments.

If the watchdog is activated (the WDGA bit is set)

and when the 7-bit timer (bits T6:T0) rolls over

from 40h to 3Fh (T6 becomes cleared), it initiates

a reset cycle pulling low the reset pin for typically

30µs.

The application program must write in the CR reg-

ister at regular intervals during normal operation to

prevent an MCU reset. This downcounter is free-

running: it counts down even if the watchdog is

disabled. The value to be stored in the CR register

must be between FFh and C0h (see Table 15, ".

Watchdog Timing (fCPU = 8 MHz)"):

– The WDGA bit is set (watchdog enabled)

– The T6 bit is set to prevent generating an imme-

diate reset

– The T5:T0 bits contain the number of increments

which represents the time delay before the

watchdog produces a reset.

Figure 23. Watchdog Block Diagram

RESET

WDGA

7-BIT DOWNCOUNTER

fCPU

CLOCK DIVIDER

WATCHDOG CONTROL REGISTER (CR)

÷49152

ST7260 датащи(PDF) 36 Page - STMicroelectronics

ST7260 датащи(HTML) 36 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL