參數(shù)資料
型號: PIC16C662T-20I/L
元件分類: 微控制器/微處理器
英文描述: 8-BIT, OTPROM, 20 MHz, RISC MICROCONTROLLER, PQCC44
封裝: PLASTIC, LCC-44
文件頁數(shù): 103/136頁
文件大?。?/td> 634K
代理商: PIC16C662T-20I/L
1996 Microchip Technology Inc.
Preliminary
DS30559A-page 69
PIC16C64X & PIC16C66X
9.7
Watchdog Timer (WDT)
The Watchdog Timer (WDT) is a free running on-chip
RC oscillator which does not require any external com-
ponents. The block diagram is shown in Figure 9-17.
This RC oscillator is separate from the RC oscillator of
the OSC1/CLKIN pin. This means that the WDT will
run, even if the clock on the OSC1 and OSC2 pins has
been stopped, for example, by execution of a SLEEP
instruction. During normal operation, a WDT time-out
generates a device RESET. If the device is in SLEEP
mode, a WDT time-out causes the device to wake-up
and continue with normal operation, this is known as a
WDT wake-up. The WDT can be permanently disabled
by clearing conguration bit WDTE (Section 9.1).
9.7.1
WDT PERIOD
The WDT has a nominal time-out period of 18 ms, (with
no prescaler). The time-out period varies with temper-
ature, VDD and process variations from part to part (see
DC specs). If longer time-outs are desired, a prescaler
with a division ratio of up to 1:128 can be assigned to
the WDT, under software control, by writing to the
OPTION register. Thus, time-out periods of up to 2.3
seconds can be realized.
The CLRWDT and SLEEP instructions clear the WDT and
the postscaler (if assigned to the WDT) and prevent it
from timing out and generating a device RESET.
The TO bit in the STATUS register will be cleared upon
a Watchdog Timer time-out (WDT Reset and WDT
wake-up).
9.7.2
WDT PROGRAMMING CONSIDERATIONS
It should also be taken in account that under worst case
conditions (VDD = Min., Temperature = Max., max.
WDT prescaler) it may take several seconds before a
WDT time-out occurs.
Note:
When the prescaler is assigned to the
WDT, always execute a CLRWDT instruction
before changing the prescale value, other-
wise a WDT reset may occur.
FIGURE 9-17: WATCHDOG TIMER BLOCK DIAGRAM
FIGURE 9-18: SUMMARY OF WATCHDOG TIMER REGISTERS
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
2007h
Cong. bits
MPEEN
BODEN(1)
CP1
CP0
PWRTE(1)
WDTE
FOSC1
FOSC0
81h
OPTION
RBPU
INTEDG
T0CS
T0SE
PSA
PS2
PS1
PS0
Legend: Shaded cells are not used by the Watchdog Timer.
Note 1: See Figure 9-1 for details of the operation of these bits.
From TMR0 Clock Source
(Figure 7-6)
To TMR0 (Figure 7-6)
Postscaler
WDT Timer
WDT
Enable Bit
0
1
M
U
X
PSA
8 - to - 1 MUX
PS2:PS0
0
1
MUX
PSA
WDT
Time-out
Note: PSA and PS2:PS0 are bits in the OPTION register.
8
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