© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
User manual
Rev. 01 — 15 August 2005
287
20.1 Features
•
Internally resets chip if not periodically reloaded.
•
Debug mode.
•
Enabled by software but requires a hardware reset or a watchdog reset/interrupt to be
disabled.
•
Incorrect/Incomplete feed sequence causes reset/interrupt if enabled.
•
Flag to indicate Watchdog reset.
•
Programmable 32-bit timer with internal pre-scaler.
•
Selectable time period from (T
PCLK
x 256 x 4) to (T
PCLK
x 2
32
x 4) in multiples of
T
PCLK
x 4.
20.2 Applications
The purpose of the watchdog is to reset the microcontroller within a reasonable amount of
time if it enters an erroneous state. When enabled, the watchdog will generate a system
reset if the user program fails to "feed" (or reload) the watchdog within a predetermined
amount of time.
For interaction of the on-chip watchdog and other peripherals, especially the reset and
boot-up procedures, please read
Section 3.10 “Reset” on page 38
20.3 Description
The watchdog consists of a divide by 4 fixed pre-scaler and a 32-bit counter. The clock is
fed to the timer via a pre-scaler. The timer decrements when clocked. The minimum value
from which the counter decrements is 0xFF. Setting a value lower than 0xFF causes 0xFF
to be loaded in the counter. Hence the minimum watchdog interval is (T
PCLK
x 256 x 4)
and the maximum watchdog interval is (T
PCLK
x 2
32
x 4) in multiples of (T
PCLK
x 4). The
watchdog should be used in the following manner:
•
Set the watchdog timer constant reload value in WDTC register.
•
Setup mode in WDMOD register.
•
Start the watchdog by writing 0xAA followed by 0x55 to the WDFEED register.
•
Watchdog should be fed again before the watchdog counter underflows to prevent
reset/interrupt.
When the Watchdog counter underflows, the program counter will start from 0x0000 0000
as in the case of external reset. The Watchdog Time-Out Flag (WDTOF) can be examined
to determine if the watchdog has caused the reset condition. The WDTOF flag must be
cleared by software.
UM10139
Chapter 20: Watchdog Timer
Rev. 01 — 15 August 2005
User manual