Philips LPC2129 - WATCHDOG

Watchdog 256 May 03, 2004
Philips Semiconductors Preliminary User Manual
LPC2119/2129/2194/2292/2294ARM-based Microcontroller
19. WATCHDOG
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
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 "Reset"
and "Boot Control on 144-pin Package" sections of this document.
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 0x00000000 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.