WWDT General description

The purpose of the Watchdog Timer is to reset or interrupt the microcontroller within a programmable time if it enters an erroneous state. When enabled, a watchdog reset is generated if the user program fails to feed (reload) the Watchdog within a predetermined amount of time.

When a watchdog window is programmed, an early watchdog feed is also treated as a watchdog event. This allows preventing situations where a system failure may still feed the watchdog. For example, application code could be stuck in an interrupt service that contains a watchdog feed. Setting the window such that this would result in an early feed will generate a watchdog event, allowing for system recovery.

The Watchdog consists of a fixed (divide by 4) pre-scaler and a 24-bit counter which 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_ WDCLK × 256 × 4) and the maximum Watchdog interval is (T_ WDCLK × 2^ 24 × 4) in multiples of (T_ WDCLK × 4). The Watchdog should be used in the following manner:

When the Watchdog Timer is configured so that a watchdog event will cause a reset and the counter reaches zero, the CPU will be reset, loading the stack pointer and program counter from the vector table as for an 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.

When the Watchdog Timer is configured to generate a warning interrupt, the interrupt will occur when the counter matches the value defined by the WARNINT register.

Block diagram

The synchronization logic (PCLK - WDCLK) is not shown in the block diagram.

Windowed Watchdog timer block diagram

Clocking and power control

The watchdog timer block uses two clocks: PCLK and WDCLK. PCLK is used for the APB accesses to the watchdog registers and is derived from the system clock. The WDCLK is used for the watchdog timer counting and is derived from the watchdog oscillator.

The synchronization logic between the two clock domains works as follows: When the MOD and TC registers are updated by APB operations, the new value will take effect in 3 WDCLK cycles on the logic in the WDCLK clock domain.

When the watchdog timer is counting on WDCLK, the synchronization logic will first lock the value of the counter on WDCLK and then synchronize it with PCLK, so that the CPU can read the WDTV register.

Note: Because of the synchronization step, software must add a delay of three WDCLK clock cycles between the feed sequence and the time the WDPROTECT bit is enabled in the MOD register. The length of the delay depends on the selected watchdog clock WDCLK.

Using the WWDT lock features

The WWDT supports several lock features which can be enabled to ensure that the WWDT is running at all times:

  • Disabling the WWDT clock source
  • Changing the WWDT reload value