36. System window watchdog (WWDG)

36.1 Introduction

The system window watchdog (WWDG) is used to detect the occurrence of a software fault, usually generated by external interference or by unforeseen logical conditions, which causes the application program to abandon its normal sequence.

The watchdog circuit generates an MCU reset on expiry of a programmed time period, unless the program refreshes the contents of the down-counter before the T6 bit is cleared. An MCU reset is also generated if the 7-bit down-counter value (in the control register) is refreshed before the down-counter reaches the window register value. This implies that the counter must be refreshed in a limited window.

The WWDG clock is prescaled from the APB clock and has a configurable time window that can be programmed to detect abnormally late or early application behavior.

The WWDG is best suited for applications requiring the watchdog to react within an accurate timing window.

36.2 WWDG main features

• • Programmable free-running down-counter
• • Conditional reset
  • – Reset (if watchdog activated) when the down-counter value becomes lower than 0x40
  • – Reset (if watchdog activated) if the down-counter is reloaded outside the window (see Figure 531 )
• • Early wake-up interrupt (EWI): triggered (if enabled and the watchdog activated) when the down-counter is equal to 0x40

36.3 WWDG functional description

If the watchdog is activated (the WDGA bit is set in the WWDG_CR register), and when the 7-bit down-counter (T[6:0] bits) is decremented from 0x40 to 0x3F (T6 becomes cleared), it initiates a reset. If the software reloads the counter while the counter is greater than the value stored in the window register, then a reset is generated.

The application program must write in the WWDG_CR register at regular intervals during normal operation to prevent an MCU reset. This operation can take place only when the counter value is lower than or equal to the window register value, and higher than 0x3F. The value to be stored in the WWDG_CR register must be between 0xFF and 0xC0.

Refer to Figure 530 for the WWDG block diagram.

36.3.1 WWDG block diagram

Figure 530. Watchdog block diagram

36.3.2 Enabling the watchdog

When the user option WWDG_SW selects “Software window watchdog”, the watchdog is always disabled after a reset. It is enabled by setting the WDGA bit in the WWDG_CR register, then it cannot be disabled again, except by a reset.

When the user option WWDG_SW selects “Hardware window watchdog”, the watchdog is always enabled after a reset, it cannot be disabled.

36.3.3 Controlling the down-counter

This down-counter is free-running, counting down even if the watchdog is disabled. When the watchdog is enabled, the T6 bit must be set to prevent generating an immediate reset.

The T[5:0] bits contain the number of increments that represent the time delay before the watchdog produces a reset. The timing varies between a minimum and a maximum value, due to the unknown status of the prescaler when writing to the WWDG_CR register (see Figure 531 ). The WWDG configuration register (WWDG_CFR) contains the high limit of the window: to prevent a reset, the down-counter must be reloaded when its value is lower than or equal to the window register value, and greater than 0x3F. Figure 531 describes the window watchdog process.

Note: The T6 bit can be used to generate a software reset (the WDGA bit is set and the T6 bit is cleared).

36.3.4 How to program the watchdog timeout

Use the formula in Figure 531 to calculate the WWDG timeout.

Warning: When writing to the WWDG_CR register, always write 1 in the T6 bit to avoid generating an immediate reset.

Figure 531. Window watchdog timing diagram

The formula to calculate the timeout value is given by:

where:

• • \( t_{WWDG} \) : WWDG timeout
• • \( t_{PCLK} \) : APB clock period measured in ms
• • 4096: value corresponding to internal divider

As an example, if APB frequency is 48 MHz, WDGTB[2:0] is set to 3, and T[5:0] is set to 63:

Refer to the datasheet for the minimum and maximum values of \( t_{\text{WWDG}} \) .

36.3.5 Debug mode

When the device enters debug mode (processor halted), the WWDG counter either continues to work normally or stops, depending on the configuration bit in DBG module. For more details, refer to Section 47.16.2: Debug support for timers, RTC, watchdog and I ² C .

36.4 WWDG interrupts

The early wake-up interrupt (EWI) can be used if specific safety operations or data logging must be performed before the reset is generated. To enable the early wake-up interrupt, the application must:

• • Write EWIF bit of WWDG_SR register to 0, to clear unwanted pending interrupt
• • Write EWI bit of WWDG_CFR register to 1, to enable interrupt

When the down-counter reaches the value 0x40, a watchdog interrupt is generated, and the corresponding interrupt service routine (ISR) can be used to trigger specific actions (such as communications or data logging), before resetting the device.

In some applications, the EWI interrupt can be used to manage a software system check and/or system recovery/graceful degradation, without generating a WWDG reset. In this case the corresponding ISR must reload the WWDG counter to avoid the WWDG reset, then trigger the required actions.

The watchdog interrupt is cleared by writing 0 to the EWIF bit in the WWDG_SR register.

Note: When the watchdog interrupt cannot be served (for example due to a system lock in a higher priority task), the WWDG reset is eventually generated.

36.5 WWDG registers

Refer to Section 1.2: List of abbreviations for registers for a list of abbreviations used in register descriptions.

The peripheral registers can be accessed by halfwords (16-bit) or words (32-bit).

36.5.1 WWDG control register (WWDG_CR)

Address offset: 0x000

Reset value: 0x0000 007F

Bits 31:8 Reserved, must be kept at reset value.

Bit 7 WDGA : Activation bit

This bit is set by software and only cleared by hardware after a reset. When WDGA = 1, the watchdog can generate a reset.
0: Watchdog disabled
1: Watchdog enabled

Bits 6:0 T[6:0] : 7-bit counter (MSB to LSB)

These bits contain the value of the watchdog counter, decremented every \( (4096 \times 2^{\text{WDGTB}[2:0]}) \) PCLK cycles. A reset is produced when it is decremented from 0x40 to 0x3F (T6 becomes cleared).

36.5.2 WWDG configuration register (WWDG_CFR)

Address offset: 0x004

Reset value: 0x0000 007F

Bits 31:14 Reserved, must be kept at reset value.

Bits 13:11 WDGTB[2:0] : Timer base

The timebase of the prescaler can be modified as follows:

• 000: CK counter clock (PCLK div 4096) div 1
• 001: CK counter clock (PCLK div 4096) div 2
• 010: CK counter clock (PCLK div 4096) div 4
• 011: CK counter clock (PCLK div 4096) div 8
• 100: CK counter clock (PCLK div 4096) div 16
• 101: CK counter clock (PCLK div 4096) div 32
• 110: CK counter clock (PCLK div 4096) div 64
• 111: CK counter clock (PCLK div 4096) div 128

Bit 10 Reserved, must be kept at reset value.

Bit 9 EWI : Early wake-up interrupt enable

Set by software and cleared by hardware after a reset. When set, an interrupt occurs whenever the counter reaches the value 0x40.

Bits 8:7 Reserved, must be kept at reset value.

Bits 6:0 W[6:0] : 7-bit window value

These bits contain the window value to be compared with the down-counter.

36.5.3 WWDG status register (WWDG_SR)

Address offset: 0x008

Reset value: 0x0000 0000

Bits 31:1 Reserved, must be kept at reset value.

Bit 0 EWIF : Early wake-up interrupt flag

This bit is set by hardware when the counter has reached the value 0x40. It must be cleared by software by writing 0. Writing 1 has no effect. This bit is also set if the interrupt is not enabled.

36.5.4 WWDG register map

The following table gives the WWDG register map and reset values.

Table 336. WWDG register map and reset values

Refer to Section 2.2 for the register boundary addresses.