RM0008 Independent watchdog (IWDG)
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16.3.1 Hardware watchdog
If the “Hardware watchdog” feature is enabled through the device option bits, the watchdog
is automatically enabled at power-on, and will generate a reset unless the Key register is
written by the software before the counter reaches end of count.
16.3.2 Register access protection
Write access to the IWDG_PR and IWDG_RLR registers is protected. To modify them, you
must first write the code 0x5555 in the IWDG_KR register. A write access to this register
with a different value will break the sequence and register access will be protected again.
This implies that it is the case of the reload operation (writing 0xAAAA).
A status register is available to indicate that an update of the prescaler or the down-counter
reload value is on going.
16.3.3 Debug mode
When the microcontroller enters debug mode (Cortex-M3 core halted), the IWDG counter
either continues to work normally or stops, depending on DBG_IWDG_STOP configuration
bit in DBG module. For more details, refer to Section 26.15.2: Debug support for timers,
watchdog, bxCAN and I2C.
Figure 154. Independent watchdog block diagram
Note: The watchdog function is implemented in the V
DD
voltage domain that is still functional in
Stop and Standby modes.
IWDG RESET
Prescaler
12-bit Down-counter
IWDG_PR
Prescaler Register
IWDG_RLR
Reload Register
8-bit
LSI
IWDG_KR
Key Register
1.8 V voltage domain
VDD voltage domain
IWDG_SR
Status Register
12-bit reload value
(40 kHz)
Table 63. Watchdog timeout period (with 40 kHz input clock)
(1)
1. These timings are given for a 40 kHz clock but the microcontroller’s internal RC frequency can vary from
30 to 60 kHz. Moreover, given an exact RC oscillator frequency, the exact timings still depend on the
phasing of the APB interface clock versus the RC oscillator 40 kHz clock so that there is always a full RC
period of uncertainty.
Prescaler divider PR[2:0] bits
Min timeout (ms)
RL[11:0]= 0x000
Max timeout (ms)
RL[11:0]= 0xFFF
/4 0 0.1 409.6
/8 1 0.2 819.2
/16 2 0.4 1638.4
/32 3 0.8 3276.8
/64 4 1.6 6553.6
/128 5 3.2 13107.2
/256 6 (or 7) 6.4 26214.4
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