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Independent watchdog (IWDG)
RM0008
17.3.1
17.3.2
17.3.3
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.
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.
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 29.16.2: Debug support for timers,
watchdog, bxCAN and I 2 C .
Figure 158. Independent watchdog block diagram
1.8 V voltage domain
Prescaler register
IWDG_PR
8-bit
LSI
(40 kHz) prescaler
Status register
IWDG_SR
Reload register
IWDG_RLR
12-bit reload value
Key register
IWDG_KR
IWDG RESET
12-bit downcounter
V DD voltage domain
Note:
Table 81.
The watchdog function is implemented in the V DD voltage domain that is still functional in
Stop and Standby modes.
Watchdog timeout period (with 40 kHz input clock)Min/max IWDG timeout
period at 32 kHz (LSI) (1)
Prescaler divider
/4
/8
/16
/32
/64
/128
/256
PR[2:0] bits
0
1
2
3
4
5
6 (or 7)
Min timeout (ms) RL[11:0]= 0x000
0.1
0.2
0.4
0.8
1.6
3.2
6.4
Max timeout (ms) RL[11:0]= 0xFFF
409.6
819.2
1638.4
3276.8
6553.6
13107.2
26214.4
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 LSI clock so that there is always a full RC period of uncertainty.
400/995
Doc ID 13902 Rev 9
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