Preventive techniques

AN1015

6/19

DocID5833 Rev 2

2

Preventive techniques

You can implement preventive techniques in existing designs to improve product robustness

and immunity against external or internal EMC disturbance.

2.1

Using the watchdog and time control techniques

The watchdog is the most efficient tool available to ensuring that the MCU can recover from

software runaway failures. Its principle is very simple: it is a timer which generates an MCU

reset at the end of count. Once the watchdog is started, the only way of preventing the

watchdog from resetting the microcontroller is to update the counter periodically in the

program.

But to make the watchdog work at its full potential, you have to insert the enable and refresh

instructions in your software at the right place of code execution.

Figure 1 shows two classic examples of bad watchdog implementation.

To do it the right way (see Figure 2), the following rules should be implemented:



Enable the watchdog as soon as possible after reset, or use the Hardware option if

available.



Never refresh the watchdog during an interrupt routine or inside any local loop not

guarded by timeout at code.

It is very important to optimize the period between the two refresh instructions according to

the duration of the various routines, including the interrupt routines.

The minimum use of the watchdog resets the MCU, this means that the program execution

context is lost as well as the application data's integrity.

After reset, in addition to enabling the watchdog, on some MCUs you can use the reset flags

to distinguish between Power On or Low Voltage reset or watchdog reset (refer to

Section 3.3: Using the reset flags to identify the reset source. for more details).