System features

AN2945

10/18

Doc ID 15468

3

System features

Today’s MCUs are complex SoCs (systems on chip) that not only include a lot of peripherals,

but also advanced-system features aiming at reducing the bill of material or enhancing the

products’ safety and robustness. This is true for both 8- and 32-bit platforms.

3.1

Reset

As shown in Figure 3, the STM8S and STM32 devices have the same reset circuitry, with

only slight differences.

The NRST pin is both an input and an open-drain output with a built-in pull-up resistor. For

EMS (electromagnetic sensitivity) robustness purposes, a filter is inserted to avoid glitch

propagation into the digital circuitry. There are three advantages with having a bidirectional

reset:

●

for multi-MCU systems, bidirectional reset ensures than all subprocessors are correctly

synchronized at startup or in case of a warm reset

●

the voltage supervisors (power-on reset and brownout reset) embedded in the MCU

can also be used at system level for other ICs

●

it is of a great help during debugging when spurious internal resets are generated

Figure 3.

STM8S and STM32 reset circuitries

NRST

WWDG reset

IWDG reset

Pulse

generator

Power reset

External

Reset

(min 20 µs)

System reset

Filter

Software reset

Low-power management reset

NRST

SWIM reset

External

reset

(min 20 µs)

System reset

Illegal opcode reset

EMS reset

IWDG/WWDG/software reset

POR/BOR reset

Filter

(typ 45k

Ω)

STM8S

STM32

Pulse

generator

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