AN2945

System features

Doc ID 15468

11/18

3.2

Clock

From the clock system standpoint, the two products have three main clock sources in

common, that share similar electrical characteristics. See Table 5 for details.

The oscillator handles both the crystal and resonators, and is called the HSE (for high-

speed external). It can also be bypassed to feed the MCU with an external clock. This is

used for applications that have stringent requirements in terms of accuracy and stability, for

communication purposes for instance.

An application can run at a high frequency without an external crystal by using the HSI clock

(for high-speed internal). This source has a consumption 10 times lower than the HSE and a

very low percentage of accuracy error. It can also be used as a PLL input on the STM32 to

increase the internal frequency to up to 64 MHz.

Finally, the low-speed internal clock (LSI) is an ultralow internal power source (a few µA),

that can be permanently enabled to clock an auto-wakeup peripheral during the Halt or Stop

mode. It can also clock a secondary on-board watchdog (refer to Section 3.4: Safety for

further details), and be used as the CPU clock on the STM8 products. It is not accurate

(error of a few tens of percents), but it can be measured periodically using the precise HSI

clock to compensate for chip manufacturing variations or the drift due to temperature for

instance.

3.3

Memory

Both product lines are based on non-volatile memories and have an option byte loader. This

mechanism replaces the legacy fuses for MCU power-up configuration: the user can select

several options at programming time, which are written alongside the program binary image.

Several features are available on all news microcontrollers:

●

Reset in Halt, Stop or Standby mode: this is to avoid a deadlock situation in case the

MCU enters a low power mode by accident, for applications not designed to handle

such a configuration

●

Hardware/Software watchdog, to have the possibility of starting the watchdog by

hardware, right after the reset sequence

●

Memory readout protection, to prevent any piracy on the program content

●

Memory write protection, to protect part of the memory, if it contains a critical code.

Usually, this applies to the boot code or an IAP (in application programming) driver

Table 5.

1.

Refer to product datasheet for detailed electrical characteristics.

System clock source

Frequency

Accuracy error

Consumption

STM8S

STM32

High-speed external (HSE)

1-24 MHz

4-16 MHz

Crystal dependent,

down to a few tens of

ppm

1 to 2 mA

High-speed internal (HSI)

16 MHz

8 MHz

1% typical

100 to 250 µA

Low-speed internal

110-146 kHz

30-60 kHz

20 to 50%

1 to 5 µA