Page 84 - ARM Based Microcontroller Projects Using MBED

70 6. ARCHITECTURE OF THE STM32F411RET6 MICROCONTROLLER

FIG. 6.4 System clock sources.

Low-Speed External (LSE): This is a 32,768Hz clock driven from an external crystal and
feeding the internal RTC module.

Internal Clock Sources
High-Speed Internal (HSI): This is an accurate RC-based 16MHz internal clock with a fac-
tory calibrated tolerance of 1%.
Multispeed Internal (MSI RC): This is a multispeed RC-based clock source providing
clock in the range 100kHz–48MHz. This clock can be trimmed by software and is able to gen-
erate 12 different clock frequencies.
PLL: The PLL is fed by HSE, HSI16, or MSI clocks and it can generate system clocks up to
100MHz.

Microcontroller Clock Output (MCO)
A clock output is possible from a special pin called MCO. This clock output can be used as a
general purpose clock or as a clock for another microcontroller.

Low-Speed Clock Output (LSCO)
A low-speed (32,768kHz) clock (LSCO) is available as a general purpose clock output.
Several prescalers are available in the clock circuit to configure the AHB, APB1, and APB2
bus clock frequencies for the system and the peripheral devices. The maximum frequency of
the AHB and APBx is 80MHz.

Configuring the Clock
As shown in Figs. 6.5 and 6.6, the clock circuit consists of a number of multiplexers,
prescalers, and a PLL. The multiplexers are used to select the required clock source. The
prescalers are used to divide the clock frequency by a constant. Similarly, the PLL is used
to multiply the clock frequency with a constant in order to operate the chip at higher
frequencies.
It is important to select the correct clock source for an application. Configuring the clock
sources by programming the internal clock registers is a complex task and detailed