Datasheet

Description STM32F105xx, STM32F107xx
14/104 Doc ID 15274 Rev 6
2.3.6 External interrupt/event controller (EXTI)
The external interrupt/event controller consists of 20 edge detector lines used to generate
interrupt/event requests. Each line can be independently configured to select the trigger
event (rising edge, falling edge, both) and can be masked independently. A pending register
maintains the status of the interrupt requests. The EXTI can detect an external line with a
pulse width shorter than the Internal APB2 clock period. Up to 80 GPIOs can be connected
to the 16 external interrupt lines.
2.3.7 Clocks and startup
System clock selection is performed on startup, however, the internal RC 8 MHz oscillator is
selected as default CPU clock on reset. An external 3-25 MHz clock can be selected, in
which case it is monitored for failure. If failure is detected, the system automatically switches
back to the internal RC oscillator. A software interrupt is generated if enabled. Similarly, full
interrupt management of the PLL clock entry is available when necessary (for example with
failure of an indirectly used external oscillator).
A single 25 MHz crystal can clock the entire system including the ethernet and USB OTG
FS peripherals. Several prescalers and PLLs allow the configuration of the AHB frequency,
the high speed APB (APB2) and the low speed APB (APB1) domains. The maximum
frequency of the AHB and the high speed APB domains is 72 MHz. The maximum allowed
frequency of the low speed APB domain is 36 MHz. Refer to Figure 55: USB OTG FS +
Ethernet solution on page 96.
The advanced clock controller clocks the core and all peripherals using a single crystal or
oscillator. In order to achieve audio class performance, an audio crystal can be used. In this
case, the I
2
S master clock can generate all standard sampling frequencies from 8 kHz to
96 kHz with less than 0.5% accuracy error. Refer to Figure 56: USB OTG FS + I2S (Audio)
solution on page 96.
To configure the PLLs, please refer to Table 63 on page 97, which provides PLL
configurations according to the application type.
2.3.8 Boot modes
At startup, boot pins are used to select one of three boot options:
● Boot from User Flash
● Boot from System Memory
● Boot from embedded SRAM
The boot loader is located in System Memory. It is used to reprogram the Flash memory by
using USART1, USART2 (remapped), CAN2 (remapped) or USB OTG FS in device mode
(DFU: device firmware upgrade). For remapped signals refer to Table 5: Pin definitions.
The USART peripheral operates with the internal 8 MHz oscillator (HSI), however the CAN
and USB OTG FS can only function if an external 8 MHz, 14.7456 MHz or 25 MHz clock
(HSE) is present.
For full details about the boot loader, please refer to AN2606.