ST STM32F20 Series - Boot Configuration; Boot Mode Selection Methods; Boot Pin External Connections

Boot configuration AN3320
16/29 Doc ID 18267 Rev 2
3 Boot configuration
3.1 Boot mode selection
In the STM32F20xxx/21xxx, three different boot modes can be selected by means of the
BOOT[1:0] pins as shown in Tab l e 1 .
The values on the BOOT pins are latched on the 4
th
rising edge of SYSCLK after a reset. It
is up to the user to set the BOOT1 and BOOT0 pins after reset to select the required boot
mode.
The BOOT pins are also resampled when exiting the Standby mode. Consequently, they
must be kept in the required Boot mode configuration in the Standby mode. After this startup
delay has elapsed, the CPU fetches the top-of-stack value from address 0x0000 0000, and
starts code execution from the boot memory starting from 0x0000 0004.
3.2 Boot pin connection
Figure 10 shows the external connection required to select the boot memory of the
STM32F20xxx/21xxx.
Figure 10. Boot mode selection implementation example
1. Resistor values are given only as a typical example.
Table 1. Boot modes
BOOT mode selection pins
Boot mode Aliasing
BOOT1 BOOT0
x 0 Main Flash memory
Main Flash memory is selected as boot
space
0 1 System memory
System memory is selected as boot
space
1 1 Embedded SRAM
Embedded SRAM is selected as boot
space
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