Boot configuration AN3216
18/30 Doc ID 17496 Rev 5
4 Boot configuration
4.1 Boot mode selection
In the STM32L1xxx, three different boot modes can be selected by means of the BOOT[1:0]
pins as shown in Table 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.
BOOT0 is a dedicated pin while BOOT1 is shared with a GPIO pin. Once BOOT1 has been
sampled, the corresponding GPIO pin is free and can be used by the application.
The BOOT pins are also resampled when exiting Standby mode. Consequently, they must
be kept in the required Boot mode configuration in 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.
4.2 Boot pin connection
Figure 11 shows the external connection required to select the boot memory of the
STM32L1xxx.
Figure 11. 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
AIB
6
$$
34-,XXX
"//4
"//4
6
$$
K½
K½
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