This application note presents the clock system configuration tool for the STM32L1xx microcontroller family. This purpose of this tool is to help user configure the microcontroller clocks, taking into consideration the product parameters (power supply, product voltage range, the Flash access mode.). The configuration tool is implemented in the “STM32L1xx_Clock_Configuration_VX.Y.Z.xls” file which is supplied with this application note and can be downloaded from www.st.com. It makes it easy to take care of all the following system aspects and interdependencies: Configure the system clock, HCLK source and output frequency; Configure the Core voltage range (VCORE); Configure the Flash latency (number of wait states depending on the HCLK frequency); Set the PCLK1, PCLK2 and TIMCLK (timer clocks) frequencies; Generate a ready-to-use system_stm32l1xx.c file with all the above settings (STM32L15x CMSIS Cortex-M3 Device Peripheral Access Layer System Source File).