RTC prescaler adjustment with LSI Measurements example

  ******************** (C) COPYRIGHT 2014 STMicroelectronics *******************
  * @file    RTC/RTC_LSI/readme.txt 
  * @author  MCD Application Team
  * @version V1.4.0
  * @date    24-July-2014
  * @brief   Description of the RTC prescaler adjustment with LSI Measurements example.
  ******************************************************************************
  *
  * Licensed under MCD-ST Liberty SW License Agreement V2, (the "License");
  * You may not use this file except in compliance with the License.
  * You may obtain a copy of the License at:
  *
  *        http://www.st.com/software_license_agreement_liberty_v2
  *
  * Unless required by applicable law or agreed to in writing, software 
  * distributed under the License is distributed on an "AS IS" BASIS, 
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
  ******************************************************************************
   

Example Description

This example demonstrates and explains how to use the LSI clock source auto calibration to get a precise RTC clock. As an application example, it demonstrates how to configure the TIM14 timer internally connected to LSI clock output, in order to adjust the RTC prescaler.

The Low Speed Internal (LSI) clock is used as RTC clock source. After reset, the RTC prescaler (asynchronous and synchronous) is set with the default LSI frequency (40KHz). The RTC Alarm is configured to generate an interrupt each 1s. LED1 is toggled inside the RTC WakeUp interrupt each 1s.

The inaccuracy of the LSI clock causes the RTC Alarm Interrupt to be inaccurate. The RTC CK_SPRE signal can be monitored by LED1 which is toggled into the RTC Alarm_A interrupt service routine.

The program waits until TAMPER Push button is pressed to begin the auto calibration procedure:

• Configure the TIM14 to remap internally the TIM14 Channel 1 Input Capture to the LSI clock output.
• Enable the TIM14 Input Capture interrupt: after one cycle of LSI clock, the period value is stored in a variable and compared to the HCLK clock to get its real value.
• The RTC prescaler is adjusted with this LSI frequency value so that the RTC CK_SPRE value become more accurate.
• When calibration is done LED2 is turned ON to indicate the end of this operation.

Directory contents

• RTC/RTC_LSI/stm32f0xx_conf.h Library Configuration file
• RTC/RTC_LSI/stm32f0xx_it.c Interrupt handlers
• RTC/RTC_LSI/stm32f0xx_it.h Interrupt handlers header file
• RTC/RTC_LSI/main.c Main program
• RTC/RTC_LSI/main.h Main program header file
• RTC/RTC_LSI/system_stm32f0xx.c STM32F0xx system source file

Note:

The "system_stm32f0xx.c" is generated by an automatic clock configuration tool and can be easily customized to meet user application requirements. To select different clock setup, use the "STM32F0xx_Clock_Configuration_VX.Y.Z.xls" provided with the AN4055 package available on ST Microcontrollers

Hardware and Software environment

• This example runs on STM32F0xx devices.

• This example has been tested with STMicroelectronics STM320518-EVAL and STM32072B-EVAL including respectively STM32F051R8T6 and STM32F072VBT6 devices and can be easily tailored to any other supported device and development board and development board.

• STM320518-EVAL Set-up
  • Use TAMPER push button for RTC calibration

• STM32072B-EVAL Set-up
  • Use TAMPER push button for RTC calibration

How to use it ?

In order to make the program work, you must do the following :

• Copy all source files from this example folder to the template folder under Project
• Open your preferred toolchain
• If the used device is STM32F051R8T6 choose STM32F051 project
  • Add the following files to the project source list
    • Utilities\STM32_EVAL\STM320518_EVAL\stm320518_eval.c

• If the used device is STM32F072VBT6 choose STM32F072 project
  • Add the following files to the project source list
    • Utilities\STM32_EVAL\STM32072B_EVAL\stm32072b_eval.c
• Rebuild all files and load your image into target memory
• Run the example

© COPYRIGHT STMicroelectronics