Counter with STM8L101xx low-power microcontroller
Contents
1. only 2 capacitors on top of mandatory parts battery LCD MCU buttons Low cost PCB single layer RoHS compliant Used components 1x STM8L101 1x 3 digit LCD glass 23 segments single common electrode 1x ceramic capacitor 47 nF on supply lines 1x ceramic capacitor 10 nF on RESET line 2x button 1x battery CR1220 1x PCB 1x SWIM connector only for evaluation purpose 1x jumper only for evaluation purpose ammeter connection Doc ID 16240 Rev 1 3 11 Main features UM0789 1 2 Circuit schematic Figure 2 STEVAL IAS003V1 schematic diagram pe ig ta 31a CECR Div E 2 4 Bn ET P S Cl IE 1 a na Aig GC ee N x7 Ze e Speer 2C DE o Ba a NG ap 40 1E a2 Ve ae D F a G S i 1c aa 3 GE a A o Bum B OM 1 ER GE P Ei W Ve Vs SR S D ul SMa wer EE E E 3 C2 p Al D P a eo KT vad Po 1 e 7 Ms e m A NEST Ons Batey Header P 066 DR i it Ve 2 3 4 Header AM03833v1 4 11 Doc ID 16240 Rev 1 D UM0789 Basic principles 2 Basic principles The main task of this demonstration board is to drive an LCD display There are different ways to drive an LCD the LCD can feature an integrated driver which communicates via an SPI bus or it can be controlled directly through its segments This application directly drives the LCD segments LCD glasses have plenty of segment electrode2. er UM0789 vd User manual Counter with STM8L101xx low power microcontroller and LCD Introduction The demonstration board STEVAL IASO03V1 has been designed to be used as example for applications where directly driven LCD and associated programmable functionality are needed Targeted applications are medical healthcare battery operated counters metering among many others The board demonstrates STM8L101xx microcontroller low power consumption The demonstration firmware is consequently written to take advantage of the low power modes available on this device It allows to achieve an average current consumption lower than 1 6 pA for the whole application including the direct LCD software driver Figure 1 STEVAL IAS003V1 demonstration board LYS Leet lil CO xO yes PET 2 4 Gd gt Q LJ ONLY FO 1 DEMOBOA September 2009 Doc ID 16240 Rev 1 1 11 www st com Contents UM0789 Contents 1 Main features 0 00 cee ee 3 1 1 Used Keefer enn EEN 3 1 2 Circuit schematic 0 00 00 ee ee ees 4 2 Basic principles EE 5 3 Additional features EEN 7 3 1 ege EEN 8 4 CONCIUSION een reel E EA E 9 5 Revision history da ch ER AANER EE eb ena e e 10 2 11 Doc ID 16240 Rev 1 ky UM0789 Main features 1 1 Main features 1 6 pA consumption 36 Hz refresh rate 3 digit LCD glass driven by software 2 buttons CR1220 battery operated Minimum external components
3. a button check Once the MCU has entered the deepest low power mode Active halt or Halt slowing down the core results in higher consumption than executing the code at high speed Faster code execution changes the ratio between Run mode and Active halt Halt mode resulting in an overall lower consumption The left button is associated with ISR code located in RAM memory To slow down the core and achieve lower consumption the clock frequency is divided by a factor of 8 The ISR consequently contains a single while loop performing a sequence of NOP instructions and button check The approximate consumption in this mode ranges from 350 to 380 pA The right button is associated with ISR code located in Flash memory main loop The core runs at high frequency while the instructions are executed To save power a brand new Wait for event mode is used In this mode the core is stopped and waits for an event which is associated with a peripheral in this case timer 2 TIM2 TIM2 generates a delay which replaces the while loop implemented in the Left button ISR Once the MCU has entered low power mode the clock frequency is divided In this mode the approximate consumption ranges from 240 to 270 pA Once button is pressed it will ground an associated pin This pin has an internal pull up of around 45 kQ connected to Vcc It means that a current of 67 pA is drawn from the battery by the I Os To save on power consumption a dynamic connection
4. of this pull up is implemented The pull up is set only for a short period of time when the software performs a check on the button It is then disabled This sequence is repeated every 6 4 ms It dramatically reduces the current consumption on the I Os The usual problem of extra consumption generated by the toggling of pins left floating does not occur in this situation a proper level is guaranteed while the button is pressed except for a few ms when the button is released This time does not exceed one period of button state check Timing is managed by TIM2 or by a software delay Doc ID 16240 Rev 1 7 11 Additional features UM0789 3 1 8 11 LCD decoding To allow using a single layer PCB the LCD display is connected directly to IOs in the simplest way from design point of view As a result the software decodes directly the characters to control the corresponding lOs The following define statements must be used to associate the active segments with each character to be shown on the LCD define LCD_CHAR_0 LCD_A LCD_B LCD_C LCD_D LCD_E LCD_F The above definitions are stored in the LCD_VALUE array which contains all possible character definitions Simple indexing could be used to read the character definitions These definitions are then used by the decoding function to set the appropriate STM8L101xx pins to 0 invisible segment or to 1 visible segment To update the LCD a simple inversion of al
5. TM8 pipeline prefetch buffers efficiently Doc ID 16240 Rev 1 5 11 Basic principles UM0789 6 11 To save power a new feature has been implemented on STM8L microcontrollers when the AL bit is set in the global configuration register CFG_GCR and the MCU is woken up from low power mode it goes back to low power mode as soon as the interrupt service routine ISR execution is complete There is no need to restore the context return to the main program and execute a HALT instruction again Doc ID 16240 Rev 1 ky UM0789 Additional features 3 Additional features Each button corresponds to a different ISR which allows to compare two possible solutions The functions performed by each ISR are identical but their implementation differs Both ISRs update the value shown on the LCD refresh the LCD as long as the button is pressed AWU is disabled during this time and perform a periodic test on the button Once the button is pressed the MCU is woken up as long as the button remains pressed The MCU is consequently in full operating mode Several procedures to decrease power consumption have been used in demo application e slowing down core frequency when it brings an advantage moving code to the RAM when it brings an advantage e using WFE WFI mode dynamic pull up configuration for buttons Slowing down the core frequency brings an advantage mainly if the MCU performs periodically the same operation such as
6. l IOs is required The common electrode is set to 0 during LCD update For LCD update a simple inversion of all I O s is used Doc ID 16240 Rev 1 ky UM0789 Conclusion 4 Conclusion This example could be used for many different applications User could modify the attached software easily to implement any additional functionalities and features Doc ID 16240 Rev 1 9 11 Revision history UM0789 5 10 11 Revision history Table 1 Document revision history Date Revision Changes 17 Sep 2009 1 Initial release Doc ID 16240 Rev 1 UM0789 Please Read Carefully Information in this document is provided solely in connection with ST products STMicroelectronics NV and its subsidiaries ST reserve the right to make changes corrections modifications or improvements to this document and the products and services described herein at any time without notice All ST products are sold pursuant to ST s terms and conditions of sale Purchasers are solely responsible for the choice selection and use of the ST products and services described herein and ST assumes no liability whatsoever relating to the choice selection or use of the ST products and services described herein No license express or implied by estoppel or otherwise to any intellectual property rights is granted under this document If any part of this document refers to any third party products or services it sha
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8. nd the ST logo are trademarks or registered trademarks of ST in various countries Information in this document supersedes and replaces all information previously supplied The ST logo is a registered trademark of STMicroelectronics All other names are the property of their respective owners 2009 STMicroelectronics All rights reserved STMicroelectronics group of companies Australia Belgium Brazil Canada China Czech Republic Finland France Germany Hong Kong India Israel Italy Japan Malaysia Malta Morocco Philippines Singapore Spain Sweden Switzerland United Kingdom United States of America www st com ky Doc ID 16240 Rev 1 14 11
9. s and one or several common electrodes To make a character or segment visible a voltage level has to be applied between the segment and the appropriate common electrode To keep the character or segment invisible a voltage level equal to the common electrode voltage must be applied to segment 1 To prevent electrolytic damage on liquid crystals an AC voltage has to be applied on the electrodes As only DC supply voltage is available on the board the MCU must change regularly the voltage polarity between the segment and the common electrode Figure 3 COM stands for common segment SEG1 for segment 1 and SEG2 for segment 2 COM Vdd Vss SEG1 Vdd Vss SEG2 Vdd Vss AM03835v1 For more details about LCD drive please refer to AN1048 and AN2687 In order to minimize current consumption the STM8L101xx is kept in Active halt low power mode most of the time The MCU is periodically woken up from this mode by an auto wakeup timer This timer is configured to wake up the MCU by generating an interrupt every 28 ms to achieve an LCD refresh rate of 36 Hz This frequency is the best compromise between consumption which increases with the frequency and flickering which appears at low refresh rates The interrupt service routine performs the toggling of LCD control signals to make sure there is no DC bias between segments and common electrodes The routine placement is aligned with memory organization to fill S
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