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september 2013 docid15625 rev 2 1/1 AN2970 application note stm8t14x proxsense? charge transfer capacitive sensing technology introduction this application note describes the proxsense? (a) charge transfer capacitive sensing technology used in stmicroelectr onics stm8t14x capacitive sensors. stm8t14x devices offer a state of the art capa citive sensing engine supporting both touch and proximity sensing. they are designed to easily replace conventional electromechanical switches in cost-sensitive applications. a. proxsense is a trademark from azoteq. table 1. applicable products type applicable products microcontrollers stm8t141, stm8t142 and stm8t143 www.st.com
contents AN2970 2/2 docid15625 rev 2 contents 1 capacitive sensing overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 sensing electrode capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 principles of charge transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 transfer rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
docid15625 rev 2 3/3 AN2970 list of tables 3 list of tables table 1. applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 table 2. charge transfer sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 table 3. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
list of figures AN2970 4/4 docid15625 rev 2 list of figures figure 1. coupling with hand increases the capacitance of the sensing electrode . . . . . . . . . . . . . . . 5 figure 2. depiction of channel charge-transfer hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. charge cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 4. voltage across c x over a charge cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 5. ideal transfer cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 6. ideal charge cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 7. non-ideal transfer cyc les . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 8. charge cycle resulting from non-ideal transfer cycle s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
docid15625 rev 2 5/10 AN2970 capacitive sensing overview 10 1 capacitive sensing overview 1.1 sensing electrode capacitance a capacitance exists between any reference point relative to ground as long as they are electrically isolated. if this reference point is a sensing electrode, it helps to think of it as a capacitor. the positive electrode of the capaci tor is the sensing electrode, and the negative elerode is formed by the surrounding area (virtual ground reference, labeled 1 in figure 1 ). figure 1. coupling with hand increases the capacitance of the sensing electrode when a conductive object is brought into pr oximity of the sensing electrode, the coupling between the object and the electrode increases, together with the capacitance of the sensing electrode relative to ground. for example, a human hand will increase the sensing electrode capacitance as it approaches it. touc hing the dielectric panel that protects the electrode increases its capacitance significantly. the sensing electrode can be made of any electr ically conductive material, such as copper on pcbs, or transparent conductive material like indium tin oxide (ito) deposited on glass or plexiglas. 1.2 principles of charge transfer the sensing electrode is connected to the c x pin of the stm8t14x device. the equivalent capacitance of the sensing electrode is referred to as c x . c x is fully charged with a stable reference voltage v dd . the charge on c x is then transferred to a sampling capacitor (c s ). c s capacitance is typically from 1000 to 100,000 times bigger than c x . the process is repeated until the voltage on c s reaches a threshold. this threshold is referred to as v trip . the number of transfer cycles required to reach the threshold represents the size of c x . refer to figure 4 and table 2 for a representation of the charge-transfer equivalent hardware and charge-transfer sequence for a given channel. 1 ai15544v2 sensing electrode lower capacitance higher capacitance 1 c x c t
capacitive sensing overview AN2970 6/10 docid15625 rev 2 figure 2. depiction of channel charge-transfer hardware 1. this pin is only available on the stm8t141. on stm8t142 and stm8t143 devices, the sampling capacitor is integrated within the product. the transfer cycle refers to the charging of c x and the transfer of the charge to the c s capacitor. the charge cycle refers to process of charging c s to v trip using a sequence of transfer cycles. the charge cycle duration refers to the time needed to complete one c s charge cycle when no proximity or touch (thu s the longest duration of a charge cycle with the current system parameters). it is graphically illustrated in figure 3 . table 2. charge transfer sequence (1) 1. step 2 to 7 are repeated until the voltage across c s reaches v trip threshold. step switch s3 switch s2 switch s1 description 1101 c s discharge 2000 deadtime 3010 charge cycle (c x charge) 4000 deadtime 5 0 0 1 transfer cycle (charge transferred to c s ) 6000 deadtime 7101 c x discharge ms33000v2 stable reference c x stm8t14x capacitive sensing block s1 s2 s3 trip generated c s sensing electrode sampling capacitor stable v dd (1)
docid15625 rev 2 7/10 AN2970 capacitive sensing overview 10 figure 3. charge cycle the charge cycles can be probed from the c s pin (stm8t141 only) or from the c x pin. figure 4 and figure 5 show the evolution of the c x voltage during the charge and transfer cycles. figure 4. voltage across c x over a charge cycle figure 5. ideal transfer cycles in addition, the devices can compensate to environmental changes by tracking the average capacitance of the sensing electr ode. this average value is compared to the latest charge cycle to determine whether a proximity or touch occurred. t charge cycle duration ai15546 v trip cs pin voltage sampling period
capacitive sensing overview AN2970 8/10 docid15625 rev 2 1.3 transfer rate the transfer cycles can be perf ormed at a default ra te depending on the internal oscillator frequency. according to the device, this frequency can be modified either through otp option bytes. the oscillator is used to determ ine the rate at which the char ge transfers is performed. a maximum efficiency is achieved when eno ugh time is allowed to fully charge c x to v dd and completely transfer th is charge to the c s capacitor. a transfer rate between 100 khz and 250 khz is a good choice in normal operating conditions. the default transfer rate is in this range. a serial resistor r x connected to the c x pin negatively influences the transfer cycle. this resistor improves the conductive object electr ical isolation from the sensing electrode and provides additional esd protection for the device. typical r x value ranges from 1 to 2k ? . figure 5 and figure 6 show ideal charge cycles probed from the c x pin. in figure 5 , it can be noted that the sensing electrode charges up to v dd . in figure 6 , the charge cycle halts when v trip is reached. figure 6. ideal charge cycles figure 7 and figure 8 shows non-ideal charge transfers and the resulting charge cycle. it can be noted that the sensing electrode is charged up to 2.12 v instead of v dd . by comparing figure 8 to figure 6 , it can be noted that the offs et is due to the fact that a fraction of the sensing electrode c harge is not transferred to the c s capacitor. this can be corrected by decreasing either the transfer rate or r x .
docid15625 rev 2 9/10 AN2970 capacitive sensing overview 10 figure 7. non-ideal transfer cycles figure 8. charge cycle resulting from non-ideal transfer cycles note: attaching a probe to the sensing electrode increases the sensing electrode capacitance by a few picofarads, depending on the probe. this has an instant negative influence on the sensitivity of the system. after a short period of time, the system automatically adjusts to compensate this change.
revision history AN2970 10/10 docid15625 rev 2 2 revision history table 3. document revision history date revision changes 04-may-2009 1 initial release. 12-sep-2013 2 replaced ?stm8t and stm8ts? with ?stm8t14x?. updated introduction . updated figure 2: depiction of channel charge-transfer hardware . added table of content (toc), list of tables (lot), list of figures (lof). added table 1: applicable products . updated titles: figure 1: coupling with hand increases the capacitance of the sensing electrode . figure 7: non-ideal transfer cycles . figure 8: charge cycle resultin g from non-ideal transfer cycles . transfer rate updated as 250 khz in section 1.3: transfer rate .
docid15625 rev 2 11/11 AN2970 11 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 he rein 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 as sumes 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. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a parti cular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. st products are not authorized for use in weapons. nor are st products designed or authorized for use in: (a) safety critical applications such as life supporting, active implanted devices or systems with product functional safety requirements; (b) aeronautic applications; (c) automotive applications or environments, and/or (d) aerospace applications or environments. where st products are not designed for such use, the purchaser shall use products at purchaser?s sole risk, even if st has been informed in writing of such usage, unless a product is expressly designated by st as being intended for ?automotive, automotive safety or medical? industry domains acco rding to st product design specifications. products formally escc, qml or jan qualified are deemed suitable for use in aerospace by the corresponding governmental agency. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and 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. ? 2013 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - philippines - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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