TB044 Sensing Air Flow with the PIC16C781 Author: Ward Brown Microchip Technology Inc. INTRODUCTION Programmable Switch Mode Controllers (PSMC) are not just for switching power supplies. This technical brief describes how to use the PIC16C781 PSMC in combination with the Integrated Operational Amplifier, Digital-to-Analog Converter (DAC), and gated timer to construct a thermally operated air flow sensor with minimum external components. Theory of Operation Air flow is detected by the cooling effect of air movement across a heated resistor. The circuit schematic is shown in Figure 1. R5 and R7 are thin film platinum Resistance Temperature Detectors (RTD). These are essentially thermistors with a very linear temperature response. The flow sensor is comprised of R6 and R7. The bias on R7 is intentionally set below the bias on R5. R6 and R7 are thermally linked so that when R7 is heated by R6, the resistance of R7 increases. As R7 resistance increases, the voltage across R7 also increases until it matches the voltage across R5, at which time the Op Amp output will shut down the Programmable Switch Mode Controller (PSMC) and cease heating R6. As moving air cools R6, more power is required to heat the R6-R7 pair to maintain the same R7 resistance and voltage. Changes in ambient temperature conditions are compensated by two voltage dividers, R2-R5 and R1-R7. R2 and R5 form a voltage divider between the Op Amp output and the Op Amp inverting input. Similarly, R1 and R7 form a voltage divider between the variable DAC reference and the non-inverting Op Amp input. Since R5 and R7 are identical RTD's, resistance variations due to self heating, as well as changes in the ambient conditions, cancel out at the Op Amp inputs. R6 heat is controlled by a closed loop comprised of: • • • • • R7 Voltage Op Amp Comparator PSMC R6 driver Q1 2002 Microchip Technology Inc. R7 is heated by R6. If moving air cools R6, the amount of heat transferred to R7 is reduced. The resistance of R7 falls with the temperature. As R7 resistance falls, the voltage drop across R7 also falls. The Op Amp output is directly proportional to the voltage across R7. When the Op Amp output goes below VR, the comparator output goes high. The PSMC responds to the high comparator output by supplying drive pulses to Q1, thereby heating up R6. The temperature rise of R6 overcomes the cooling effect of moving air, and heat is transferred to R7 closing the loop. The PSMC is configured for pulse skipping. The control loop generates pulses until the temperature of R7, and the corresponding resistance, is high enough to disable the pulse drive. At equilibrium, the number of drive pulses match the heating requirement to keep the voltage at R7 equal to the voltage at R6. The DAC output is used to adjust the equilibrium point in still air by varying the bias on R7. At high bias levels, less heat is required by R6 to reach the equilibrium resistance level. Low required heating in still air means that there is plenty of headroom in the potential drive output, but this also means less variation due to cooling and thus low sensitivity. At low bias levels, more heat is required by R7. Greater heat means the effect of cooling is greater and, in turn, higher sensitivity. There is a limit to the drive available to R6 so that if the bias level is low enough the equilibrium resistance and voltage cannot be obtained. In other words, at low bias levels there is better sensitivity but less head room in potential heating drive. It was determined empirically that a good bias point is obtained when the Op Amp output is 100 mV below VR when R6 heating is inhibited. The power being delivered to R6 is proportional to the cooling effect of moving air. This power is measured by counting the average time that the R6 driver is enabled. The PIC16C781 has an integral Timer1 count enable input (Timer1 Gate). By connecting the PSMC output to the Timer1 Gate input, Timer1 will count only when the PSMC output is low. Average PSMC drive time is determined by clearing Timer1 then using Timer0 to wait a fixed period and reading Timer1 at the end of that period. Since the gate is low true, higher counts indicate that less power is being delivered to R6. A 10-segment LED bar graph is used to display relative air flow. The circuit shows how to drive ten segments with five outputs. Each microcontroller output is tied to two segments. When the output is high, one LED is DS91044A-page 1 TB044 driven. When the output is low, the other LED is driven. When the output is high-impedance, neither LED is driven. FIGURE 1: CIRCUIT SCHEMATIC FOR SENSING AIR FLOW WITH PIC16C781 TP3 RB0 DAC VR INTERNAL PIC16C781 SHOWN FOR REFERENCE RB1 R1 TIMER1 VR 100K RA0 RA1 C1 1000 pF + + OPA - PSMC - T1G RB7 PSMC1A RB6 RB3 R2 TP1 Thermally coupled 100K +5 VDC CYNTEC Part #SE102 R5 1K RTD R7 1K RTD R6 51 TP2 R3 Q1 2N2222 D1 2.2K 470 Ω(TYP) +5 VDC +5 VDC (ANALOG) +5 VDC PIC16C781 RA2 VDD C2 10 µF RA6 + 1 +5 VDC RA7 R8 47 K R9 470 Ω RB2 C6 RN1 RB4 0.1 µF 2 7 8 3 4 17 18 5 RN2 Bar Graph RESET SW Zeroing and Calibration The integral DAC makes automatic zeroing of the R7 bias current possible. While this process is in progress, the sensor should be in still air (no air flow). One LED flashes as a calibration-in-progress indicator. When the LED stops flashing, air flow may be resumed and measurements can begin. The first task after power-on initialization is to calibrate the Op Amp offset using the built-in Op Amp calibration utility of the PIC16C781. After Op Amp calibration, the DAC is initially set for about 3.0 volts output. The RTD temperatures are allowed to settle for 6 seconds, then the average PSMC drive time is measured using Timer1 and the Timer1 gate input. If the measured value is within plus or minus one display resolution of the expected zero value, then the zeroing routine is exited and measurement and display commences. If the measured value is outside of the expected window, the DAC is adjusted DS91044A-page 2 AVDD C3 0.1 µF C4 0.1 µF + C5 10 µF 9 10 19 20 11 12 13 RA0 RA1 RA2 RA3 RA4 MCLR RA6 RA7 RB0 RB1 RB2 RB3 RB4 RB5 RB6 RB7 VSS AVSS 6 14 Analog and Digital Grounds connect at single point up or down to compensate for the offset and, after the six second settling time, another measurement is taken. This process repeats until the desired R7 bias level has been obtained. SUMMARY This technical brief demonstrates how temperature changes resulting in milliohm differences can be measured quickly and accurately using only the built-in peripherals of the PIC16C781. This is the first of the mixed-signal PICmicro® microcontrollers with integral DAC, operational amplifier, comparators, PSMC and gated timer inputs which, when used in harmony, make such measurements possible. Source code for this application is available for free. Download it from the Microchip web site. 2002 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, KEELOQ, MPLAB, PIC, PICmicro, PICSTART and PRO MATE are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. FilterLab, microID, MXDEV, MXLAB, PICMASTER, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. dsPIC, dsPICDEM.net, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2002, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999 and Mountain View, California in March 2002. The Company’s quality system processes and procedures are QS-9000 compliant for its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, non-volatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001 certified. 2002 Microchip Technology Inc. DS91044A - page 3 WORLDWIDE SALES AND SERVICE AMERICAS ASIA/PACIFIC Corporate Office Australia 2355 West Chandler Blvd. Chandler, AZ 85224-6199 Tel: 480-792-7200 Fax: 480-792-7277 Technical Support: 480-792-7627 Web Address: http://www.microchip.com Microchip Technology Australia Pty Ltd Suite 22, 41 Rawson Street Epping 2121, NSW Australia Tel: 61-2-9868-6733 Fax: 61-2-9868-6755 Rocky Mountain China - Beijing 2355 West Chandler Blvd. Chandler, AZ 85224-6199 Tel: 480-792-7966 Fax: 480-792-4338 Atlanta 3780 Mansell Road, Suite 130 Alpharetta, GA 30022 Tel: 770-640-0034 Fax: 770-640-0307 Boston 2 Lan Drive, Suite 120 Westford, MA 01886 Tel: 978-692-3848 Fax: 978-692-3821 Chicago 333 Pierce Road, Suite 180 Itasca, IL 60143 Tel: 630-285-0071 Fax: 630-285-0075 Dallas 4570 Westgrove Drive, Suite 160 Addison, TX 75001 Tel: 972-818-7423 Fax: 972-818-2924 Detroit Tri-Atria Office Building 32255 Northwestern Highway, Suite 190 Farmington Hills, MI 48334 Tel: 248-538-2250 Fax: 248-538-2260 Kokomo 2767 S. Albright Road Kokomo, Indiana 46902 Tel: 765-864-8360 Fax: 765-864-8387 Los Angeles 18201 Von Karman, Suite 1090 Irvine, CA 92612 Tel: 949-263-1888 Fax: 949-263-1338 San Jose Microchip Technology Inc. 2107 North First Street, Suite 590 San Jose, CA 95131 Tel: 408-436-7950 Fax: 408-436-7955 Toronto 6285 Northam Drive, Suite 108 Mississauga, Ontario L4V 1X5, Canada Tel: 905-673-0699 Fax: 905-673-6509 Microchip Technology Consulting (Shanghai) Co., Ltd., Beijing Liaison Office Unit 915 Bei Hai Wan Tai Bldg. No. 6 Chaoyangmen Beidajie Beijing, 100027, No. China Tel: 86-10-85282100 Fax: 86-10-85282104 China - Chengdu Microchip Technology Consulting (Shanghai) Co., Ltd., Chengdu Liaison Office Rm. 2401-2402, 24th Floor, Ming Xing Financial Tower No. 88 TIDU Street Chengdu 610016, China Tel: 86-28-86766200 Fax: 86-28-86766599 China - Fuzhou Microchip Technology Consulting (Shanghai) Co., Ltd., Fuzhou Liaison Office Unit 28F, World Trade Plaza No. 71 Wusi Road Fuzhou 350001, China Tel: 86-591-7503506 Fax: 86-591-7503521 China - Shanghai Microchip Technology Consulting (Shanghai) Co., Ltd. Room 701, Bldg. B Far East International Plaza No. 317 Xian Xia Road Shanghai, 200051 Tel: 86-21-6275-5700 Fax: 86-21-6275-5060 China - Shenzhen Microchip Technology Consulting (Shanghai) Co., Ltd., Shenzhen Liaison Office Rm. 15-16, 13/F, Shenzhen Kerry Centre, Renminnan Lu Shenzhen 518001, China Tel: 86-755-82350361 Fax: 86-755-82366086 China - Hong Kong SAR Microchip Technology Hongkong Ltd. Unit 901-6, Tower 2, Metroplaza 223 Hing Fong Road Kwai Fong, N.T., Hong Kong Tel: 852-2401-1200 Fax: 852-2401-3431 India Microchip Technology Inc. India Liaison Office Divyasree Chambers 1 Floor, Wing A (A3/A4) No. 11, O’Shaugnessey Road Bangalore, 560 025, India Tel: 91-80-2290061 Fax: 91-80-2290062 Japan Microchip Technology Japan K.K. Benex S-1 6F 3-18-20, Shinyokohama Kohoku-Ku, Yokohama-shi Kanagawa, 222-0033, Japan Tel: 81-45-471- 6166 Fax: 81-45-471-6122 Korea Microchip Technology Korea 168-1, Youngbo Bldg. 3 Floor Samsung-Dong, Kangnam-Ku Seoul, Korea 135-882 Tel: 82-2-554-7200 Fax: 82-2-558-5934 Singapore Microchip Technology Singapore Pte Ltd. 200 Middle Road #07-02 Prime Centre Singapore, 188980 Tel: 65-6334-8870 Fax: 65-6334-8850 Taiwan Microchip Technology (Barbados) Inc., Taiwan Branch 11F-3, No. 207 Tung Hua North Road Taipei, 105, Taiwan Tel: 886-2-2717-7175 Fax: 886-2-2545-0139 EUROPE Austria Microchip Technology Austria GmbH Durisolstrasse 2 A-4600 Wels Austria Tel: 43-7242-2244-399 Fax: 43-7242-2244-393 Denmark Microchip Technology Nordic ApS Regus Business Centre Lautrup hoj 1-3 Ballerup DK-2750 Denmark Tel: 45 4420 9895 Fax: 45 4420 9910 France Microchip Technology SARL Parc d’Activite du Moulin de Massy 43 Rue du Saule Trapu Batiment A - ler Etage 91300 Massy, France Tel: 33-1-69-53-63-20 Fax: 33-1-69-30-90-79 Germany Microchip Technology GmbH Steinheilstrasse 10 D-85737 Ismaning, Germany Tel: 49-89-627-144 0 Fax: 49-89-627-144-44 Italy Microchip Technology SRL Centro Direzionale Colleoni Palazzo Taurus 1 V. Le Colleoni 1 20041 Agrate Brianza Milan, Italy Tel: 39-039-65791-1 Fax: 39-039-6899883 United Kingdom Microchip Ltd. 505 Eskdale Road Winnersh Triangle Wokingham Berkshire, England RG41 5TU Tel: 44 118 921 5869 Fax: 44-118 921-5820 11/15/02 DS91044A-page 4 2002 Microchip Technology Inc.