M TB052 Multi-Zone Temperature Monitoring with the TCN75 Thermal Sensor Author: Ken Dietz Microchip Technology Inc. INTRODUCTION This Technical Brief presents an example of a simple, multi-zone thermal monitoring system using the Hardware mode of the Master Synchronous Serial Port (MSSP) module of a PICmicro® microcontroller. The design uses several TCN75 thermal sensors and the PIC16LF872 microcontroller. Typical applications where this design may be implemented include power supplies, aquariums, personal computers, work stations, ovens, hot-swappable thermal sensing systems, office electronics, and electronic test equipment. This type of system design is also optimum for monitoring temperatures in the electronic systems of various home appliances, such as microwave ovens, blenders, toasters, refrigerators, washers, dryers, vacuum cleaners, and mixers. SENSOR OVERVIEW The TCN75 is a solid state, serially programmable digital temperature sensor. Environment temperature is output as a 9-bit, 2’s complement serial digital signal. The device has an operating range of -55°C to +125°C (represented as 192h to 0FAh), with a typical resolution of 0.5°C. The TCN75 has the ability to notify the host controller when a temperature measurement exceeds a user programmable set-point through its interrupt feature. It also incorporates a comparator function that can be used in place of the interrupt. These two functions are multiplexed through the same pin (INT/CPL) on the chip. Selectable addressing is implemented by pulling the three address pins to either VDD (binary 1), or ground (binary 0), providing for a total of eight devices that can be addressed on the same two-wire serial bus. The device is available in MSOP and SOIC packaging, which also makes it an ideal choice for low cost/low profile thermal management systems. CIRCUIT OVERVIEW nodes are connected to the host processor through an I2C™ interface. Only three TCN75 sensors are shown, to demonstrate the binary addressing scheme using pins A0 through A2. Since the original design was prototyped using Microchip’s PICDEMTM 2 Demonstration Board, some minor changes to the board were required to accommodate the 3.3V power supply and the additional resistors (noted in the shaded areas of Figure 1). When RB0 is communicating with the LCD, the pin needs to be isolated from the sensors, regardless of the state of INT/ CMP. For this reason, R3 was designed into the circuit as an isolation resistor. R4 and R5 are typical pull-up resistors for the I2C bus, while R6 is a weak pull-up when RB0 is configured as an input waiting for INT/ CMP to change state. The circuitry for implementing over-temperature conditions is shown using R3 through R6 (this feature is not included in the firmware, however). Users who also wish to use the demonstration board as a basis for their designs are advised to review the overall schematic, which may be downloaded from the Microchip corporate website. To create a more economical design, the PIC16LF872 oscillator is configured for RC mode. Because I2C bus timing is critical, and to accommodate minimum and maximum clock rate specifications that may be compromised due to running the processor in RC mode, the I2C clock rate was derated to 49 kHz. (This is compared to the maximum 100 kHz clock rate for the controller port running in this I2C mode.) After powering up and completing its initialization sequence, the microcontroller enters the main software routine where I2C addresses are continuously polled based on a timer interrupt and a counter. Detecting the sensors at various addresses is accomplished by sending address bytes to slave addresses, then testing the Acknowledge status bit in the MSSP module. When a slave acknowledges, the system identifies the device at the address as a TCN75 and interrogates it for temperature data. Once collected, the data is processed and displayed on the LCD. Each sensor is polled for data, approximately every 1 to 2 seconds; if the sensor acknowledges the master, its data is sequentially displayed on the LCD. Polling and display frequencies are controlled by a delay loop in the firmware. Figure 1 shows the overall design of the system. The circuit is divided into two sections: the remote sensors and the microprocessor board. The thermal sensing 2001 Microchip Technology Inc. DS91052A-page 1 DS91052A-page 2 C3 20pF 19 8 9 1 20 28 21 22 23 24 25 26 27 R/W E RS RB6 RB7 RB5 RB4 RB1 RB2 RB3 RB0 Sensor 0 8 VDD 7 A0 6 A1 5 A2 +3.3V 8x2 LCD Dot Matrix Sensor 1 3 INT/CMP 1 SDA 2 SCL 4 GND TCN75 8 VDD 7 A0 6 A1 5 A2 TCN75 8 DB1 9 DB2 10 DB3 11 DB4 12 DB5 13 DB6 14 DB7 LCD1 DMC50448 U3 +3.3V RC4 15 SDA RC3 14 SCL 2 RA0 3 RA1 4 RA2 RB0 RB1 RB2 RB3 RB4 RB5 RB6 RB7 PICDEM 2 Board U2 VSS VSS OSC1 MCLR VDD 3 INT/CMP 1 SDA 2 SCL 4 GND R2 4.7 kΩ U1 PIC16LF872 7 6 5 2 3 1 R/W 4 RS VCC VEE VSS DB0 E Sensor 7 +3.3V R8 680 Ω R7 10 kΩ +3.3V 8 VDD 7 A0 6 A1 5 A2 R6 100 kΩ TCN75 U9 R5 1 kΩ Board Modifications 3 1 INT/CMP SDA 2 SCL 4 GND R4 1 kΩ R3 1 kΩ +5V FIGURE 1: R1 4.7 kΩ +3.3V TB052 CIRCUIT DIAGRAM FOR THE TEMPERATURE SENSOR 2001 Microchip Technology Inc. TB052 APPENDIX A: SOFTWARE DISCUSSED IN THIS TECHNICAL BRIEF The source code for the design discussed in this Technical Brief is not provided here. The firmware for this design is available as a single WinZip archive file. The archive may be downloaded from the Microchip corporate Web site at www.microchip.com 2001 Microchip Technology Inc. DS91052A-page 3 TB052 NOTES: DS91052A-page 4 2001 Microchip Technology Inc. Note the following details of the code protection feature on PICmicro® MCUs. • • • • • • The PICmicro family meets the specifications contained in the Microchip Data Sheet. Microchip believes that its family of PICmicro microcontrollers is one of the most secure products 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 PICmicro microcontroller in a manner outside the operating specifications contained in the data sheet. The person doing so may be 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 product. If you have any further questions about this matter, please contact the local sales office nearest to you. 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, FilterLab, KEELOQ, MPLAB, PIC, PICmicro, PICMASTER, PICSTART, PRO MATE, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microID, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. Serialized Quick Term 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. © 2001, 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. The Company’s quality system processes and procedures are QS-9000 compliant for its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001 certified. 2001 Microchip Technology Inc. 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