Abstract :Maxim has introduced the MAX6675 is a cold junction compensation with K type thermocouple amplifier with single digital converter . In this paper, the characteristics of the device , working principle and interface timing , and given with the microcontroller interface circuit and the temperature reading , the conversion process . Keywords :Thermocouple cold junction compensation digital amplifier output Thermocouple temperature as a main component , has a simple structure , easy to manufacture, easy to use , wide range temperature measurement , temperature measurement precision . However, the thermocouple used in the field of Embedded System Based on the time , but there are the following issues. ? nonlinear : thermocouple output of thermoelectric power and temperature relationship between the nonlinear relationship , it must be linear in the application processing . ? cold compensation: the output of the thermoelectric power for the thermocouple cold junction maintained at 0 ?, and the measurement of electrical potential difference between terminal and, in the practical application of cold -side temperature is temperature as the environment changes , so in need of cold- junction compensation . output : and embedded system interface is bound to use the digital output and digital interfaces and, as the analog components of the thermocouple temperature measurement of small signal method is clearly directly meet this requirement. Hence, if the thermocouple used in embedded systems that require complex signal amplification , A / D conversion, look-up table of linear lines, temperature compensation and digital output interface hardware and software design . If the above functions can be integrated into an integrated circuit chip, that is a single chip to perform signal amplification , cold junction compensation , linearization and digital output , it will greatly simplify the application of the thermocouple in the field of embedded design . Maxim 's MAX6675 that the company recently introduced an integrated thermocouple amplifier , cold junction compensation , A / D converter and SPI serial thermocouple amplifier with digital converter . 1 Features MAX6675 's main features are as follows : ? simple SPI serial output temperature ; ? 0 ? ~ +1024 ? the temperature range; ? 12?0.25 ? resolution ; ? chip cold-junction compensation; ? high impedance differential input ; ? Thermocouple break detection; ? single +5 V power supply voltage ; ? low power consumption ; ? Operating temperature range -20 ? ~ +85 ?; ? 2000V of ESD signal . The device posts an 8-pin SO -chip package . Pin arrangement shown in Figure 1 , pin functions as listed in Table 1 . 2 Work MAX6675 's internal structure shown in Figure 2 . The device is a complex monolithic thermocouple digital converter , an internal amplifier with signal conditioning , 12-bit analog / digital converter thermocouple , cold junction compensation sensor and correction, digital controller , an SPI compatible interface And an associated control logic . Figure 2 MAX6675 block diagram of the internal structure 2.1 Temperature change MAX6675 is a cold junction compensation by detection and correction of the ambient temperature changes . Ambient temperature of the device can be detected by the internal temperature diode is converted to temperature compensation voltage , in order to produce the actual thermocouple temperature measurements , MAX6675 output from the thermocouple measurement and detection diode output voltage . The device internal circuitry to the diode voltage and thermocouple voltage sent to the ADC converted to calculate the temperature of the hot end of thermocouple . When the thermocouple cold junction temperature is equal to the chip , MAX6675 for best accuracy. Therefore, the actual temperature measurement application, should be avoided in the MAX6675 placed near the heating device or component , as this will cause the cold side error . 2.5 Accuracy improvement MAX6675 internal thermocouple signals with the ADC input channels compatible with the voltage of the signal conditioning amplifier , T + and T- input terminal connected to the low noise amplifier A1, to ensure accurate detection of the input , while connecting to the thermocouple wire and Interference isolation . Thermocouple output of thermoelectric power amplified by low noise amplifiers A1 , A2 voltage follower in another buffer after being sent to the ADC input . In the temperature voltage is converted to equivalent temperature value price before it needs to thermocouple cold junction temperature compensation , that is, MAX6675 cold temperature 0 ? ambient temperature and the difference between the actual reference value . For K -type thermocouple , voltage was 41µV /?, voltage can be linear formula Vout = (41µV /?) × (tR-tAMB) to approximate the characteristics of the thermocouple . On the formula , Vout for the thermocouple output voltage (mV), tR is the temperature measurement point ; tAMB is the ambient temperature . 2.2 Cold Junction Compensation Function is to test the thermocouple hot and cold ends of the temperature difference , the thermocouple hot junction temperature at 0 ? ~ +1023.75 ? range of changes . Cold side of the circuit board that is installed around the MAX6675 temperature than the temperature at -20 ? ~ +85 ? range of changes . When the cold -side temperature fluctuations , MAX6675 hot end can accurately detect temperature changes . Figure 3 MAX6675 SO -ended output data format .3 Thermal Compensation In temperature measurement applications, the chip self-heating will reduce the MAX6675 temperature measurement accuracy , error depends on the MAX6675 package thermal conductivity, installation techniques and ventilation . To reduce the chip from the heat caused by measurement error , can be used in routing technology to improve MAX6675 large area ground temperature measurement accuracy. 2.4 Noise Compensation MAX6675 measurement accuracy of the power supply coupled noise sensitive . To reduce the power supply noise can MAX6675 power pins near the access to a 0.1µFCeramic bypass capacitor . Thermocouple measurement accuracy Ketong Guo Ji Tong of the following preventive measures to improve : ? Jinliangcaiyong Bu Neng heat from the measuring area of Tai Jiemiandaoxian ; ? such as Bi Xu Yong Small Sectional Area Wire , then Zhinengyingyong in the measuring area , and the temperature gradient in the Wu Region with extended wire ; ? avoid taut wire extrusion and vibration of machinery ; ? When the thermocouple distance should be used for thermocouple twisted pair connections ; ? In the temperature range of the thermal rating Thermocouple wire ; ? avoid rapid temperature changes ; ? to harsh environments in the use of appropriate protective sleeve to ensure the thermocouple wire ; ? only small changes in temperature and the rate of expansion of the regional use of wire ; ? events to keep the thermocouple resistance Records and recorded continuously . 2.6 SPI serial interface MAX6675 uses a standard SPI serial peripheral bus and the MCU interface, and the MAX6675 only as a slave . MAX6675 SO -side format of the output temperature data shown in Figure 3 , MAX6675 SPI interface timing shown in Figure 4 . MAX6675 SPI serial interface output data from the process as follows : MCU to CS going low and to provide the clock signal to the SCK, SO read from the measurement results . CS going low to stop any conversion process ; CS becomes higher to start a new conversion process. A complete serial interface read requires 16 clock cycles , the falling edge of the clock read 16 output bits , the first one and 15 is a false flag , and the total is 0 ; No. 14 to No. 3 as MSB to LSB order to the conversion temperature ; the first two usually low , when the thermocouple input is high when open , open thermocouple detection circuit fully realized by the MAX6675 , as an open thermocouple detector operation , T- Must be grounded, and to GND pin as close as possible to place ; the first one to provide the MAX6675 devices for low status code , the first 0 for the three-state . 3 Temperature Applications Here are MAX6675 specific methods used in embedded systems . AT89C2051 microcontroller here to an example, the MAX6675 The interface circuit composed of temperature and the corresponding temperature reading , the conversion process . MAX6675 single-chip digital thermocouple amplifier , its work without any external peripheral devices , where power coupling to reduce noise in their power supply pin and ground -side access to the one before the capacity of 0.1µFCapacitance . MAX6675 and the AT89C2051 MCU interface circuit shown in Figure 5 . As AT89C2051 do not have the SPI bus interface , so here the method of simulated SPI bus interface to implement and MAX6675 . Of which P1.0 simulation SPI data input (MISO), P1.1 analog output SPI serial clock SCK, P1.2 simulation SPI slave select terminal SSB. Here are the corresponding temperature reading program and data conversion . ; temperature reading program ; bit definitions SO BIT T1.0; data entry CS BIT P1.1; from the machine selection SCK BIT P1.2; clock ; data bytes defined DATAH DATA 30H; read data high DATAL DATA 31H; read data low TDATAH DATA 32H; temperature high TDATAL DATA 33H; temperature low ; reading temperature subroutine READY: CLR CS; stop the conversion and output data CLR CLK; clock goes low MOV R2, # 08H READH: MOV C, SO RLC A; read D15 ~ D8 high 8-bit data SETB CLK NOP CLR CLK DJNZ R2, READH MOV DATAH, A; will read the high 8-bit data storage MOV R2, # 08H READL: MOV C, SO; Reading D7 ~ D0 low 8-bit data RLC A SETB CLK NOP CLR CLK DJNZ R2, READL MOV DATAL, A; will read the low 8 bits of data stored SETB CS ; start another conversion process RET ; data conversion routines will be read with 16-bit data into 12-bit temperature value , remove the unwanted bits . D16T12: MOV A, DATAL CLR C RLC A MOV DATAL, A ; data as a whole shifted to the right one , MOV A, DATAH; to remove the pseudo- Shii D15 RLC A SWAP A; will DATAH the data exchange level 4 MOV B, A; data temporarily stored in B, MOV A, # 0FH; get temperature value D11 ~ D8 bit position 0 and D15 ~ D12 MOV TDATAH, A; converted the data to send high temperature MOV A, B; remove the temperature of the D7 ~ D4 bit ANL A, # 0F0H MOV B, A; buffer B, MOV A, DATAL ANL A, # 0F0H; remove the temperature of the D3 ~ D0 SWAP, A ORL A, B; merged into the low byte MOV TDATAL, A; converted the data to send high temperature RET Conclusion MAX6675 Thermocouple application to complex linear , cold junction compensation and issues such as digital output concentrated in one chip solution simplifies the thermocouple temperature measurement scheme is applied when the field of embedded systems hardware and software complex Design , so the device is used in thermocouple temperature measurement program ideal for embedded systems .