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Application Note

SMBus communication with MLX90614

390119061402 Page 1 of 32 Jan-2008

Rev 004

1 Scope

This document introduces the users in SMBus communication protocol and especially how it can

be used to communicate with MLX90614 infrared thermometers. The MLX90614 is an Infra Red

thermometer for non contact temperature measurements. Both the IR sensitive thermopile

detector chip and the signal conditioning ASSP are integrated in the same TO-39 can. Thanks to

its low noise amplifier, 17-bit ADC and powerful DSP unit, a high accuracy and resolution of the

thermometer is achieved. The thermometer comes factory calibrated with a digital PWM and

SMBus output. As a standard, the 10-bit PWM is configured to continuously transmit the

measured temperature in range of -20 to 120 °C, with an output resolution of 0.14 °C and the

POR default is SMBus.

The original purpose of the SMBus was to define the communication link between an intelligent

battery, charger for the battery and a microcontroller that communicates with the rest of the

system. However, SMBus can also be used to connect a wide variety of devices including

power-related devices, system sensors, inventory EEPROMs communications devices and

more. The original specification of the SMBus protocol can be found on http://www.smbus.org/specs/

2 Related Melexis Products

EVB90614 is the evaluation board which supports the MLX90614 devices.

3 Table of contents

Scope ....................................................................................................................................................................1

Related Melexis Products .....................................................................................................................................1

Table of contents ..................................................................................................................................................1

General SMBus protocol discription ....................................................................................................................2

4.1

Definitions of terms .........................................................................................................................................2

4.2

SMBus overview..............................................................................................................................................2

4.3

Electrical characteristics of SMBus devices ....................................................................................................5

4.4

Timeouts ..........................................................................................................................................................6

4.5

Slave device timeout definitions and conditions..............................................................................................6

4.6

Master device timeout definitions and conditions............................................................................................7

4.7

Low-power DC specifications .........................................................................................................................7

4.8

High-power DC specifications.........................................................................................................................7

4.9

Bit transfer .......................................................................................................................................................8

Comparing the I2C Bus to the SMBus ...............................................................................................................10

5.1

Timeout and Clock Speed differences ...........................................................................................................10

5.2

DC specifications differences ........................................................................................................................10

5.3

Other differences............................................................................................................................................11

SMBus comunication with MLX90614..............................................................................................................12

6.1

Overview........................................................................................................................................................12

6.2

Timings ..........................................................................................................................................................12

6.3

Detailed Communication description.............................................................................................................13

Sleep Mode .........................................................................................................................................................25

Electrical considerations of SMBus applications with MLX90614....................................................................26

Conclusion ..........................................................................................................................................................28

♦ APPENDIX – SMBus exemplary functions for PIC18 using microchip MCC18 compiler .....................................29

Summary of content (32 pages)

PAGE 1
Application Note SMBus communication with MLX90614 1 Scope This document introduces the users in SMBus communication protocol and especially how it can be used to communicate with MLX90614 infrared thermometers. The MLX90614 is an Infra Red thermometer for non contact temperature measurements. Both the IR sensitive thermopile detector chip and the signal conditioning ASSP are integrated in the same TO-39 can.
PAGE 2
Application Note SMBus communication with MLX90614 4 General SMBus protocol discription 4.1 Definitions of terms ACK - Acknowledgement from receiver Address Resolution Protocol - A protocol by which SMBus devices with assignable addresses on the bus are enumerated and assigned non-conflicting slave addresses. ASSP - Application Specific Standard Product Bus Master - Any device that initiates SMBus transactions and drives the clock. Bus Slave - Target of a SMBus transaction which is driven by some master.
PAGE 3
Application Note SMBus communication with MLX90614 Devices may be powered by the bus VDD or by another power source Vbus (Fig.1). Fig.1: SMBus Topology VDD may be 3 to 5 volts +/- 10% and there may be SMBus devices powered directly by the bus VDD. Both SDA and SCL lines are bi-directional, connected to a positive supply voltage through a pull-up resistor or a current source or other similar circuit. When the bus is free, both lines are high.
PAGE 4
Application Note SMBus communication with MLX90614 Fig.2 shows a CRC calculation example. Fig.2 For more information about calculation of CRC refer to the next document: http://www.sbs-forum.org/marcom/dc2/20_crc-8_firmware_implementations.pdf Fig.3 shows the common structure of an SMBus transaction Fig.
PAGE 5
Application Note SMBus communication with MLX90614 4.3 Electrical characteristics of SMBus devices The diagram bellow illustrates the various SMBus timings Fig.4: SMBus timing measurements The table below describes all timings. Table 1 Symbol fSMB tBUF tHD:STA tSU:STA tSU:STO tHD:DAT tSU:DAT tTIMEOUT tLOW tHIGH tLOW:SEXT tLOW:MEXT tF tR T POR Parameter SMBus Operation frequency Bus free time between Stop and Start condition Hold time after(Repeated)Start Condition.
PAGE 6
Application Note SMBus communication with MLX90614 Note 3: tHIGH,MAX provides a simple guaranteed method for masters to detect bus idle conditions. A master can assume that the bus is free if it detects that the clock and data signals have been high for greater than tHIGH,MAX. Note 4: tLOW:SEXT is the cumulative time a given slave device is allowed to extend the clock cycles in one message from the initial START to the STOP.
PAGE 7
Application Note SMBus communication with MLX90614 4.6 Master device timeout definitions and conditions tLOW: MEXT is defined as the cumulative time a master device is allowed to extend its clock cycles within one byte in a message as measured from: START to ACK ACK to ACK ACK to STOP A system host may not violate tLOW:MEXT except when caused by the combination of its clock extension with the clock extension from a slave device or another master.
PAGE 8
Application Note SMBus communication with MLX90614 specifications are an alternative to the lower power specifications stated above and may be used in environments where necessary. Table 3 Symbol VIL VIH VOL ILEAK-BUS ILEAK-PIN VDD IPULLUP CBUS CI VNOISE Parameter SMBus signal Input low voltage SMBus signal Input high voltage SMBus signal Output low voltage Input Leakage per bus segment Input Leakage per device pin Nominal bus voltage Current sinking, VOL=0.
PAGE 9
Application Note SMBus communication with MLX90614 acknowledge a byte, the receiver must pull the SDA line LOW during the HIGH period of the clock pulse according to the SMBus timing specifications. A receiver that wishes to NACK a byte must let the SDA line remain HIGH during the acknowledge clock pulse. A SMBus device must always acknowledge (ACK) its own address.
PAGE 10
Application Note SMBus communication with MLX90614 5 Comparing the I2C Bus to the SMBus The I2C bus and the SMBus are popular 2-wire buses that are essentially compatible with each other. Normally devices, both masters and slaves, are freely interchangeable between both buses. Both buses feature addressable slaves (although specific address allocations can vary between the two). The buses operate at the same speed, up to 100kHz, but the I2C bus has both 400kHz and 2MHz versions.
PAGE 11
Application Note SMBus communication with MLX90614 Low I2C VDD Dependent 0.3*VDD I2C Fixed SMBus 5.3 1.5V 0.8V Other differences ACK and NACK usage: The differences in the use of the NACK bus signaling follow: In I2C, a slave receiver is allowed not to acknowledge the slave address, if for example is unable to receive because it’s performing some real time task.
PAGE 12
Application Note SMBus communication with MLX90614 6 SMBus comunication with MLX90614 6.1 Overview The MLX90614 can only be used as a slave device. Generally, the master initiates the start of data transfer by selecting a slave through the Slave Address (SA). The MLX90614 meets all the timing specifications of the SMBus (refer to Electrical characteristics of SMBus devices above). MLX90614 has 32x17 RAM. It is not possible to write into the RAM memory.
PAGE 13
Application Note SMBus communication with MLX90614 used – MD when the master device is making acknowledge; SD when the slave device is making acknowledge). For other timings see Electrical characteristics of SMBus devices above. Fig.8:MLX90614 specific timings In Table 5 are given the values of the specific timings. Table 5 Symbol Parameter tREQ SMBus Request tsuac (MD) tsuac(SD) thdac(MD) thdac(SD) 6.3 Min 2 0.5 1.5 1.5 0.
PAGE 14
Application Note SMBus communication with MLX90614 Table 7 describes most important RAM registers. For other registers in RAM and EEPROM memory refer to MLX90614 datasheet. Table 7 Name Address (hexadecimal) Ambient sensor data 0x03 IR sensor 1 data 0x04 IR sensor 2 data 0x05 Linearized ambient temperature TA 0x06 Linearized object temperature Tobj1 0x07 Linearized object temperature Tobj2 0x08 All bytes are sent and received with MSb first.
PAGE 15
Application Note SMBus communication with MLX90614 see Fig.11. After this first temperature calculation flow from the module, IIR refreshes its value from EEPROM ConfigRegister<2..0>). See AppNote ”Understanding MLX90614 on-chip digital signal filters” on http://www.melexis.com/Asset.aspx?nID=5272 to understand how to calculate this delay time.
PAGE 16
Application Note SMBus communication with MLX90614 Read RAM Block Diagram 390119061402 Rev 004 Page 16 of 32 Jan-2008
PAGE 17
Application Note SMBus communication with MLX90614 The format of SMBus reading from EEPROM is: EEPROM memory is accessible for reading without restriction. Pseudo code example: Reading EEPROM address 0x0E (SMBus Address) 1. Send START bit 2. Send Slave Address (0x00* for example) + Rd\-Wr bit** 3. Send Command (0b001x_xxxx + 0b0000_1110 -> 0b0010_1110) 4. Send Repeated START_bit 5. Send Slave Address + Rd\-Wr bit** 6. Read Data Byte Low (master must send ACK bit) 7.
PAGE 18
Application Note SMBus communication with MLX90614 Read EEPROM Block Diagram 390119061402 Rev 004 Page 18 of 32 Jan-2008
PAGE 19
Application Note SMBus communication with MLX90614 The format of SMBus writing in EEPROM is: In Application mode only 9 cells are accessible for writing. An attempt to write not accessible EEPROM cell results in no change. Before writing an erasing operation must be done. An erasing operation is just a writing of zeros in an EEPROM cell. After a/an writing/erasing 5ms are need the new value to be written/erased.
PAGE 20
Application Note SMBus communication with MLX90614 Write/Erase EEPROM Block Diagram Note* : For an erasing operation Low and High data byte must be zero Note**: For an erasing operation no need to restart MLX90614 390119061402 Rev 004 Page 20 of 32 Jan-2008
PAGE 21
Application Note SMBus communication with MLX90614 The format of SMBus transaction which enters Sleep mode is: Pseudo code example: Put MLX90614 in Sleep mode 1. Send START bit 2. Send Slave Address (0x00* for example) + Rd\-Wr bit** 3. Send command 0xFF 4. Send PEC byte 0xF3 5. Send STOP bit 6.
PAGE 22
Application Note SMBus communication with MLX90614 Enter in Sleep Mode Block Diagram Begin Send STOP bit Send START bit Send Slave Address ACK from Slave ? No Yes Send command ACK from Slave ? No Yes Send PEC byte ACK from Slave ? No Yes Send STOP bit Put SCL line in Low level End 390119061402 Rev 004 Page 22 of 32 Jan-2008
PAGE 23
Application Note SMBus communication with MLX90614 The format of a SMBus transaction which reads flags is: Flags read are: Data[7] - EEBUSY – the previous write/erase EEPROM access is still in progress. High active. Data[6] - Unused Data[5] - EE_DEAD – EEPROM double error has occurred. High active. Data[4] - INIT – POR initialization routine is still ongoing. Low active. Data[3] - Not implemented. Data[2..0] - All zeros. Data[8..15] - All zeros. Flags read is a diagnostic feature.
PAGE 24
Application Note SMBus communication with MLX90614 Read Flags Block Diagram 390119061402 Rev 004 Page 24 of 32 Jan-2008
PAGE 25
Application Note SMBus communication with MLX90614 7 Sleep Mode MLX90614 can enter Sleep Mode via command “Enter SLEEP mode” sent via the SMBus interface. This mode is not available for the 5V supply version. To limit the current consumption to 2.5uA (typ), the SCL pin should be kept low during sleep (Fig.9). MLX90614 goes back into power-up default mode (via POR reset) by setting SCL pin high and then PWM/SDA pin low for at least tDDq=14ms (Fig.10). Fig.9 Fig.
PAGE 26
Application Note SMBus communication with MLX90614 8 Electrical considerations of SMBus applications with MLX90614 Vdd Vdd PP/-OD ; SMBus Vdd +Vdd Din PWM/SDA SCL/Vz Dout Synthesized Zener diode for building external HV voltage regulator Vss Vss Vss SCLin Vss Vss Fig.12: Input/Output pin schematics of MLX90614 For reliability reasons, the MLX90614 incorporates ESD clamp diodes to Vdd and from Vss. Therefore a powered-down MLX90614 will load the SMBus. This differs from the SMBus specification.
PAGE 27
Application Note SMBus communication with MLX90614 mode is available with the 3V version only, while the zener diode function can be used with the 5V version. However, the zener diode is present in all MLX90614 versions, so the leakage will be seen on the 3V version too. It is recommended to disable the pull-up on the SCL line in order to prevent the leakage from increasing the overall power-down power drain of the SMBus system. 1 U1 5.
PAGE 28
Application Note SMBus communication with MLX90614 3.00 VinL, VinH, V Vin,L,-40°C,min Vin,L,-40°C,max 2.50 Vin,L,+27°C,min Vin,L,+27°C,max 2.00 Vin,L,+125°C,min Vin,L,+125°C,max 1.50 Vin,H,-40°C,min Vin,H,-40°C,max 1.00 Vin,H,+27°C,min Vin,H,+27°C,max 0.50 Vin,H,+125°C,min Vdd,V (3V vers ion) Vin,H,+125°C,max 0.00 2.4 2.8 3 3.2 3.6 Fig.15: Input voltage levels versus power supply voltage and temperature Both temperature and supply voltage increase both low and high thresholds.
PAGE 29
Application Note SMBus communication with MLX90614 ♦ APPENDIX – SMBus exemplary functions for PIC18 using microchip MCC18 compiler //SMBus control signals #define _SCL_IO TRISCbits.TRISC3 #define _SDA_IO TRISCbits.TRISC4 #define _SCL PORTCbits.RC3 #define _SDA PORTCbits.
PAGE 30
Application Note SMBus communication with MLX90614 //************************************************************************************************************ // STOP CONDITION ON SMBus //************************************************************************************************************ //Name: STOP_bit //Function: Generates STOP condition on SMBus //Parameters: No //Return: No //Comments: Refer to "System Managment BUS(SMBus) specification Version 2.
PAGE 31
Application Note SMBus communication with MLX90614 //-----------------------------------------------------------------------------------------------------------------------------void send_bit(unsigned char bit_out) { if(bit_out==0) // Check bit mSDA_LOW(); // Set SDA if bit_out=1 else mSDA_HIGH(); // Clear SDA if bit_out=0 Nop(); // Nop(); // Tsu:dat = 250ns minimum Nop(); // mSCL_HIGH(); // Set SCL line Delay10TCYx( HIGHLEV ); // High Level of Clock Pulse mSCL_LOW(); // Clear SCL line Delay10TCYx( LOWLEV )
PAGE 32
Application Note SMBus communication with MLX90614 //-----------------------------------------------------------------------------------------------------------------------------unsigned char Receive_bit(void) { unsigned char bit; _SDA_IO=1; mSCL_HIGH(); Delay10TCYx( HIGHLEV ); // SDA-input // Set SCL line // High Level of Clock Pulse if(_SDA) bit=1; else bit=0; // Read bit, save it in bit mSCL_LOW(); Delay10TCYx( LOWLEV ); // Clear SCL line // Low Level of Clock Pulse return bit; }//End of Receive_bi