User Manual

Application Note

SMBus communication with MLX90614

390119061402 Page 11 of 32 Jan-2008

Rev 004

Low I

C VDD Dependent

0.3*V

C Fixed 1.5V

SMBus 0.8V

5.3 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. SMBus requires devices to

acknowledge their own address always, as a mechanism to detect a removable device’s

presence on the bus (battery, docking station, etc.).

I2C specifies that a slave device, although it may acknowledge its own address, some time later

in the transfer it may decide that it cannot receive any more data bytes. The I2C specifies, that

the device may indicate this by generating the not acknowledge on the first byte to follow.

Besides to indicate a slave device busy condition, SMBus is using the NACK mechanism also to

indicate the reception of an invalid command or data. Since such a condition may occur on the

last byte of the transfer, it is required that SMBus devices have the ability to generate the not

acknowledge after the transfer of each byte and before the completion of the transaction. This is

important because SMBus does not provide any other resend signaling.

More information about the differences between I

C and SMBus can be found on:

http://www.maxim-ic.com/appnotes_frame.cfm/appnote_number/476

http://www.smbus.org/specs/