I am trying to read Melexis MLX90614 infrared temperature sensor through SMBus (I2C or TWK like protocol).

The protocol (SMBus) implement an special type of command or sequence called repeated start, that means we send a start condition without a previous stop condition.

I am using a XMEGA microcontroller using TWI libraries used in application note AN1308

In order to read the sensor data, I need to send 1 byte to write the address to read, and next read 3 bytes of data. For that reason is used repeated start.

Reading data example

The problem is I get ACK from 2 first bytes (write address and command write) but i get NACK when i send the address after repeated start condition

I have 3 captures from Oscilloscope:

General sequence General sequence The repeated start condition appears OK, we have a start condition in the middle without a previous stop condition. The address is right and the command (address to read) also is ok (0b00000110 - 6)

The initialization of module is:

// ************** TWI Master config ****************

And the usage is:


MLX90614 is 0x5A and bufferTX is a unsigned char array with a 0x06 in the first position. Also the baudrate is set to 100kHz

I don't know why i get NACK in 2nd address byte to read the data from sensor, the repeated start condition looks right. Also, the hardware configurations must be ok because I get ACK from 1st and 2nd byte

Thanks in advance

  • \$\begingroup\$ Why are there two peaks after the slave address? There should be a single ACK - which is expected to be the second peak. You should have a "zero" instead of the first peak - which is the "write" command bit. Do you use pull ups on both lines? \$\endgroup\$
    – Nazar
    Dec 17, 2016 at 14:47
  • \$\begingroup\$ I use a 5V version of MLX90614 and a XMEGA D microcontroller (3.3V power supply), so I need to use a level shifter. I am using a circuit that is "I2C safe" for level shifting, based on NMOS transistor (BSS138) \$\endgroup\$
    – mapedraza
    Dec 17, 2016 at 16:24
  • \$\begingroup\$ I am not sure how your level shifter works, but it must maintain two-way signalling, to allow the master and the slave to pull down the SCL and SDA lines. \$\endgroup\$
    – Nazar
    Dec 19, 2016 at 13:19

1 Answer 1


It is hard to tell without being zoomed in further, but it looks as though you may have inadvertently sent a Stop immediately after the Repeated Start. SDA looks like it went high a little bit before SCL went low. This would certainly prevent the slave from ACKing.

The first Start has the same behavior, but it looks like it wasn't quite as bad and so wasn't perceived as a Stop.


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