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I'm trying to communicate in I2C between a MSP430FR2433 and an MPU6050. I can send the start bit then the slave address (0x68) followed by the write bit (0) but the slave send me a NACK. I'm using the GY-521 board and there are two pull-ups of 4k7. The clock is at 100 kHz which is under the maximum. I could once make the communication but since I always get a NACK

I2C communication red clock blue data

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    \$\begingroup\$ Schematic? Are you sure that your MPU is powered correctly and pin AD0 is LOW? \$\endgroup\$ – mic May 2 '19 at 10:35
  • \$\begingroup\$ @mic The GY-521 board has a pulldown on AD0. \$\endgroup\$ – CL. May 2 '19 at 10:45
  • \$\begingroup\$ On the I²C bus, the high voltage level is the idle state. A NACK means that the slave device did not react at all. Maybe it isn't connected properly to the MSP or to the power supply. \$\endgroup\$ – CL. May 2 '19 at 10:47
  • \$\begingroup\$ @CL. Yes, just found the GY-521 schematic :-) Anyway, the pulldown may be broken ;-) But I agree it's more likely that there's a connection issue. \$\endgroup\$ – mic May 2 '19 at 10:52
  • \$\begingroup\$ personally, I don't see the need for R6, I'd just put AD0 straight to GND \$\endgroup\$ – pm101 May 2 '19 at 12:38
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Have you considered the often confused issue of the slave address being considered a 7-bit quantity versus an 8-bit quantity?

When a slave address is considered an 8-bit quantity the least significant bit (LSB) of the address is the R/W bit so the slave device looks like it responds to two addresses, one for read and the other for write.

When specified as a 7-bit quantity the slave address is encoded as a value independent from the R/W bit.

Your waveform picture shows the 0x68 address encoded as 7-bits. But it could be that the device data sheet may have specified as an 8-bit in which case you may need to send out that first byte of the I2C protocol as a value of 0x68 instead of the 0xD0 value you have used now.

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  • \$\begingroup\$ The MPU-6050 datasheet shows that the I²C address is 1101000. \$\endgroup\$ – CL. May 2 '19 at 12:01
  • \$\begingroup\$ @CL. - So seven bits then. Good to know. Likely a connection issue then. \$\endgroup\$ – Michael Karas May 2 '19 at 12:05
  • \$\begingroup\$ Thank you for the answer, after having checked your suggestion, I finially change the sensor and it works. \$\endgroup\$ – jessica schmid May 6 '19 at 14:23

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