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ADT7470 two byte I2C write supported?

ADT7470 datasheet rev. E says on page 10 that "In the ADT7470, write operations contain either one or two bytes".  When I try to do an I2C write with two bytes of data to the tach limit registers, I am seeing a NAK after the 2nd byte of write data.  Other places in the datasheet it only lists byte writes (Write Operations, p11-12).  Is two byte access supported?  If not what conditions would cause a nak on a write?

An example i2c write I see looks like this (adt7470 in bold):

<start><0x2c address><0 write><ack><0x62><ack><0xC1><ack><0x01><nak><stop>

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  • Schematic looks like this:

    The i2c lines also go to a Xilinx FPGA(master), pullups, AD7414 temp sensor and an LTC3350 supercap controller.

    The scope shot shows a write to I2C address 0x2c, subaddress 0x58 of 0xff, 0x07.  The tek0004 shot is the start of the transaction, tek0005 is the end (zoomed in a notch more).  SCL in yellow, SDA in blue.

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  • Schematic looks like this:

    The i2c lines also go to a Xilinx FPGA(master), pullups, AD7414 temp sensor and an LTC3350 supercap controller.

    The scope shot shows a write to I2C address 0x2c, subaddress 0x58 of 0xff, 0x07.  The tek0004 shot is the start of the transaction, tek0005 is the end (zoomed in a notch more).  SCL in yellow, SDA in blue.

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