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DAC7678EVM Output Voltage Not Detected

Mueen Hakak
Prodigy 10 points
Other Parts Discussed in Thread: LM3S8962, DAC7678EVM

Hello,

We are using DAC7678EVM which is connected to a Stellaris LM3S8962 Microcontroller. Ths system uses I2C protocol. We can detect the input (SDA) and the clock (SCL) that is being sent to the DAC from the MCU.

Jumpers: All default

Input power supply: 5 V input to pin J3.3

Slave Address: 0x90

Command/Access Byte: 0x00 (write into DAC register - channel A) (Aside: We also used 0x3F which writes and updates all DAC channels).

Speed: Standard (100 kbps)

Data: Our data is an integer from 0 to 1023. Data is coming from ADC of another LM3S8962. Data is packaged as MSDB 00xxxxxx and LSDB xxxx0000.

 

We expect an output that changes according to the data, but all we see at the pin J1.2 is a constant 2.048 V (which seems to be the half of the internal ref. voltage of 4.096 V).

 

What are we missing? What should we check?

 

 

  • 0
    TI__Mastermind 43915 points

    Hi Mueen,

    From the looks of things, it looks like you have the RSTSEL pin high (resetting the part to mid-scale) and may either have the /CLR pin held low or are not properly communicating with the device. The first thing I would do is check all your static pins by probing the pins on the J2A header to make sure they are as you expect. The next thing I would do is look at the state of your address pins on the EVM to make sure you are using the correct address to communicate with the I2C device. If the jumpers are as shown on the front page of the User's Guide  (shown here: http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=sbau168a), then the address will be 1001 000 followed by the R/W bit. Next, I would remove the JP3 jumper to disconnect the external reference from the DAC and place a multimeter on the middle pin of the JP3 jumper header. Finally, I would try to write to the DAC to turn on the internal reference per page 25 of the data sheet. I recommend you use a oscilloscope to verify that you are receiving ACK bits following each 8 bit transmission to verify that the DAC is responding to the data word.

    Regards,

    Tony Calabria