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Original question:

DAC8750: SPI communication, daisy chain configuration

DAC8750: Problems with Daisy-chain DAC8750

Melissa Wetter
Prodigy 90 points
Part Number: DAC8750

Hello,

after successfully putting one DAC8750 into operation I encountered a problem with the daisy-chain mode.

Our goal is a daisy-chain of 8 DACs.

My routine is following:

  • First I send every DAC a reset --> 8x24 bit 
  • then a NOP
  • after that every other register

The Control register is 0x100D -->  4-20mA range, Daisy-chain enable, output enable

The sending routine is following:

CLR LOW

Latch Low

Sending the data for one register for every DAC ( 8x 24 bit) //Reset register

Latch high

again

Latch Low

Sending the data for one register for every DAC ( 8x 24 bit)//NOP Register

Latch high

and 6x again

So my DACs won't work but why?

My other routines for one DAC are working and I think my daisy-chain routine matches the datasheet.

  • 0
    TI__Guru 52713 points

    I take it from this that you have 8 devices in the daisy-chain?

    Note that each device DOES NOT latch the command as it shifts it through the SDI/SDO buffer.  Instead, think of it has a single shift register that is 8×24 bits wide.  When CS/SYNC goes high, all 8×24bits are latched at the same time.  Each one of the devices latches the 24 bits currently in its buffer, so for example, the first device in the change latches last 24 bits shifted.

    So if want to send the same command to all 8 devices, you must send that command 8 times.

    To enable daisy-chain mode, it is the same idea, but CS/Latch must be asserted between each 24-bits.  So you should send the control register (24bits) command 8 times in a row to ensure that all devices are in daisy-chain mode, then you can start streaming the 8×24bit commands.

    Does that help?

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 90 points

    Thank you very much!

    Now everything works fine :)