In my design,i use the DAC8734,now i met a problem that i cannot sure how can i write the data to Common Register,DAC input Register,Zero Register and Gain Register.As the datasheet says,if we want to fish a DA trasform from Channel A to Channel D,we must write all of the Registers once,as the Time Diagrams mode below.If the data of input Register is variable,others are fixed ,may i write other Registers every time?
From my understanding, you plan to use all of the channels on the DAC and need to set the registers accordingly. The easiest way to complete this task is to power up the device, then write what you would like to all the Zero Register-x and Gain Register-x. Now you can set the command register as you would like. These 9 registers will remain fixed and will not change until you write to them again or power down the device.
Now you can write to the DAC channels DAC-x. There are two ways to update your output. You can either tie /LDAC low and have each output update as soon as /CS returns high (Case 1 on page 9) or you can set /LDAC high, write to all four DACs individually and then pulse /LDAC low to update all four outputs at once (Case 2 on page 9). All of the registers in the DAC8734 will remain at the values you set them to until you write over them. Therefore, you do not need to write to the Command Register, Zero Register-x, or Gain Register-x each time you want to do a conversion.
As you change the data of the input registers the outputs can be updated in the two ways described above.
Another question,if I want to write Zero Register,this Register is 16 data bits ,but the SPI Shift Register is 24 data bits,does it mean every code must be 24 data bits,even the Zero Register or the Gain Register is 16 data bits?
Use page 25 of the data sheet as a guide.
The first 8 bits indicates if it is a read or write and what register you would like to communicate with.
The next 7 bits are 'dont care' bits. Writing to them has no effect.
The last 9 bits (Z8:Z0) are used to set the Zero Register.
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