17.GENERATING AN ANALOG OUTPUT

This chapter describes the steps involved in generating an analog output (also called performing a D/A conversion) on a selected output channel using direct programming (not with the driver software).

There are three steps involved in performing a D/A conversion:

1.Compute the D/A code for the desired output voltage

2.Write the value to the selected output channel

3.Wait for the D/A to update

17.1Compute the D/A code for the desired output voltage

Use the formulas on the preceding page to compute the D/A code required to generate the desired voltage.

⇒Note: The DAC cannot generate the actual full-scale reference voltage; to do so would require an output code of 4096, which is not possible with a 12-bit number. The maximum output value is 4095. Therefore the maximum possible output voltage is always 1 LSB less than the full-scale reference voltage.

17.2Write the value to the selected output channel

First use the following formulas to compute the LSB and MSB values:

LSB = D/A Code & 255 ;keep only the low 8 bits

MSB = int(D/A code / 256) ;strip off low 8 bits, keep 4 high bits

Example:

Output code = 1776

LSB = 1776 & 255 = 240 (F0 Hex); MSB = int(1776 / 256) = int(6.9375) = 6

The LSB is an 8-bit number in the range 0-255. The MSB is a 4-bit number in the range 0-15. The MSB is always rounded DOWN. The truncated portion is accounted for by the LSB.

Now write these values to the selected channel. The LSB is written to Base + 6. The MSB and channel number are written to Base + 7. The 2-bit channel no. (0-3) is written to bits 7 and 6, and the MSB is written to bits 3-0.

outp(Base + 6, LSB);

outp(Base + 7, MSB + channel << 6);

17.3 Wait for the D/A to update

Writing the MSB and channel number to Base + 7 starts the D/A update process for the selected channel. The update process requires approximately 30 microseconds to transmit the data serially to the D/A chip and then update the D/A circuit in the chip. During this period, no attempt should be made to write to any other channel in the D/A through addresses Base + 6 or Base + 7.

The status bit DACBUSY (Base + 3 bit 4) indicates whether the D/A is busy updating (1) or idle

(0). After writing too the D/A, monitor this bit until it is zero before proceeding to the next D/A operation.

Prometheus CPU User Manual V1.44

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Diamond Systems PR-Z32-EA-ST, PR-Z32-E-ST Generating AN Analog Output, Write the value to the selected output channel
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