Diamond Systems PR-Z32-E-ST, PR-Z32-EA-ST Perform an A/D conversion on the current channel

14.4 Perform an A/D conversion on the current channel

After the above steps are completed, start the A/D conversion by writing to Base + 0. This write operation only triggers the A/D if AINTE = 0 (interrupts are disabled). When AINTE = 1, the A/D can only be triggered by the on-board counter/timer or an external signal. This protects against accidental triggering by software during a long-running interrupt-based acquisition process.

outp(base,0x80);

14.5 Wait for the conversion to finish

The A/D converter chip takes up to 5 microseconds to complete one A/D conversion. Most processors and software can operate fast enough so that if you try to read the A/D converter immediately after starting the conversion, you will beat the A/D converter and get invalid data. Therefore the A/D converter provides a status signal to indicate whether it is busy or idle. This bit can be read back as bit 7 in the status register at Base + 3. When the A/D converter is busy (performing an A/D conversion), this bit is 1 and the program must wait. When the A/D converter is idle (conversion is done and data is available), this bit is 0 and the program may read the data. Here are examples:

while (inp(base+3) & 0x80); // Wait for conversion to finish before proceeding

This method could hang your program if there is a hardware fault and the bit is stuck at 1. Better is to use a loop with a timeout:

int checkstatus()	// returns 0 if ok, -1 if error
int i;
for (i = 0; i < 10000; i++)
{
if !(inp(base+3) & 0x80) then return(0);		// conversion completed
}
return(-1);	// conversion didn’t complete

14.6 Read the data from the board

Once the conversion is complete, you can read the data back from the A/D converter. The data is a 16-bit value and is read back in two 8-bit bytes. The LSB must be read from the board before the MSB, because the data is inserted into the board’s FIFO in that order. Unlike other registers on the board, the A/D data may only be read one time, since each time a byte is read from the FIFO, the FIFO’s internal pointer advances, and that byte is no longer available. Note that reading data from an empty FIFO returns unpredictable results.

The following pseudo-code illustrates how to read and construct the 16-bit A/D value:

LSB = inp(base);

MSB = inp(base+1);

Data = MSB * 256 + LSB; // combine the 2 bytes into a 16-bit value

The final data is interpreted as a 16-bit signed integer ranging from –32768 to +32767.

⇒Note: The data range always includes both positive and negative values, even if the board is set to a unipolar input range. The data must now be converted to volts or other engineering units by using a conversion formula as shown on the next page.

In scan mode, the behavior is the same except that when the program initiates a conversion, all channels in the programmed channel range will be sampled once, and the data will be stored in the FIFO. The FIFO depth register will increment by the scan size. When STS goes low, the program should read out the data for all channels.

Prometheus CPU User Manual V1.44 Page 49