9. PERFORMING AN A/D CONVERSION

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

The A/D FIFO

All A/D conversions are stored in an on-board FIFO (first in first out memory). The FIFO can hold up to 1024 samples. Each time an A/D conversion is finished, the data is stored in the FIFO, and the FIFO counter increments by 1. Each time you read A/D data, you are actually reading it out of the FIFO, and the FIFO counter decrements by 1. When the FIFO is empty the data read from it is undefined – you may continue to read the last sample, or you may read all 1s.

You can read each A/D sample as soon as it is ready, or you can wait until you take a collection of samples (up to 1024 maximum) and then read them all out at once.

To be sure that you are getting only current A/D data, be sure to reset the FIFO each time before you start any A/D operation. This will prevent errors caused by leaving data in from a previous operation. To reset the FIFO, write a 1 to bit 2 of register 7 (see page 20). This bit is not a real register bit; instead it triggers a command in the board’s controller chip. Therefore you do not need to write a 1 and then a 0, just write a 1. However writing to the FIFORST bit affects the values of other bits in this register as well:

outp(Base+7, 0x02); // resets the FIFO and clears SCANEN and FIFOEN

outp(Base+7, 0x0A); // resets the FIFO and SCANEN but leaves FIFOEN set

Note that this register also contains a FIFO enable bit, FIFOEN.

This bit only has meaning during A/D interrupt operations. The FIFO is always enabled and is always in use during A/D conversions.

There are seven steps involved in performing an A/D conversion:

1. Select the input channel or input channel range

2. Select the analog input range

3. Wait for the analog circuit to settle

4. Start an A/D conversion on the current channel

5. Wait for the conversion to finish

6. Read the A/D data

7. Convert the numerical data to a meaningful value

If you are going to sample the same channel multiple times or sample multiple consecutive channels with the same input range, you only need to perform steps 1-3 once, and then you can repeat steps 4-6 or 4-7 as many times as desired.

Diamond Systems also provides sample register level code, downloadable at http://diamondsystems.com/files/binaries/SourceCodeExamples.zip