4.9 A/D State Machine
The DAQP card has an internal state machine that controls A/D operation, (see Figure 4-1). The state machine defaults to S0 after power up or reset. The normal state flow would be first S0 to S3, initiated by a scan list (queue) flush command (RSTQ). Then the queue must be programmed by writing into the queue (base + 1). With the queue being programmed, the next step is moving the state machine from S3 back to S0. This is done by issuing a flush data FIFO command (RSTF), which sets up the gain and channel selections for the first channel in the scan list and then waits for a trigger to start the scan. When the trigger (ADCLK) comes, the state machine moves from S0 to S1 and then A/D conversion is started. The state machine will wait at S2 until the conversion is completed. It then moves to S4, where the A/D conversion result is written into the data FIFO. The scan rate is determined by the time the state machine moves from S1 to S4, which can be programmed as either 10, 20 or 40 ms. If there are more channels left in the scan list, the state machine will skip to S1 for another conversion loop. Otherwise it will return to S0, waiting for another trigger (or a sampling pulse from the pacer clock if in continuous trigger mode). Any time during data acquisition, an A/D stop command will stop the data acquisition by moving the state machine back to S0.
ADCLK (trigger) | Wait (A/D conversion) |
|
| S0 | S1 | S2 |
RSTF > | < RSTQ | | < Channels |
scan | | remaining |
data | list flush | | |
FIFO | | | |
flush | S3 | | S4 |
| |
| | | < Scan list completed |
Figure 4-1. Transition Diagram of A/D Conversion Process
It is important that the sequence of S0-S3-S0 be followed as described above. The user must issue two commands to the DAQP card: the flush scan list command (RSTQ) and the flush data FIFO command (RSTF). This guarantees that the scan list and the data FIFO are flushed properly for the expected data acquisition. Once the flush data FIFO command is issued, the DAQP card will prepare the first channel in the scan list and then return to state S0 waiting for the first trigger. Anytime the data FIFO is flushed, the default threshold setting will be restored (7 bytes to full) by the hardware. The data FIFO threshold should always be programmed after flushing if the required threshold is different from the default threshold.
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