There are several different A/D sampling modes available on Poseidon. The mode in use is selected with the FIFO enable and Scan enable bits at the FIFO control register at Base+7 as well as the A/D interrupt enable bit in the Interrupt control register at Base+9.
Note: If interrupts are not enabled, the FIFO should not be enabled. FIFO storage is only useful when interrupts are used. Otherwise, the FIFO has no effect.
All of these features may be selected as arguments to function calls in the driver software. The control register details are provided for completeness and for programmers not using the driver.
| SCAN | FIFO | Interrupt | Mode | Description |
| No | No | No | Single Conversions | The most basic sampling method. Used for low-speed sampling (typically up to about 100Hz) under software control where a precise rate is not required, or under external control where the rate is slow. Consists of either one channel or multiple channels sampled one at a time. |
| Yes | No | No | Scan Conversions | Used to sample a group of consecutively numbered channels in rapid succession, under software or external control. The time between samples in a scan is programmable between 5 to 20 microseconds, while the time between scans depends on the software or external trigger and may be very short or very long, but is usually less than about 100Hz (above this rate use interrupt scans below). |
| No | No | Yes | Interrupt Single Conversion, Low Speed | Used for controlled-rate sampling of single channels or multiple channels in round-robin fashion, where the frequency of sampling must be precise but is relatively slow (less than 100Hz). The sampling clock comes from the on-board counter/timer or from an external signal. The interval between all A/D samples is identical. |
| Yes | No | Yes | Interrupt Scans, Low Speed | Used for controlled-rate sampling a group of channels in low-speed mode (less than 500Hz per channel). Each sampling event consists of a group of channels sampled in rapid succession. The time between scans is determined by the sample rate. |
| No | Yes | Yes | Interrupt Single Conversions, High Speed | Intended for medium- to high-speed operation (recommended above about 500Hz). Can support sampling rates up to the board’s maximum of 250,000Hz. May also be used at slower rates if desired. The sampling clock comes from the on-board counter/timer or from an external signal. |
| Yes | Yes | Yes | Interrupt Scan Conversions | Used for high-speed sampling of a group of channels where the scan rate is high. The sampling clock comes from the on-board counter/timer or from an external signal. |
Poseidon uses a 1024-sample FIFO (First In First Out) memory buffer to manage A/D conversion data. It is used to store A/D data between the time it is generated by the A/D converter and the time it is read by the user program. In enhanced mode, the entire 1024-sample FIFO is available. In normal mode only 512 samples are available. The FIFO may be enabled and disabled under software control.
In single-conversion mode, the FIFO features are not generally needed so FIFO use should not be selected, although, the FIFO is still actually being used. Each A/D sample is stored in the FIFO. When the software reads the data, it reads it out of the FIFO. In low-speed sampling, each time a conversion occurs, the program reads the data, so there is always a one-to-one correspondence between sampling and reading. Thus, the FIFO contents never exceed one sample.
For high-speed sampling or interrupt operation, the FIFO substantially reduces the amount of software overhead in responding to A/D conversions as well as the interrupt rate on the bus because it enables the program to read multiple samples rather than one at a time. In addition, the FIFO is required for sampling rates in excess of the maximum interrupt rate possible on the bus. Generally, the fastest sustainable interrupt rate on the ISA bus running DOS is around 40,000 per second. Since Poseidon can sample up to 250,000 times per second, the FIFO is needed to reduce the interrupt rate at high speeds. When the interrupt routine runs, it reads multiple samples from the FIFO. The interrupt rate is equal to the sample rate divided by the number of samples read each interrupt. On Poseidon, this number is programmable using the register at Base+6. The usual value is 1/2 the maximum FIFO depth, or 512 samples. Therefore, the maximum interrupt rate for Poseidon is reduced to 488 per second, which easily sustainable on any popular operating system.
Note:: If both Scan and FIFO operation are enabled, the interrupt still occurs at the programmed FIFO threshold and the interrupt routine reads the indicated number or samples and then exits. This happens even if the number of samples is not an integral number of scans. For example, if you have a scan size of 10 and a FIFO threshold of 256, the first time the interrupt routine runs, it reads 256 samples, consisting of 25 full scans of all 10 channels followed by 6 samples from the next scan. The next time the interrupt routine runs, it reads the next 256 samples, consisting of the remaining 4 samples from the last scan it started to read, the next 25 full scans of 10 samples, and the first 2 samples of the next scan. This continues until the interrupt routine ends in either one-shot or recycle mode. In one-shot mode, the last time the interrupt routine runs it reads the entire contents of the FIFO, making all data available.
A scan is defined as a quick burst of samples of multiple consecutive channels. For example, you may want to sample channels 0-15 all at once, and repeat the operation each second. This would be a scan at a frequency of 1Hz. Each time the A/D clock occurs (software command, timer, or external trigger), all 16 channels are sampled in high-speed succession. There is a short delay of 4 – 20 microseconds between each sample in the scan. Since each clock pulse causes all channels to be sampled, the effective sampling rate for each channel is the same as the programmed rate, and the total sampling rate is the programmed sampling rate times the number of channels in the scan range.
Scan sampling is independent of FIFO operation. Either or both can be enabled independently.
In sequential sampling, each clock pulse results in a single A/D conversion on the current channel. If the channel range is set to a single channel (high channel = low channel), each conversion is performed on the same input channel. If the channel range is set to more than one channel (high channel > low channel), then the channel counter increments to the next channel in the range, and the next conversion is performed on that channel. When a conversion is performed on the high channel, the channel counter resets to the low channel for the next conversion. The intervals between all samples are equal. Since each clock pulse results in only one channel being sampled, the effective sampling rate is the programmed sampling rate divided by the number of channels in the channel range.