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Transferring Data from Hardware to System Memory
The transfer of acquired data from the hardware to system memory follows these steps:
- Acquired data is stored in the hardware's first-in first-out (FIFO) buffer.
- Data is transferred from the FIFO buffer to system memory using interrupts or DMA.
These steps happen automatically. Typically, all that's required from you is some initial configuration of the hardware device when it is installed.
The FIFO Buffer
The FIFO buffer is used to temporarily store acquired data. The data is temporarily stored until it can be transferred to system memory. The process of transferring data into and out of an analog input FIFO buffer is given below:
- The FIFO buffer stores newly acquired samples at a constant sampling rate.
- Before the FIFO buffer is filled, the software starts removing the samples. For example, an interrupt is generated when the FIFO is half full, and signals the software to extract the samples as quickly as possible.
- Because servicing interrupts or programming the DMA controller can take up to a few milliseconds, additional data is stored in the FIFO for future retrieval. For a larger FIFO buffer, longer latencies can be tolerated.
- The samples are transferred to system memory via the system bus (for example, PCI bus or AT bus). After the samples are transferred, the software is free to perform other tasks until the next interrupt occurs. For example, the data can be processed or saved to a disk file. As long as the average rates of storing and extracting data are equal, acquired data will not be missed and your application should run smoothly.
Interrupts
The slowest but most common method to move acquired data to system memory is for the board to generate an interrupt request (IRQ) signal. This signal can be generated when one sample is acquired or when multiple samples are acquired. The process of transferring data to system memory via interrupts is given below:
- When data is ready for transfer, the CPU stops whatever it is doing and runs a special interrupt handler routine that saves the current machine registers, and then sets them to access the board.
- The data is extracted from the board and placed into system memory.
- The saved machine registers are restored, and the CPU returns to the original interrupted process.
The actual data move is fairly quick, but there is a lot of overhead time spent saving, setting up, and restoring the register information. Therefore, depending on your specific system, transferring data by interrupts might not be a good choice when the sampling rate is greater than around 5 kHz.
DMA
Direct memory access (DMA) is a system whereby samples are automatically stored in system memory while the processor does something else. The process of transferring data via DMA is given below:
- When data is ready for transfer, the board directs the system DMA controller to put it into in system memory as soon as possible.
- As soon as the CPU is able (which is usually very quickly), it stops interacting with the data acquisition hardware and the DMA controller moves the data directly into memory.
- The DMA controller gets ready for the next sample by pointing to the next open memory location.
- The previous steps are repeated indefinitely, with data going to each open memory location in a continuously circulating buffer. No interaction between the CPU and the board is needed.
Your computer supports several different DMA channels. Depending on your application, you can use one or more of these channels, For example, simultaneous input and output with a sound card requires one DMA channel for the input and another DMA channel for the output.
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