Contec CNT-3204MT-LPE, Руководство пользователя

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4. Functions
CNT-3204MT-LPE
49
Sampling function
Sampling function
The sampling function obtains count data periodically using the internal or external clock.
The sampling data is transferred to the PC's memory using bus master transfer. The sampling speed can
be up to 20MHz using the internal clock (for 1ch). As bus master transfer is used, an error occurs and
transfer halts if this product cannot obtain bus access in time for a transfer. Note that whether or not
continuous transfer at 20MHz is possible depends on factors such as what other applications are running
on the PC.
This product can perform sampling at various different timings based on the start condition, clock
condition, and stop condition combination. See the "Sampling Control" section later in this manual for
details of how to setup the sampling conditions.
Bus mastering
The bus master transfer function on this product performs DMA transfer between the board and the
application's memory space by utilizing times when the bus is idle. Use a standard variable declaration
in your program to define a static area to use as the application's memory space. As application memory
space is defined by a logical address in operating systems such as Windows, the area may not be
continuous in the physical address space. However, this product can still perform continuous data
transfer to such discontinuous physical address spaces. Bus master transfer can transfer data to physical
address spaces up to 64Mbytes. When setting up the transfer area in your application, note that the
actual available memory size depends on the operating system you are using and on the size of memory
installed on the PC.
The bus master transfer function can transfer data to memory as a one-time transfer or as a ring (cyclic)
transfer. For a one-time transfer, the transfer halts when the end of the specified memory area is
reached. For ring transfer, transfer starts again from the start of the specified memory area when the end
of the memory area is reached. The transfer continues until the stop condition is detected or the transfer
is halted by software.
Interrupt (During bus mastering)
The following interrupt features are available during bus mastering:
- Generating an interrupt upon completion of transfer of the specified number of data items
- Generating an interrupt upon completion of transfer
These interrupts can be passed to the application using the "API-CNT(98/PC)" routines
in API-PAC(W32).
If transfer halts due to an error such as not being able to obtain bus access in time to perform a transfer,
this product halts the transfer and generates the transfer completion interrupt. Whether or not an error
has occurred can be determined by checking the status.