Datasheet

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4.2.3 FPGA

The design employs a Xilinx Virtex-4 FPGA for capturing the digital data. While the board is

powered up and configured, the FPGA is continually receiving data from the ADC. In response

to a user command through the WV-5 software, the ADC captures the desired amount of data in

its on-chip buffer (up to a maximum of 32K samples per-channel). The user can then command

the FPGA to upload the captured data to the PC through the USB interface for further

processing.

This board can support the ability to program the FPGA for specific requirements. A standard

JTAG connector is provided for downloading FPGA object code from the Xilinx development

environment.

Please note that National Semiconductor does not provide support for any user-designed FPGA

functionality beyond the standard functionality that is shipped with the board.

Hardware Trigger: The external trigger feature of the Reference Board is designed to enable

applications which trigger a data capture. When the hardware trigger is enabled, an acquisition

can be selected from the software, but the actual beginning of data capture will be postponed

until the external trigger pulse is applied to the J26, the EXT_TRIG SMA.

Note: This only applies to the data which is captured and displayed in the WaveVision GUI; the

streaming data to the FMC connector still runs continuously.

Apply the trigger signal at J26, the EXT_TRIG SMA. If the voltage applied, Vtrigger is less than

the threshold, Vth, of the MC100EP16, then the system will not capture. If Vtrigger is greater

than Vth, then a data capture will occur. Vth is approximately 2.1V, so it is recommended that

high trigger voltage is in the range of {2.5V, 3.3V}. The low trigger voltage should be 0V. Note

that this may be a single shot data capture or a continuous trigger. If the trigger is armed, but

Vtrigger is not greater than Vth within approximately 3 minutes, then the software will time out

and show the error message: “Board failed to collect samples.”

Figure 18: Trigger circuit on reference board

1. Connect a signal source to the EXT_TRIG SMA (J26).

2. In the WaveVision5 GUI, select Registers – Settings – check the H/W Trigger box.

3. Select single or continuous capture.

4. Apply Vtrigger above the threshold, Vth, when ready to capture.