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Oscilloscope Measurements

The Digital Systems & Microprocessor Lab is equipped with HP digital oscilloscopes. These CROs have features which make it straightforward to measure signal frequencies and to capture one-shot events (such as a serial byte transmission).

Figure 59 shows the CRO displaying the 19,200 Hz clock signal provided by the digital trainer board.

  

Figure 59: CRO with receiver clock signal and idle channel signal.

Because the frequency dial on the trainer board is not very accurate, you can use the CRO to set the frequecy as follows:

• Connect, say, channel 2 of the CRO to the clock signal (FPGA ribbon cable pin 40).

• Connect the CRO ground to your GROUND rail.

• Press the source button and select channel 2.

• Press the autoscale button to automatically detect the clock signal.

• Adjust the volts/div to 2V, and the time/div as appropriate to see the waveform as shown.

• Press the cursor button, and use the cursors, as shown, to measure the frequency (and period) of the signal. In the figure cursor t1 is set to the beginning of a clock cycle, while cursor t2 is set to the end; the period is displayed on the screen.

Figure 60 shows the transmitted serial signal.

  

Figure 60: Serial signal showing the byte $34.

You can capture the signal as follows:

• Connect, say, channel 1 of the CRO to the serial signal (SBC cable pin txd1).

• Adjust the trace of channel 1 with 2V/div and a suitable time/div.

• Press the source button and select channel 1.

• Turn the level knob and select a voltage level of about 2.5 V.

• Press the mode button and select single.

• Press the run button.

• Transmit a byte from the SBC. It should appear as shown on the CRO screen.

• You may need to adjust the time/div to see the whole byte.

• The sample signal is also shown in Figure 60. You can see the trace by connecting the signal to channel 2.

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