Hello, I am a masuo.
When I was doing my own board wiring, a multimeter was indispensable. I used it to measure current and voltage on the board and to check for opens and shorts. However, I needed to be careful when using the multimeter on a board with an FPGA.
LEDs do not light up...
In the production exercise, a board that controls a 7-segment LED to realize a clock is being produced. The 7-segment LED lights up when activated low (Figure 1 and Figure 2).
Figure 1: FPGA outputs High = LED turns off Figure 2: FPGA outputs Low = LED lights up
However, the 7-segment LED did not light up as specified.
When measuring the voltage, for some reason, some of the 7-segment LEDs remained off even though the FPGA was low (IO pin indicating 0V), contrary to Figure 2.
A continuity check was performed between the FPGA IO pins connected to the LEDs not lit by the multimeter and the board GND. Since no buzzer sound was heard, I thought the wire was disconnected and discussed this fact with my senior.
Masuo : "There seems to be a disconnection between the IO pin of the FPGA and the board GND..."
Senior : "First of all, did you check the specifications of the multimeter to see if it is safe to measure continuity to the FPGA?"
Masuo : "Oh, I'm sorry, I didn't check. What are you checking?
Senior : "When the multimeter is in continuity check mode, the voltage at both probe terminals should not exceed the absolute maximum rating of the FPGA.
I found that the voltage at both probe pins with the multimeter in continuity check mode was 7V, which exceeded the absolute maximum rating.
The absolute maximum rating of the Cyclone® IV used in this case is 4.2V.
Senior : "It is possible to damage the FPGA's IO pins by applying a voltage higher than the absolute maximum rating or by allowing a large current to flow in the FPGA, so be careful when using a multimeter with an FPGA board."
Mass man : "When an IO pin shows 0V, how do I know if it is 0V due to GND or open?"
Senior : "The current-voltage characteristics of semiconductors are measured with a curve tracer.
What I learned
- The voltage at both terminals of the probe when the multimeter used by Masuo was in continuity check mode exceeded the absolute maximum rated voltage, so it was wrong to measure with the multimeter.
- The voltage over the absolute maximum rating on the IO pins of the FPGA or high current in the FPGA is a cause of damage, so be careful when using a multimeter with an FPGA board.
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