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Week 6 - Group assignment - Electronics design

Assignment

Use the test equipment in your lab to observe the operation of a microcontroller circuit board (in minimum, check operating voltage on the board with multimeter or voltmeter and use oscilloscope to check noise of operating voltage and interpret a data signal)

Equipment we used to test

  1. Laboratory Power Supply
    2. TENMA 72-10480
  2. Multimeter
  3. Oscilloscope
    4. Memory prime GDS-1152a
  4. Logic analyzer
    5. Saleae Logic 16 (knockoff)
  5. Alligator Clips

For our group assignment we tested a multimeter, a powerbench and an oscilloscope. Next to that Henk demonstrated a logical analyzer to us. For the testing device we used a board with an ATTiny44 chip containing a LED and a button.

Goal

See the output to the led on ATtiny44 via an oscilloscope. The Led supposed to be blinking. But this was so fast it was not visible by the eye, so the only way to see the output was by using a Oscilloscope. Next to that we wanted to know the currency drawing of the board.

Setup

Power the ATTiny the Lab Supply, and observe attach the Leg of the LED to the oscilloscope and read the voltage change in time on the oscilloscope.

Step 1: The multimeter

To connect the board to the power bench we first had to find the VCC and GND on the board. We used a multimeter to find the GND and VCC. First we looked up the image of the board and looked where the GND was. We looked in the datasheet for the Attiny44 to find the GND leg on the Tiny44. It is leg number 14. We needed a magnifying glass to find the Dot on the component and find the leg numbers.

We then checked with the multimeter what pins of the FTDI was connected to leg 14. We measured with the multimeter in continuity mode. We then put one pen on the GND and the other pen on one of the FTDI pins. One beeped, so that must be the ground. Our educated assumption that it was pin 14 was correct.

Step 2: Laboratory Power Supply

This device has its own power source and can be used to feed power to the device you are measuring. You can adjust the values by a turnable knob. We looked up the voltage needs for the Tiny board and saw it is 5V. We set the power bench to 5V and tried to clamp the alligator clamps to the device. This was hard. The clamps are made for larger electronic components then the tiny board we were testing. The overly large clamps remained a struggle throughout the test. We clipped the clamps to the VCC and GND on the FTDI header that we located in the previous step.

After we connected the power bench at 5V to the board we had a read out stating that the board was drawing 5mA. This information you can use to calculate how long a battery will last when it is powering your board.

image of the alligator Clips connected to the FTDI pins (gnd & VCC) and powered by the Lab power supply

We then measured the voltage over the LED with the multimeter. We held each probe pen on one side of the LED. We set the multimeter at setting 2V because the datasheet said the LED is 1.8V. We measured 0.8V. We figured that since the LED is blinking fast, the measured voltage was lower than the LEDs specifications as it oscillated between on and off. We located the VCC side of the LED by looking for the green stripe on the component that indicates the VCC. We used a magnifying glass to see it.

We measured the voltage over the LED. We could not see the led going on and off using a multimeter. The voltage was 0.813 volt

Step 3: Oscilloscope

An oscilloscope is a device used for testing electronics. Its main function is to display voltages in time. From Wikipedia "Oscilloscopes display the change of an electrical signal over time, with voltage and time as the Y- and X-axes, respectively, on a calibrated scale. The waveform can then be analyzed for properties such as amplitude, frequency, rise time, time interval, distortion, and others."

Next we wanted to see the voltage over the LED. The LED was programmed to blink really fast, we didn't know how fast and couldn't see if it was really blinking. It looked DImmed. Our challenge was to get a read out of the voltage over the led. We connected the oscilloscope to the GND and to the leg of the ATtiny44 that is connected to the leg. The LED leg was easy to find as we could just see with our eyes and follow the copper trace that connects the LED to the chip. We Connected the GND of the Oscilloscope to the GND on the FTDI pin on the board.

We then used the oscilloscope's alligator clamps to connect the GND of the oscilloscope to the GND of the board. And then made contact between the probe pen of the oscilloscope and the leg of the ATtiny44 that connects to the LED.

We turned on the oscilloscope. Then turned the voltage setting until we saw a flat line. Twice we made a short. This happened because the large pen of the oscillator was hard to keep in the right spot. The board stopped blinking and we thought we'd broken it. But it was the powerbench that had shut itself off. We surmised that the powerbench has a build-in safety feature that it stops powering when there is a short.

We first set the oscilloscope on 100 mV. We saw the the jumps between the voltage going from flat to top. We then changed the setting to 2V with the turning the knob Volts/Div turning knob. And then altered the timing with the Time/Div button until we saw the squares.

The best representation of current flowing through the LED was at setting 2V and timing 2.5 milliseconds. We could now clearly see the representation of the blinking LED as we saw the line on the oscilloscope go up and down.

One square on the grid of the display depicts the measured unit. In our case we had set it to 2 Volts. So 2.5 squares in the picture refers to 5 Volt. The time was set at 2.5 milliseconds. And the panel said 50mA.

Logical analyzer

Henk demonstrated a logical analyzer to us. This is a nifty device that can read out bits. In the final instance bits are nothing other than a transistor switching on and off. The logical analyzer measures this on and off switching and represents them in software. It groups the bits in bytes (8 bits). In our case the analyzer was tracking an echo operation on the board. When connected to a computer and you type in something, the board echo's that message back.

In the software of the logical analyzer we could see the bits represented as on (current flows or 1) and off (current does not flow or 0) in the form of visual stripes high (on) of low (off). Eight bits were grouped together visually divided by a vertical blue stripe in the software. Above the bytes the software translated the bits in to more easily humanly readable representations. Since the chip was sending letters (echoing what Henk typed on his laptop) the bits formed ASCII code. Each byte represents a single character. The software showed the characters above the bytes in the visualizer.