SE3910 Week 2, Class 1 Today Basic Circuits Other ??? Tomorrow Lab 2, S365 (Complete prelab BEFORE) Thursday See ScheduleSchedule SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 1

Warning… Originally from: SE-2811 Dr.Yoder 2

Cartoon One source: ohmslaw.htm SE-2811 Dr.Yoder 3

Equipment (Reminder) This week: Purchase 4GB or greater micro SD card Used Lab 3 (Optional) Tools – e.g. needle-noise pliers Check out short-term Kit (breakout board, breadboard cape, LCD cape, USB Oscilloscope) Small parts (also may desire to purchase) SE-2811 Dr.Yoder 4

Ex: Compute voltage across C1 If S1 is closed, and S2 is opened, what is the voltage across C1? If S2 is closed, and S1 is opened? If both are opened? If both are closed? SE-2811 Dr.Yoder 5

Ex: Compute voltage into GPIO If S1 is open and S2 is closed, compute the voltage from GPIO 12 to ground. (Assume GPIO 12 is not connected to anything) SE-2811 Dr. Yoder 6

A slightly better assumption if connected to a GPIO input SE-2811 Dr. Yoder 7

Warning 1: (Low risk) Input is like a (very small) capacitor Modern processors use CMOS technology These gates can look something like this on the inside: If left “floating,” can be any voltage If two switches use, can short source, so … SE-2811 Dr. Yoder 8

Solution 1: Use a pull-up or pull-down resistor (This is from Lab 2, you will analyze it there.) ( (See pp in E Exploring Beaglebone b by Derek Molloy,. 2015) SE-2811 Dr.Yoder 9

If you apply 3.3 V to a 2.10 V LED, it will be like applying 1.2 V directly to a short circuit Use a limiting resistor to fill up the rest of the voltage and set current. Warning 2: (Low risk) LEDs have constant voltage SE-2811 Dr. Yoder 10 Solution 2: Use limiting resistor

Warning 3: (High risk) Do not short the GPIO pins to ground or 3.3V when they are in output mode. SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 11

Solution 3: Do not place beaglebone on metal surface (e.g. aluminum-finish laptop) [EB, p. 21] Ensure beaglebone is correctly configured before powering If circuit provides input, beaglebone MUST NOT provide output When using output, your circuit should not Source more than 4mA (current out of bone) Sink more than 8mA (current into bone) … for any given GPIO pin. SE-2811 Dr.Yoder 12

Warning 4: (High risk) Beaglebone is 3.3 V Supplying 5 V to any GPIO pin could damage it. Furthermore, supplying even 3.3 V to a pin without powering the board could damage it. SE-2811 Dr.Yoder 13

Solution 4: Always use the 3.3 V supply Never use external power – use the 3.3 V supplied Unless you know what you are doing! May need to use voltage converters from external signals. Or at least external logic “enabled” by the 3.3V supply SE-2811 Dr.Yoder 14

Do not unplug beaglebone while running Residual currents can result in higher residual voltages Memory might be saved in inconsistent state Hold down power button until all lights turn off If you must, hold down reset button (which forces the bone off) while unplugging. Warning 5: (Medium risk) SE-2811 Dr.Yoder 15 Solution 5:

You can, e.g. easily plug into 5V when you mean to plug in to 3.3V. Print labels for your pins tiny.cc/ebb105 [EB, p. 21] tiny.cc/ebb105 Use the breakout cape It has labels already printed Google beaglebone black pinout Warning 6: Hard to pick the right pin SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 16 Solution 6:

No more than 1.8 V into analog pin Use the supplied 1.8 V supply as power for analog devices Ensure devices are not configured in analog before plugging into POTENTIALLY analog pins Warning 7: Analog limit SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 17 Solution 7:

If you pull out the card, it could cause damage Push in the card, and it will push itself out (never fails…) Warning 8: Don’t pull SD card SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 18 Solution 8:

If you are working on a very complicated circuit, please see the (un)official System Reference ManualSystem Reference Manual And read all the red text (I have a summary file, if you want …) Warning 9: Other things SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 19 Solution

GPIO safety (summary of a few key points) Sourcing limit: 4mA Limit if current is coming OUT OF Beaglebone Sinking limit: 8mA Limit if current is flowing INTO Beaglebone Digital Voltage: 3.3 V Analog input max voltage: 1.8 V SE-2811 Dr.Yoder 20

Ex: What is the resistance of this resistor? SE Dr. Josiah Yoder Slide style: Dr. Hornick Much Material: Dr. Schilling 21

References EB: Derek Malloy, Exploring Beaglebone, Wiley, 2015 SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder 22