DEBUGGING TECHNIQUES

Wiring Type of wire to use Solid core for breadboard Multiple-strand for flying leads and connections to power supply unit (PSU) Be consistent with colour coding of wires, especially power supply signal to differentiate different levels. Suggested colours of red for +5V and orange for +3V Being consistent as is easier to help debug each other’s circuits and reduces the chance of accidentally applying wrong voltages. Be consistent with colour coding signal for scope channels as we easily confuse the written colours with the channel colour. This helps when looking at the images collected from the scope later. Known the Stroop effect

Circuits Know what the expected voltages are at intermediate points in the circuit. This is what circuit simulation and hand calculations provide you. Begin with simple measurements. Is there power to all of the components in the circuit? Are all of your grounds connected to one another? Start at the very beginning of the circuit. Does the input signal exist? Move from one circuit to the next to make sure that the data flow is correct. Where possible, disconnect subcircuits and inject known input signals in and measure output. For example, disconnect the amplifier from the optical sensor and make sure that the amplifier works properly with a known input signal. This is why we breadboard a circuit before soldering the components on to a PCB. Feel (carefully) if the major components in the subcircuit are hot, but not too hot. Record your measurements! Other members of your team may want to look at them – Use their help.

mbed Electrical Make sure that the voltage supplied to power the mbed is the correct voltage, particularly when the other components in the circuit are consuming power. Voltage droop Suggestion: Connect the mbed into the pulse meter circuit after you have determined that all of the other subcircuits work properly. There are few replacement mbed available and none are free!

mbed Software Be neat and structured Split the code into small understandable sections Comments need to be accurate Ideally, there is a comment with every line of code to explain what that line does. Industry standard is that a software engineer writes about 10 lines of good code per day. Naming variables, functions, and subroutines be consistent There are some cases where a decision may be made whether hardware or software should be used to perform certain functions. Identifying the blocks of code that act like a hardware block allows others to insert or remove the software. Debugging Without a Debugger Toggle a pin and note whether there was a reaction to an input – switch between features or modes on a serial port Save old versions of your code Have a naming convention that allows you to determine which code is the latest one. Other suggestions on debugging are posted at

Debugging on a PCB Put test points at appropriate places on the PCB Test points are places where there is an a pin (or an exposed metal run, but this is not really good practice) at an input or output terminal of a component so that you can measure the voltage at these points. Test points can also be used to inject known input signals into subcircuits when disconnected from the rest of the circuit. Construct circuit on PCB in blocks Build the subcircuit that caused the most problems on the breadboard first. Characterise it with a known signal injected at a test point and power supplied to the ICs. Once it is working, continuing building the rest of the circuit. Stop to verify that the subsequent blocks function properly.

Header pins

Use IC sockets Only solder an IC directly on to your PCB if: You are exceptional at soldering and won’t overheat the component accidently solder two pins together solder the part in backwards and will never damage the part while testing the circuit. Otherwise, use an IC socket

IC socket for mbed Two female header strips 1x20 You will have to layout the holes for the header strips correctly so that the two rows of pins on the mbed line up with the two header strips. You will also have to solder the two header strips in well so that they are perpendicular to the PCB and will not flex when the mbed is pushed in place. 1x10 female header strip as an example

Label Component Locations Use your PCB as more than a way to run metal wires from one component to another. Put your names on it. Label the locations of the components With the full name of the component if possible or a code that you give to the component. Number the holes for pins or location of notch to remember how the IC and electrolytic capacitors should be placed on the board l

If you are frustrated STOP!!! Bad things happen to good circuits when the person debugging is ticked off. Same is true if you are rushing because you have other things that you must do Friday evening tend to be poor time to debug circuits if your friends are waiting for you somewhere else. Don’t wait until the end of the semester to get this project done, time pressures make for bad decision making and major errors. Also, you and your teammates will not get a good grade as there will be due dates for sections of the project throughout the semester. Debug with someone else on your team. Discussing what is observed can help you both think through the problem and find a solution.

This is a team project Figure out how to keep your teammates informed about the status of the project. It is a waste of time if you debug the software, find the errors, and fix it and then a teammate repeats the same process using the original code. Store the components, breadboards, PCBs, and software in places where the other team members can access them. It is not an acceptable excuse to say that your teammate took the circuit home over the weekend to show the parents and then forgot to bring it back to school. When there is a problem and/or a solution, everyone on the team should be aware of it. There should be note of this in the minutes from team meetings recorded by the scribe (stapled in each lab notebook) or a hand written note/copy of an in each team member’s lab notebook.