Digital Debugging

You will use the debugger board (see below) to test whether your circuit is working or not.

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Another version looks like this:

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The debugger board is set up with either two or four banks of eight signals. Here's a closer look at a single bank:


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Each bank can be configured to either "display" logic levels in your circuit or to "control" logic levels in your circuit. In "display" mode, you can connect a signal to its corresponding connection on the debugger board and the LED will be ON if the signal is HIGH and OFF if it is LOW.

Here's what the debugger board looks like connected:


These are the power connections:


This is a bank in "control" mode (i.e. you can use it for inputs to a circuit):

Hint: If you're not using all of the pins in one bank, spread them out so it's easier to tell which is which.

This is a bank in "display" mode (i.e. you can use it to see the outputs from a circuit):

Note: The debugger board has TTL gates on its input. These gates, like most TTL gates, tend to float HIGH if unconnected. Thus, if you have nothing connected to a pin in a bank set for "display" mode, the LED will usually be ON. No connection does not mean LOW; pins must be tied LOW if you want to ensure that they are LOW.

In "control" mode, the DIP switches for a bank of connectors will control the logic level on the pins.

Note: Make sure that you do not try and have two different things trying to control the same signal. For instance, only use the debugger board in "control" mode to connect to inputs of your circuit which are not connected to anything else.

Switching a bank from one mode to the other is accomplished either by a single DIP switch or by a jumper. (You can tell which mode you're in by whether the bank of DIP switches change the LEDS. If so, you're in "control" mode. If not, you're in "display" mode.) The circuits on the debugger board need power and ground to work, so make sure they are connected.

Here are a few important rules to follow when handling ICs during debuging:

1. Always use red wires for power and black wires for ground. Don't use either colour for anything else!
2. Always turn off the power on your breadboard before removing/inserting any ICs.
3. Be careful to avoid reversing power connections. Typically we only need +5 V and 0 V (ground) connections for digital circuits. Ask your IA for help setting up power busses on your breadboard if in doubt.

Troubleshooting Suggestions

Your circuit will probably not work the very first time. Therefore, trouble-shooting is a basic skill that you will have to learn.

The followings are a list of helpful suggestions:

• Always use red wires for power and black wires for ground. Don't use either colour for anything else!
• Ensure you're are using the right IC and placing in the right way on the breadboard.
• Start trouble-shooting at the point at which the logic/voltage value is wrong in the circuit and continue to check your circuit backward, making sure that at each point the logic/voltage value agrees with your desired value.
• Construct your circuit neatly. Use appropriate length wires for connections; i.e. use short wires for short distance connections. Your circuit should not look like a bowl of spaghetti:
  
  A better look would be:
  .
• Check IC insertion to make sure all the pins are in the holes correctly and the IC does not have any missing pins.
• Make sure that the power and the ground are properly connected to all IC's before powering up your ciruit.
• Do not strip wire ends longer than 1/4" (1/2 cm) and don't jam long bare ends into the breadboard holes. This will cause shorts and ruin the board.
• Do not short the power supply to the ground in the breadboard. This will cause permanent chip damage.
• Do not connect an output of any gate to the output of another gate, to a switch, to the power (+5V), or to the ground. These situations will cause excessive currents and result in permanent damage to the chip.