Debugging the MUX

With Battat gone for the rest of the week, Dr. Phillips came in Wednesday to help with the development of the project again. Since we debugged the counter in our last session, we wanted to focus on the debugging of the MUX eval board and see how it worked with the 16-channel signal simulator.

Wednesday Session: Observations

After soldering wires on to the A0 and A1 inputs as well as the V+, V- and Ground inputs of the board, we hooked it up and saw that the timing of the address lines were incorrect, although we had already checked to see the counter working perfectly. We also noticed that the Vout line of the MUX showed four levels of a voltage pattern -- one about .3V, a second and a third slightly above GND level, and possibly a fourth, which would indicate that the signal simulator is working properly with the MUX and ribbon cable. The voltage pattern from Vout followed proper operation however, since the signals on the MUX inputs were mixed using jumper wires to place high potentials on inputs adjacent to low-voltage female headers.

Yet, when we delved deeper into our analysis of the system, we noticed two main issues: the counter was a full 50% off from what it should be for the address lines but it worked correctly without the MUX connected; and although we read a signal pattern at the Vout line, we could not match (or find for that matter) the input signals received at the four input channels of the MUX to the Vout signal pattern. We tested to see if the indicator LEDs were having any affect on the odd behavior by removing them, but they seemed to neither aid nor worsen the situation. We also looked at the connections to the MUX themselves and the trigger levels on the 300 MHz oscilloscope,

Potential Solutions

It is possible that the erroneous behavior lies in the wiring of the MUX, or perhaps a stray impedance due to the temporary, low-quality wiring currently used. If the problem is not solved with a very modest change such as the quality of the wiring, we should look to design the PCB (printed circuit board) with hex inverters between the counter and the MUX that will serve as buffers. Additionally, a set of 0-ohm jumpers would be used to allow the buffer to be part of the circuit or easily omitted if necessary. Moreover, the next step is to look at gEDA for PCB design and simulation.

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