Friday, September 23, 2011

Test Test and Retest

In my last post I gloated about my DFF always starting in the LOW state... Turns out that is wrong.  I was testing to see if the 7400 can be used as a tri-state buffer and noticed that once in a while the DFF would start high.  This does not happen often, maybe 1 in 10 times, which it turns out is infrequent enough for me not to see it when I was first testing it.  I should have tested the circuit more thoroughly but I was too excited to fully evaluate it.  This mistake wouldn't have killed the entire design as there is only one part that absolutely needs to start low, but It could have added a nuance to the game where the car wouldn't show up sometimes and you have to toggle the power switch.

I added a simple fix that seems to work, I need to do further testing to ensure it does but I can't until the rest of the parts come in next week.  I added a capacitor and a resistor that makes the clock start high and go low when the capacitor drains.  This makes the initial state of D the start condition for the output.

The standard DFF using a single 7400 Quad NAND

The advanced implementation of the DFF

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