0
\$\begingroup\$

I modified my Sega Master System to have a pause button on the actual controller. To process the signal, I soldered a quad NOR gate to the I/O and CPU circuits in the console. The idea is that the controller pause button sends a combined UP/DOWN signal, and the gate processes it and sends an NMI to the CPU, which is what the Pause button on the actual console does. This is the procedure recommended at the SMSPower forum, and I followed their diagrams.

Here's the thing: I installed the gate upside down, so that the packaging is resting on the circuit board, and the pins are sticking up in the air. I did this so it would be easier to solder the pins to the console, reasoning that the package is non-conductive anyway. When I tested it out, it didn't work correctly. As I was troubleshooting with my logic probe, I accidentally dislodged the Vcc connection, which I had soldered to a +5 Vcc solder point on the board. All of a sudden, it works correctly! I've checked the pin assignments over and over, and I'm positive that the Vcc pin isn't connected to anything; yet, the chip has power, and works as it is supposed to.

So, what is going on here? Was it a bad idea to install the chip package so that it touches the board? And why is the chip working when the Vcc pin is disconnected?

(edited to add requested information)

The gate is a CD4001AE Quad NOR; the datasheet is here:

http://www.datasheetlib.com/datasheet/1478457/cd4001ae_harris-corporation.html

The schematic is here, at the bottom of the page:

http://www.smspower.org/Development/JoypadPauseButton

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Schematic? Part numbers/descriptions? Datasheet references? \$\endgroup\$
    – user65586
    Commented Mar 13, 2016 at 13:54
  • \$\begingroup\$ This same question was asked just a few weeks ago---can anybody find the duplicate? \$\endgroup\$
    – The Photon
    Commented Mar 13, 2016 at 17:01

1 Answer 1

4
\$\begingroup\$

You've left out important details such as the part number and logic family of the NOR gate. By the sound of things you have an error in your Vcc connection but when that is removed the chip is able to power itself somehow - possibly through the internal input protection diodes - enough to switch the output as required.

enter image description here

TI application note Designing with Logic goes into detail for various logic families. Figure 5 shows from that document shows the input protection diodes. Note that if Vcc is not connected and one of the inputs is high then the whole chip could be powered up.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.