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I have a Yamaha DX-100 that when powering on it only displays a brief flash on the LCD display, then nothing. Proper power is present throughout the board although the chip enable input for the ROM is at around 1.20 V. Being an active low input that wouldn't activate the chip or tie it high. Aren't inputs always tied high or low?

Also, the CE input is preceded by a quad two-input NOR gate with its inputs tied together. The same IC is used elsewhere having both inputs tied together. Any technical reason for why a regular inverter wasn't used here?

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  • \$\begingroup\$ Is this 1.2V measured with a multimeter or oscilloscope? If it’s the former I expect you’re seeing an average voltage that reflects average time the rom is on the bus. \$\endgroup\$
    – Bryan
    Apr 1, 2023 at 5:28
  • \$\begingroup\$ I did measure with a multimeter, but this voltage doesn't fluctuate at all. \$\endgroup\$
    – SJ8
    Apr 1, 2023 at 6:11
  • \$\begingroup\$ If you have a spare NOR gate in your design then it's far more efficient to use that instead of adding another entire inverter IC and wasting 5 of its gates. \$\endgroup\$
    – brhans
    Apr 1, 2023 at 6:14
  • \$\begingroup\$ Aren't any spares though. Single chip, four gate, but only two are used and both have the inputs tied together. \$\endgroup\$
    – SJ8
    Apr 1, 2023 at 6:19
  • \$\begingroup\$ Could maybe be a BOM reduction effort. If you need NOR gates elsewhere in the circuit, and only 4 or fewer inverters, then using the NOR gates as inverters means one less different line item in the BOM. \$\endgroup\$
    – brhans
    Apr 1, 2023 at 6:23

1 Answer 1

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You can't expect to measure a microsecond level signal with multimeter.

Sure, the voltage could be 1.2V at DC, which is wrong, or the CPU could be executing from ROM as usual and it would have an average voltage of 1.2V and that is not an issue.

The multimeter can't tell the difference.

And the NOR being used in place of an inverter, it's just likely that it is the nearest free gate that was able to perform the required function to invert a signal.

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