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I'm working on a free-running circuit like the one here: http://www.z80.info/z80test0.htm

The only differences are that I'm using a 4049 clock circuit with a 1 uf cap, and I've added an extra LED at Address pin #3. The resistors call for 470 ohm, but I'm using what I have, which is a 1K. Now, I didn't pay attention the first time I inserted my Z80, and found it was upside-down, among other small but important wiring details, which have been fixed. However, the LEDs are not glowing at all upon power up and a few resets. And I've checked for power problems, because I have a power-indicating LED mounted for that purpose on the breadboard, to which it glows proudly. So is my Z80 fried, or could it be something else?

OrangeCalx01
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    I'm surprised this circuit works at all. The Z80 datasheet specifies a maximum clock-low time of 2 us, and says that although the chip is static by design, operation with a clock-high time "greater than 200 us is not guaranteed." In other words, the minimum clock frequency should be 1/202us, or 4950 Hz (with a 99% duty cycle). Are you using a "real" Z80, or some sort of CMOS clone? – Dave Tweed Mar 19 '14 at 13:04
  • So are you saying it has to do with my multivibrator being CMOS? It can easily be replaced with a 555 multivibrator... it's genuine - has "Zilog" on it. – OrangeCalx01 Mar 19 '14 at 16:12
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    No, I'm just saying that Zilog never intended the CPU to run that slowly. Even if you had a known-good chip, there's no guarantee that this self-test setup would work. But in any case, I think you fried it when you plugged it in backwards. – Dave Tweed Mar 19 '14 at 18:15
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    Nope! As soon as I fixed my wiring issues, as well as make a proper clock out of a 555 and some caps, address signals flashed across the LED's like it was supposed to. Thanks though! – OrangeCalx01 Mar 31 '14 at 12:25

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Putting the Z80 upside-down it's not a good thing, but the Z80 power pins are placed in a way that don't make flows reverse current (11 - 29). Unlucky this cannot give us that the poor and old CPU, even if survived to the time, will continue for the future .... it can be 'tired' about the injury and decide to abandon this cruel world.

For the clock you can use a simple 74LS14 (Schmitt trigger inverter) instead of CD 4093 that is a CMOS, not the best to drive this kind of CPU (with input impedance similar more to a TTL or LS IC than a CMOS).

To investigate more, without to go in the deeps of this archaeological device, consider to test if there are some change in the /MREQ /RD and /RFSH. You can have an help measuring the frequencies at these pins or trying to drive an high impedance headphone with a capacitor in series.

Colin
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umlibero
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