Vcc above 5V - how much is too much?

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jms2
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Vcc above 5V - how much is too much?

Post by jms2 »

In my quest to understand why my Gotek won't co-operate nicely with a real floppy drive, I have got as far as looking at the 74HC04 inverter within the Gotek itself. Keir Fraser (the author of Flashfloppy) asked me to check whether it was inverting signals correctly, and we found that it wasn't. The problem is that it is working with 3.3V signals but is powered by a 5V supply, and a logic high is taken as >0.7 * Vcc, which is 3.5V.

This is a known issue but under normal circumstances it just about works. However, my Compact's PSU is outputting 5.201V as measured by my multimeter. This would mean that a logic high value would have to exceed 3.65V, and I think that maybe this is asking a bit much. It could explain why I am seeing weird effects, some of which are repeatable and some of which aren't.

So I'm wondering whether it is better to adjust the output voltage of the PSU (which appears to be very hard given that there are no schematics and no obvious way of doing it), or changing the 74HC04 to a TTL-compatible 74HCT04.

5.2V is within a tolerance of +/- 5% so am I right in thinking it would be an acceptable value under normal circumstances?
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Re: Vcc above 5V - how much is too much?

Post by Kazzie »

5.2V would certainly be acceptable if you were using compatible logic families.

The underlying issue you have here is that TTL components are only specified to output a "logic high" voltage of 2.7V~3.3V minimum (depending on the exact family), which is well below the minimum input of 0.7 Vs for CMOS (3.5V for a 5V supply), and approaching the threshold level of 0.5. It sounds like your two systems are interfaced outside their specified parameters, so it's not too surprising that you're getting unspecified behaviour.

While I haven't looked at the Gotek's circuit, bridging between TTL and CMOS (HC) logic is exactly what the HCT family was designed for, and would be my preferred fix.

p.s. this thread may be of interest.
BBC Model B 32K issue 7, Sidewise ROM board with 16K RAM
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jms2
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Re: Vcc above 5V - how much is too much?

Post by jms2 »

Thanks - that's the reassurance I was looking for that the 5.2V isn't inherently wrong, just not helpful in this case.

Actually I don't think that the inverter in the Gotek is used to interface between CMOS and TTL. All the inputs are CMOS, and if I'm reading the schematic correctly there is a transistor on each of the outputs to switch on a 5V signal. The issue is simply that, for reasons that aren't clear to me, the inverter is powered with 5V (or 5.2V in my case).

It occurs to me that a simple solution might be to put a resistor in series with the 5V input to the Gotek board, to drop the overall value of Vcc a bit. But fitting a 74HCT04 seems to be a recognised improvement.
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Re: Vcc above 5V - how much is too much?

Post by 1024MAK »

Yes, the logical fix is to replace the 74HC04 with a 74HCT04. Assuming that will not affect any other parts of the circuitry that may need a “74HC” part.

And 5.201V is within +/-5% (4.75V to 5.25V) that TTL and microprocessor systems are happy with.

However before replacing the 74HC04, try wiring a 1N4001 (or similar) diode in series from the +5V supply from the Master Compact before it feeds the Gotek.

The 1N4001 diode may drop enough voltage (about 0.6V to 0.7V) to get the supply voltage of the 74HC04 low enough so that it sees the 3.3V input as a logic high.

If that does not work, try two 1N4001 diodes wired in series instead of just one :mrgreen:

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Re: Vcc above 5V - how much is too much?

Post by jms2 »

That's a good idea Mark.
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Re: Vcc above 5V - how much is too much?

Post by BigEd »

(This is a really interesting failure mode!)
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Re: Vcc above 5V - how much is too much?

Post by jms2 »

It's interesting now that there's a good theory on the table. Before that, it was just very frustrating! Floppy drives are one of those nice simple things which usually "just work".
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Re: Vcc above 5V - how much is too much?

Post by Phipli »

Yup - I've been advised to put a diode before the power pin on a marginal RAM chip before - same thing, knock 0.7v off the supply voltage to drop the logic thresholds down a little.

In the specific case I turned out to have another issue, but still, it is another trick in the armoury! Good call Mark.
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Re: Vcc above 5V - how much is too much?

Post by jms2 »

I looked at the diode option, but then I realised that the inverter is surface mounted - it's a SOIC-14 package. So getting a diode in there would not be easy, and I thought that putting a diode into the main 5V input to the board might affect too many other things. So I decided to extract the HC04 and put in an HCT04.

To cut a long story short, getting the old one out was tricky but I managed it. Getting the new one in was dead easy.

And now it works! :D

I found initially that the old setup had corrupted my DFS test disc, but once I found an uncorrupted disc everything worked well.

Now that it's fixed, this problem now officially qualifies as "interesting"! :lol: I can see that the HC04 randomly interpreting highs as lows would have caused all the problems that I saw:

- Drives not always responding
- One Gotek seemingly better behaved than the other (just depended on the HC04 chip's internals)
- Random corruption of discs
- Chinon drive randomly starting and stopping (don't know why specifically the Chinon...)

...etc

So I wouldn't want to start hares running by suggesting that everyone who wants to use two drives rushes out and swaps their Gotek to a 74HCT04, but I would say make sure the power supply is equal to or less than 5V, and be prepared for the possibility that you might need to do this mod.

What I have to do now is go back to my two previous threads and close them down neatly with an explanation of what happened...
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