I'm testing a device i built. It's got a lot of 74LSxx and 74HCTxx logic, a few EPROMs and a 61256 SRAM. The PSU is giving 4.7V when I apply the load of the logic to it (approx 200mA) instead of 5V. Will the .3V affect the working of the logic?
Thanks,
David
The gauranteed powersupply voltage specification is 5.0 VDC +/- 5%. That ranges out to 4.75 to 5.25 VDC, at the powersupplys rated load current.
Beyond the +/- 5%, the logic, while it may work, isn't gauranteed to work under all circumstances. Temperature, operating speed, fan-out/fan-in loading, capacitive loading, propagation delays, will all play a part (and will vary slightly) in the reliably logic operation when the powersupply is within specification limits.
Technically, your powersupply is out of specification. It may work just fine for a system using 5 or 10 logic devices but. For say, the number of devices required to make a small single board descrete logic computer system, something on the order of an Apple II, that power supply won't be reliable.
It might work, but not be very reliable.
Isn't there a way to improve the supply ?
Nard
Edit: Carl posted one minute earlier
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Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips
Yes, there is. Do not use USB. I'm using only the GND and +5V so I don't have to carry another PSU to my computer for testing, but what the heck, guess lazyness got the best of me.
For say, the number of devices required to make a small single board descrete logic computer system, something on the order of an Apple II, that power supply won't be reliable.
Well, I'm using 24 chips for it. But I guess to be on the safe side I'll use a real PSU.
Thanks Carl, Plons.
I think USB can be lower than that, like 4.3V.
Don't rely on raw USB to power 5V logic! There are switchers out there designed to provide good 5V from USB supply.
Jim
Twenty four logic chips?! Have you attempted minimization via Karnaugh maps or similar? Should be able to knock off a few chips, at the very least.
- Dean :twisted:
dean: It's a homebrew CPU. I think 24 logic chips is not that much for such an application. And most of them are 74193 (actually 9 of them).
Although, now that you have mentioned it, I got an idea for optimizing it, that it would have 2 chips less. Oh well. It's soldered already.
I mean, I could have optimized it a bit more (the IF DATA IS == 0 section is composed of 7 OR gates, whereas one magnitude comparator would do the job, but try getting one at GM electronics - my supplier)
I can sympathise. My design has had at least a dozen compromises down to chips not being available at the suppliers/in the package desired/at the right voltage/not been made for twenty years :)
Neil
Just out of curiosity - what is the combined current drawn by these '74s, EPROM and RAM?
Johan: Well, I hooked it up to a bigger supply, and realised that without the correct microcode, some things might go bad (to devices having OE=0 on the bus at the same time) but the current was 300mA. Most of the power was eaten away by a 74LS245 which is strangely heating up, but all of the LS chips are doing that. I guess if I had CMOS the power consumtion would be less, but I am currently a bit broke :-(
74LSxxx does consume a lot more power than CMOS. But, it should not get very warm. You likely have "bus conflict".
Jim
