Texas Instruments 74LS Logic Comparators are available at Mouser Electronics. Mouser offers inventory, pricing, & datasheets for Texas Instruments . December INTEGRATED CIRCUITS. 74HC/HCT 8-bit magnitude comparator. For a complete data sheet, please also download. Part Number: 74LS, Maunfacturer: National Semiconductor, Part Family: 74, File type: PDF, Document: Datasheet – semiconductor.

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Hi I doubt you’ll be able to run the output of the directly to a clock. I’m currently designing the power-on-jump circuit for my processor board. This might cause a false trigger if your address trap was at B.

(PDF) 74LS688 Datasheet download

I don’t do well with word descriptions of circuits–can you draw a datasheey A way to avoid that would be to de-glitch the circuit by adding a synchronizing flip-flop running at 16MHz just before the latching flip-flop.

It’s an 8-bit comparator, but it’s cheaper than the 74LS85 and has an enable input. As the address lines transition and settle out, they might very dxtasheet set up a glitch that agrees with the test condition, well before you intended. Glitchy output shouldn’t be a problem — glitch, I see how you got your nickname.

For example, going from 9 to A might give a false compare of B for a few nanoseconds. This is all fine.


You’re depending too much on the propagation delay of the 74 and the creating a clock pulse of the minimum required width t wclk on your datasheet. Here’s the pieces relevant to Power on Jump: It may even make it worse.

If you expect to use it, you need to turn the enable off during the time when the compare inputs are in transition.

When the system comes up from a reset, a 74LS with its inputs all tied to ground is connected to the data lines of the CPU and the data bus tranceivers are switched off — this feeds the CPU NOPs until the address equals what’s set on the DIP switch. There may be better ways to do this job so keep tinkering.

Propagation delay question [Archive] – Vintage Computer Forum

Note that this would take away 62 nS from the access time of the first instruction fetch after the proper address compare, but it should be OK if you are using newer memory devices. Any decoder, like the will create big nasty full dqtasheet glitches that can only be removed with the proper clocked gating or latching.

It’s comparing the high four address bits to a 4-position DIP switch. I’m not talking about little tiny noise spikes. This sets the D-type flip flop, which switches off the NOP generator and turns on the bus datashest. It is a glitchy signal.


This would also add about 62 nS to your latch clock which right now is only 3 gate delays long and would be difficult to see on an old low bandwidth scope.

Just to scratch my own curious itch, what does the connect to? Glitchy output shouldn’t be a problem — it needs to be inverted for the D-type flip flop’s clock anyway, so I’m using a 74LS14 inverter, which has Schmitt inputs and should clean up the signal into a nice, sharp positive-going square wave.

Hi No it won’t. I 74s688 lost reading that. I’m not that good with circuit design, but is it possible a monostable multivibrator one-shot would do the job? You could then use it as a clock. In other words, what function does it serve? I’ve never had to use one.

74LS Datasheet, PDF – Alldatasheet

You can’t use it while the inputs are changing. It might work, but I don’t like it much.

My calculations say yes, with lots of time to spare, but I’d like others’ opinions.