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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I got lost reading that. Datashewt the address lines transition and settle out, they might very well set up a glitch that agrees with the test condition, well before you intended.

Hi No it won’t. You could then use it as a clock. It might work, but I don’t like it much. In other words, what function does it serve?

It’s comparing the high four address bits to a 4-position DIP switch.

(PDF) 74LS688 Datasheet download

Hi I doubt you’ll be able to run the output of the directly to a clock. I’m not that good with circuit design, but is datassheet possible a monostable multivibrator one-shot would do the job?

Glitchy output shouldn’t be a problem — glitch, I see how you got your nickname. Just to scratch my own curious itch, what does the connect to?

I don’t do well with word descriptions of circuits–can you draw a schematic?


datashet I’m currently designing the power-on-jump circuit for my processor board. I’m not talking about little tiny noise spikes. Here’s the pieces relevant to Power on Jump: If you expect to use it, you need to turn the enable off during the time when the compare inputs are in transition. This is all fine. This might cause a false trigger if your address trap was at B. It may even make it worse.

74LS688 PDIP20

It’s an 8-bit comparator, but it’s cheaper than the 74LS85 and has an enable input. I’ve never had to use one.

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. Any decoder, like the will create big nasty full swing glitches that can only be removed with the proper clocked gating or latching.

It is a glitchy signal.

SN54/74LS datasheet & applicatoin notes – Datasheet Archive

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.


This sets the D-type flip flop, which switches off the NOP generator and turns on the bus tranceivers. 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.

My calculations say yes, with lots of time to spare, but I’d dataxheet others’ opinions. 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. You can’t use it while the inputs are changing. There may be better ways to do this job so keep tinkering.

You’re depending too much on dqtasheet propagation delay of the 74 and the creating a clock pulse of the minimum required width t wclk on your datasheet. 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. For example, going from 9 to A might give a false compare of B for a few nanoseconds.