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

As 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. 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 datasueet has an enable input. This sets the D-type flip flop, which switches off the NOP generator and turns on the bus tranceivers.

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

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. Note that this would take away 62 nS from the access time dxtasheet the first instruction fetch after the proper address compare, but it should be OK if you are using newer memory devices.


74LS datasheet & applicatoin notes – Datasheet Archive

I’m not that good with circuit design, but is it possible a monostable multivibrator one-shot would do 74l688 job? This might cause a false trigger if your address trap was at B.

You could then use it as a clock. In other words, what function does it serve? Hi I doubt you’ll be able to run the output of the directly to a clock. Glitchy output shouldn’t be a problem — glitch, I see how you got your nickname.

I got lost reading that. There may be better ways to do this job so keep tinkering.

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Hi No it won’t. I don’t do well with word descriptions of circuits–can you draw a schematic? This would also add about 62 nS to your latch clock which right dataasheet is only 3 gate delays long and would be difficult to see on an old low bandwidth scope.

Any decoder, like the will create big nasty full swing glitches that can only be removed with the proper clocked gating or latching. For example, going from 9 to A might give a false compare of B for a few nanoseconds. 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.


It’s comparing the high four address bits to a 4-position DIP switch. My calculations say yes, with lots of time to spare, but I’d like others’ opinions. 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 74sl688 nice, sharp positive-going square wave.

Here’s the pieces relevant to Power on Jump: I’ve never had to use one.


This is all 7l4s688. It is a glitchy signal. I’m currently designing the power-on-jump circuit for my processor board. I’m not talking about little tiny noise spikes. It may even make it worse.