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. You can’t use it while the inputs are changing. In other words, what function does it serve?
Glitchy output shouldn’t be a problem — it needs to be inverted for datashwet 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 eatasheet. Glitchy output shouldn’t be a problem — glitch, I see how you got your nickname. I’m not that good with circuit design, but is it possible a monostable multivibrator one-shot would do the job?
Here’s the pieces relevant to Power on Jump: It may even make it worse.
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. 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.
74LS688 Datasheet PDF
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.
This sets the D-type flip flop, which switches off the NOP generator and turns on the bus tranceivers. Any decoder, like the will create big nasty full swing glitches that can only be removed with the proper clocked gating or latching.
This is all fine. 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. This might cause a false trigger if your address trap was at B.
For example, going from 9 to A might give a false compare of B for a few nanoseconds. It’s an 8-bit comparator, but it’s cheaper than the 74LS85 74s688 has an enable input.
I don’t do well with word descriptions of circuits–can you draw a schematic? Note that this would take away 62 nS from the access time of the first instruction fetch after the proper address compare, dataxheet it should be OK if you are using newer memory devices. Hi No it won’t.
74LS688 Datasheet PDF – ETC
Hi I doubt you’ll be able to run the output of the directly to a clock. It is a glitchy signal. I’m currently designing the power-on-jump circuit for my processor board. It’s comparing the high four address bits to a 4-position DIP switch. It might work, but I don’t like it much. I’m not talking about little tiny noise spikes. You could then use it as a clock.
SN54/74LS datasheet & applicatoin notes – Datasheet Archive
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.
There may be better ways to do this job so keep tinkering. My calculations say yes, with lots of time 74ls68 spare, but I’d like others’ opinions. I’ve never had to use one. Just to scratch my own curious itch, what does the connect to?