83 lines
2.4 KiB
Markdown
83 lines
2.4 KiB
Markdown
# lec11
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_diagrams references implied for now_
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Sequential Logic: at this point we effectively are dealing with state(_state machines_). Simply put we have _memory_ now.
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## State Tables
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In Q~s~ is our _Current state_ while Q~s+1~ is the next state
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| A | Q~s~ | Q~s+1~ |
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|---|---|---|
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| 0 | 0 | 0|
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| 0 | 1 | 0|
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| 1 | 0 | 0|
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| 1 | 1 | 1|
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We can try the same thing with an `or` gate:
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Keeping in mind that our effective input here is only `A`.
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## Latches
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Namely we are going to look at set-reset latches.
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They should be able to do two things:
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* store a state
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* change state upon appropriately changed signals.
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Note that the above state machine the two rows show up as illogical; because both don't make sense in that context.
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The actualy gate implementation of the above would look like the above.
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The same can also be done with `nor` gates making the whole operation much more efficient on transistor usage.
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## Clocking & Frequency
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The period of the square wave in this case can be used to find the frequency.
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We simple note that `1/T = F`.
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This frequency is measured in cycles/second or _hertz_.
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### Setup time & Hold time
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Setup time would be some aount of time after the previous point where we wait for the combinational logic to perpetuate its results into memory.
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A short period of time in the valley would be setup time
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Hold time is the time that we wait before we start feeding input into our combinational logic(unit).
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Say we wanted to start our combinational logic at the beginning of one of our plateaus.
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## D Latches
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_D stands for data_
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Essentially we want to only read in D when the clock signal is high.
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If it's low only we want to _block_ the signal from our output state.
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The latch simply allows or disallows our input from passing through the other side based on what our clock is(high/low).
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If D was 0 then it stays 0 when the clock goes low.
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If D was 1 then it stays 1 when the clock goes low.
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### Flip-Flop & Edge Triggering
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Say we want to grab what ever D is but, only when we approach a falling edge.
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The first latch opens as grabs any changes coming off D, then the the second opens just as the first closes.
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We can reverse the two like in the next figure to acheive the opposite result: reading on the rising edge.
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