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IS 151 Lecture 11 - UDSM - 2013/2014

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- 1. Edge-Triggered Flip Flops • A flip flop is a synchronous bistable device • Synchronous - the output changes state only at a specified point on a triggering input clock • An edge-triggered flip-flop changes state either at the – positive edge (rising edge) or – negative edge (falling edge) of the clock pulse – The flip flop is sensitive to its inputs only at this transition of the clock IS 151 Digital Circuitry 1
- 2. Edge-Triggered Flip Flops • Three types of flip-flops – S-R –D – J-K IS 151 Digital Circuitry 2
- 3. Edge-Triggered S-R Flip Flop • Synchronous – because data on its inputs are transferred to the flip flop output only on the triggering edge of the clock pulse • When S = 1 and R = 0, Q = 1 on the triggering edge of the clock pulse – SET • When S = 0 and R = 1, Q = 0 on the triggering edge of the clock pulse – RESET • When S = R = 0, the output does not change • When S = R = 1, invalid condition – Same operation as the gated S-R latch • The flip flop cannot change state except on the triggering edge of a clock pulse IS 151 Digital Circuitry 3
- 4. Edge-Triggered S-R Flip Flop • Operation of the positive-edge-triggered flip-flop S Q S = 1, R = 0, flip flop SETS on the positive clock edge Q’ 1 If already SET, it remains SET C t0 0 R IS 151 Digital Circuitry 4
- 5. Edge-Triggered S-R Flip Flop • Operation of the positive-edge-triggered flip-flop S Q S = 0, R = 1, flip flop RESETS on the positive clock edge Q’ 0 If already RESET, it remains RESET C t0 1 R IS 151 Digital Circuitry 5
- 6. Edge-Triggered S-R Flip Flop • Operation of the positive-edge-triggered flip-flop S Q S = 0, R = 0, flip flop does not change Q’ 0 If SET, it remains SET; if RESET, it remains RESET C t0 0 R IS 151 Digital Circuitry 6
- 7. Edge-Triggered S-R Flip Flop • Truth table Inputs Outputs Comments S R CLK Q Q’ 0 0 X Q0 Q’0 No Change 0 1 0 1 RESET 1 0 1 0 SET 1 1 ? ? = Clock transition LOW to HIGH X = irrelevant Q0 = output level prior to clock transition Invalid IS 151 Digital Circuitry 7
- 8. Edge-Triggered S-R Flip Flop • Example – determine the Q and Q’ output waveforms of an edge-triggered flip flop for the S, R and CLOCK inputs. Assume that the FF is initially RESET CLK 1 2 3 4 5 6 S R Q Q’ IS 151 Digital Circuitry 8
- 9. Edge-Triggered S-R Flip Flop • Determining the Q and Q’ outputs – At clock pulse 1, S = 0, R = 0, Q does not change (Q = 0) – At clock pulse 2, S = 0, R = 1, Q is RESET (Q = 0) – At clock pulse 3, S = 1, R = 0, Q is SET (Q = 1) – At clock pulse 4, S = 0, R = 1, Q = RESET (Q = 0) – At clock pulse 5, S = 1, R = 0, Q = SET (Q = 1) – At clock pulse 6, S = 1, R = 0, Q = SET (Q = 1) IS 151 Digital Circuitry 9
- 10. Applications of Sequential Circuits - Counters • Flip flops can be used to perform counting operations • 2 categories: – Asynchronous – events that do not have a fixed time relationship with each other – Events do not occur at the same time – In an asynchronous FF, the first flip flop is clocked by the external clock pulse and then each successive flip flop is clocked by the output of the preceding flip flop IS 151 Digital Circuitry 10
- 11. Applications of Sequential Circuits - Counters – In a synchronous – the clock input is connected to all of the flip flops so that they are clocked simultaneously • e.g. 2-bit asynchronous binary counter requires 2 flip flops; 3-bit counter requires 3 flip flops IS 151 Digital Circuitry 11
- 12. Applications of Sequential Circuits - Counters • 2-bit Asynchronous Binary Counter HIGH J0 CLK Q0 C K0 J1 Q1 C Q0’ FF0 IS 151 Digital Circuitry K1 FF1 12
- 13. 2-bit Asynchronous Binary Counter Operation • The clock line (CLK) is connected to the clock input of only the first flip flop, FF0 • The second flip flop, FF1, is triggered by the Q’0 output of FF0 • FF0 changes state at the positive-going edge of each clock pulse • FF1 changes state only when triggered by a positive-going transition of the Q’0 output of FF0 IS 151 Digital Circuitry 13
- 14. 2-bit Asynchronous Binary Counter Operation • Timing diagram – E.g. apply 4 clock pulses to FF0 and observe the Q output of each flip flop – Both flip flops are connected for toggle operation (J = K = 1) – Q is initially RESET (0) IS 151 Digital Circuitry 14
- 15. 2-bit Asynchronous Binary Counter Operation • Timing diagram CLK 1 2 3 4 Q’0 Q0 Q1 IS 151 Digital Circuitry 15
- 16. 2-bit Asynchronous Binary Counter Operation • On the positive-going edge of CLK 1 (clock pulse 1), Q0 = HIGH, Q’0 = LOW • Q’0 being LOW, no effect on FF1 (a positive-going transition must occur to trigger the flip flop) • On the positive-going edge of CLK 2, Q0 = LOW, Q’0 = HIGH • Q’0 being HIGH triggers FF1, causing Q1 to go HIGH • On the positive-going edge of CLK 3, Q0 = HIGH, Q’0 = LOW • Q’0 being LOW, no effect on FF1, Q1 remains HIGH • On the positive-going edge of CLK 4, Q0 = LOW, Q’0 = HIGH • Q’0 being HIGH triggers FF1, causing Q1 to go LOW IS 151 Digital Circuitry 16
- 17. 2-bit Asynchronous Binary Counter Operation • • • • • • • If Q0 represents the Least Significant Bit (LSB) and Q1 represents the Most Significant Bit (MSB), the sequence of counter states is actually a sequence of binary numbers Clock Pulse Q1 Q0 Initially 0 0 binary 0 1 0 1 binary 1 2 1 0 binary 2 3 1 1 binary 3 4(recycles) 0 0 binary 0 Recycle means transition of a counter from its final state back to its original state IS 151 Digital Circuitry 17
- 18. 2-bit Asynchronous Binary Counter Operation • Propagation delay – The effect of the input clock pulse is first ‘felt’ by FF0. This effect cannot get to FF1 immediately because of the propagation delay through FF0 – The same happens to each flip flop until the last one – Total propagation delay = propagation delays in each flip flop – E.g. if each flip flop in the example given has a propagation delay of 5ns, the total propagation delay is 5ns x 2 = 10ns – Maximum clock frequency = 1/total propagation delay = 1/10ns = 10GHz IS 151 Digital Circuitry 18

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