This document discusses positive edge triggered and negative edge triggered RS flip flops. Positive edge triggered RS flip flops change state only on the positive edge of the clock signal, while negative edge triggered change state only on the negative edge. The output only changes according to the R and S inputs during the short positive or negative trigger pulse of the clock. Truth tables and output waveforms are provided as examples.

1. E D G E T R I G G E R E D R S F L I P F L O P
SEQUENTIAL LOGIC

2. EDGE TRIGGERED RS FLIP FLOP
• In the case of Flip flops (RS) that we already had discussed, the o/p Q
changes immediately when there is a change in the i/p (R, S).
• Gated or clocked FFs, o/p changes when the clock is enabled(EN) or
High(1).
• All the digital systems operates in synchronous mode also.
• If any of these FFs, are used in the Synchronous system, care must be
taken to ensure that all FF i/p change state in synchronism with the clock
(C).
• These lead to the development of edge triggered flip

3. POSITIVE EDGE TRIGGERED RS FLIP FLOP
• In the positive edge triggered FF, Clock
(C) is applied to a positive pulse forming
circuit.
• Then these positive pulses (positive
trigger, PT) are applied to a gated RS FF.
• The small triangle in the symbol indicates
that Q can change state only with
PT(positive trigger) of the clock.
• Each PT of the clock is very narrow (25
ns) and is applied to the AND gates.
• The AND gates are active only while PT
is high and thus Q can change state only
during this short period, ie; Q is in
synchronism with the PTs of the clock.

4. POSITIVE EDGE TRIGGERED RS FLIP FLOP
CLK R S Qn+1 Action
↑ 0 0 Qn No change
↑ 0 1 1 SET
↑ 1 0 0 RESET
↑ 1 1 ? Forbidden
TRUTH TABLE

5. +VE EDGE TRIGGERED RS FLIP FLOP-O/P WAVE FORMS
• At time t0 S=0
R=0 no change in Q
(Q remains 0)
• At time t1 S=1 R=0
Q changes to 1 from 0
• At time t2 S=0 R=1
Q resets to 0
• At time t3 S=1 R=0
Q sets to 1
• At time t4 S=0 R=0
No changes in Q (remains 1)
(previous o/p)

6. NEGATIVE EDGE TRIGGERED RS FLIP FLOP
• Here Q changes state according
to R & S inputs but only
during NTs of the clock.
• Symbol has a small bubble.
• The FF behaves exactly same as
that of PT RS FF except that
changes in o/p Q are
synchronised with NTs &
clock(c).
CLK R S Qn+1 Action
↓ 0 0 Qn No change
↓ 0 1 1 SET
↓ 1 0 0 RESET
↓ 1 1 ? Forbidden
TRUTH TABLE

7. -VE EDGE TRIGGERED RS FLIP FLOP-O/P WAVE FORMS
• At time t0 S=0 R=0
Q=0 (No change)
• t1 S=1 R=0
Q changes from 0 to 1
• t2 S=0 R=1
Q resets to 0
• t3 S=1 R=0
Q sets to 1
• t4 S=0 R=0
Q remains at 1