2. Latch
• Latch Function
– store a data value
• non-volatile; will not lose value over time
• –often incorporated in static memory
• building block for a master-slave flip flop
Static CMOS Digital Latch
• most common structure
• •cross-coupled inverters, in positive feedback arrangement
• circuit forces itself to maintain data value
• inverter a outputs a 1 causing inverter b to output a 0
• or, inverter a outputs a 0 causing inverter b to output a 1
3. D Latch
• When CLK = 1, latch is transparent
• – D flows through to Q like a buffer
• When CLK = 0, the latch is opaque
• – Q holds its old value independent of D
• Transparent latch or level-sensitive latch
20. Sequential
Logic
Slide
20
Latch Design
• Buffered output
+ No backdriving
• Widely used in standard cells
+ Very robust (most important)
- Rather large
- Rather slow (1.5 – 2 FO4 delays)
- High clock loading
Q
D
X
25. D Flip Flop
Master-Slave Concept
– cascade 2 latches clocked on opposite clock phases
• φ = 1, φ = 0: D passes to master, slave holds previous value
• φ = 0, φ = 1 : D is blocked from master, master holds value and
passes value to slave
Triggering
– Output only changes on clock edge; output is held when clock is at a
level value (0 or 1)
– Positive Edge
• output changes only on rising edge of clock
– Negative Edge
• output changes only on falling edge of clock
28. Sequential
Logic
Slide
28
Enable
• Enable: ignore clock when en = 0
• Mux: increase latch D-Q delay
• Clock Gating: increase en setup time, skew
D Q
Latch
D Q
en
en
Latch
D
Q
0
1
en
Latch
D Q
en
D
Q
0
1
en
D Q
en
Flop
Flop
Flop
Symbol Multiplexer Design Clock Gating Design
29. Sequential
Logic
Slide
29
Reset
• Force output low when reset asserted
• Synchronous vs. asynchronous
D
Q
Q
reset
D
Q
D
reset
Q
D
reset
reset
reset
Synchronous
Reset
Asynchronous
Reset
Symbol
Flop
D Q
Latch
D Q
reset reset
Q
reset
30. Sequential
Logic
Slide
30
Set / Reset
• Set forces output high when enabled
• Flip-flop with asynchronous set and reset
D
Q
reset
set
reset
set
34. The inputs of the RS latch are precharged and selectively discharged at
the rising edge of the clock signal. The RS latch retains the data output
during the precharge period when CLK is low.
The transistor M 5 provides a current path to ground if the input switches
after the rising edge of the clock. This prevents the sources of M 2 and M
4 to become floating, which can cause the inputs of the RS
latch to have an intermediate voltage level.
This circuit was reportedly used
in a RISC microprocessor [4.14]. The flip-flop contains a simple differential feedback
circuit that drives an RS latch. The transistors M I' M 2' M 3' M 4 form a pair of cross
coupled feedback inverters. The inputs of the RS latch are precharged and selectively
discharged at the rising edge of the clock signal. The RS latch retains the data output
during the precharge period when CLK is low. The transistor M 5 provides a current
path to ground if the input switches after the rising edge of the clock. This prevents
the sources of M 2 and M 4 to become floating, which can cause the inputs of the RS
latch to have an intermediate voltage level. Although only three transistors are connected
to the single phase clock signal, one of the nodes at the inputs of the RS latch
is always charged and discharged during evaluation. Note that the full CMOS implementation
does not consume static current when the clock is stopped.