Download Clocking-Strategies - KIT

(Neil west -p:-317-357)
Finite-state machine (FSM) which is composed of a set
of logic input feeding a block of combinational logic
resulting in a set of logic output.
 Operation: determining the “next state” as a function
of the “current state” and the external inputs.

Output
Input
Current
state bits
Combinational Logic
Next state
bits
Q
D
Clock
Single-phase clock
The time before the clock edge that the D input has to
be stable is called the Setup time (Ts)
 The time after the clock edge that the D input has to
remain stable is called the Hold time (Th)
 The delay from the positive clock input to the new value
of the Q output is called the clocked-to-Q delay(Tq)
 Clock time or Cycle time (Tc)


Cycle Time (Tc)
Clock
Setup Time (Ts)
Data
Hold Time (Th)
Q
Clock-to-Q delay(Tq)
In positive level-sensitive latch because the D input is
passed to the output when the clock is high.
 The D input must be stable for a short time before and
after the negative clock transition

By combining two level-sensitive latches, one positivesensitive and one negative-sensitive can construct edgetriggered register.


Cycle time:
Tc = Tq+Td+Ts
Td is the delay through combinational logic
Logic delay of block must satisfy
Tda<Tc1-Tqa-Tsb
Tdb<Tc2-Tqb-Tsb
Latch
A
C
Tq
Combinational
Logic
Tda
Latch
Ts
B
Combinational
Logic
Tdb
C
Clock
Tc=Tda+Tdb+[2(Tq+Ts)]
Latch
C
C
In synchronous system, if the data input to a register
dose not obey the setup and hold-time constraints, then
potential clock race problems may occur.
 Hold time violation : Tc2>(Tc1+Tq1)
 Setup time violation : (Tc1+Tq1) - Tc2 is larger than the cycle
time, Tc then the data will arrive late at M2

Reg
D
q
Tq
Combinational
Logic
Tda
Reg
D
Ts
C
C
M1
Clock
Delay
Tc1
q
M2
Delay
Tc2
PLL is used to generate internal clocks on chip for
two main reasons
1) To synchronization the internal clock of a chip with
an external clock.
2) If it is desired to operate the internal clock of a
chip at a higher rate that the external clock input.

Chip that receives an external clock that is
internally buffered
 The delay time is clock-buffer delay + the RC delay
to the register + clock to Q delay of the register and
the output buffer delay

PLL senses the internal clock at the input of the
register, it also receives the input clock.

PLL generate a clock that is in phase with the input
clock.

By including divider in the PLL loop the on chip
frequency may be increased by the divider ratio.

/n
Phase
Detector
Reference
Clock
U
D
Charge
Pump
Filter
VCO
n*fin
The phase detector detects the difference between
the reference clock and VCO clock and applies charge-up
or charge-down pulses to the charge pump.

The loop filter filters these pulses and applies the
resulting control voltage to the VCO.

The VCO changes oscillation frequency depending on
the control voltage. Thus the total system forms a
feedback system where the VCO is locked to the
reference clock.

Metastable state: The output is in an indeterminate
state between 0 and 1.
 From the probability theory, the mean time between
the failure of the output to be resolved with in some
time(tf)(MTBU) is given by

 tf=The
time after the change in clock by which the latch
output must be resolved.
fc=the frequency of the clock
fd=the frequency of data
T0=Flip flop propagation delay
τr=Time constant of resolution of the latch

Independently clocked systems
Synchronizers are used at the interface between
independently clocked systems.

Asynchronous
data in
Reg
D
q
C
Clock
Reg
D
q
C
synchronous
data out
Phi1(t) . phi2(t) =0 for all t.
During Phi1=1: charges the gate capacitance of the
inverter and the out put capacitance of the
transmission gate(c1).
 During phi2=1 the stage-1 transmission gate opens
and the inverse of the stored value on C1 is placed on
C2.


phi1
phi2
Phases must not overlap:
non-overlap region
clk signal may be passed through a transmission gate to equalize delay with
respect to - clk
 Dynamic single clock latches
 Dynamic latches storing data on gate capacitance of inverter (or logic gate)
 Can be clocked at high frequency since very little delay in latch elements
 Examples: (a) or (b) show simple transmission gate latch concept and (c )
tri-state inverter dynamic latch holds data on gate when clk is high, (d) and (e)
dynamic D register

Operation of the un buffered L1 latch:
Assume D and clock was high, then X is initially low
and Q is high.
 With the clock low ,high to low transition on D
causes X to go high, which turns P2 off, holding the
value at the Q output.
 If D was low when the clock is high, then X is high
and Q is low, if D is high when clock is low, P1 is turned
off , holding X high, then Q is holding low
 P3 feedback transistor is added to counteract noise
sources and leakage at X.


Combines NP Domino logic sections
with C2MOS latch

 n-logic block can drive p-logic block or
another n-logic block with a static
inverter
 similarly for a p-logic block
 Must end in a C2MOS latch
clk logic: (a) prechrg on clk=0, eval clk=1
-clk logic: (b) pre on clk=1, eval on clk=0
clk logic can feed –clk logic & vice-versa
can mix static logic with NP domino
logic
 Rules to avoid race conditions:




 During precharge, logic blocks are OFF
 During eval, internal inputs make only
one transition

Pipeline design:
 Even number of inversions between
C2MOS
R. W. Knepper
SC571, page 5-72
Compact implementation of of
two phase dynamic registers
shown at left using a tri-state
buffer form.

 Transmission gate and inverter
integrated into one circuit
Two phase dynamic registers
and logic is often preferred over
single phase because

 Due to finite rise and fall times, the
CLK and CLK’ are not truly nonoverlapping
 Clock skew often is a problem due
to the fact that CLK’ is usually
generated from CLK using an
inverter circuit and also due to the
practical problem of distributing
clock lines without any skew

n- transistor can replace the transmission gate

p feed-back transistor to provide fully logic levels

Single clock version using p and n transistor

Clock qualification technique
Top figure shows n type
logic stages with two phase
non-overlapping clocks

 phi1 high: precharge phi1
logic, evaluate phi2 logic
 phi2 high: precharge phi2
logic, evaluate phi1 logic
Bottom figure shows use
of Domino logic having both
phi1 and phi2 logic stages

 Each block is separated from
other by a clocked pass gate
register/latch to store the
logic result
 Note that inverters must be
used between successive
stages of the same clock logic

Simple four-phase structure operation:
 during clk1=0 node n1 precharges
 When clk2=1 and ck1=1, node n1 conditionally discharges.
 When clk2 falls to 0, this value is held on the node n1 regardless of the state of
the input D.
 During clk3=0 ,Q is precharge ,
 During clk4=1, clk3=1, Q node is conditionally discharged according to the state
of node n1
Due to charge sharing during the clk2 phase, clk2 is sometimes
replaced by clk12 (and clk4 is replaced by clk34) by keeping clk12 high
during both clk1 and clk2 phases.

Four phase logic structure shown using transmission gate to isolate data on z
during hold time
 Operation:

 during clk1 time, -clk12 is down causing Pz to be precharged to Vdd
 during clk2 time, -clk12 is still down keeping precharge active, but clk23 goes high
thus precharging node z
 during clk3 time, precharge of node Pz ends and evaluation begins with Xgates still
closed
 during clk4 time, the transmission gate opens and the correct data is isolated on
node z
Using four different type logic
gates as shown in previous chart
(where Type refers to the
evaluation phase time), four
phase logic can be used in
pipelined logic structures where
each type must be used per the
allowable
interconnection
diagram at the left

 a Type 1 gate can feed Type 2
or Type 3 gates
 a Type 2 gate can feed Types 3
and 4
 a Type 3 gate can feed Types 4
and 1
 a type 4 gate can feed Types 1
and 2
Single phase clocking scheme is preferable
Two phase clocking scheme is used in RAMs, ROMs,
and PLAs
 Single-phase clocking for complex, high speed
CMOS circuits.
 Special clocks are normally generated using self
timed logic circuits. Alternative clocking scheme may
be of used in special situations



1)
2)
Two main technologies:
A single large buffer
A distributed clock tree approach