Physical Methods of SpeedIndependent Module Design
Physical Methods of SpeedIndependent Module Design
Physical Methods of SpeedIndependent Module Design
Oleg Izosimov
INTEC Ltd, Room 321, 7a Myagi Street, Samara 443093, Russia
1. Introduction
Any method of logic circuit design is based on using formal models
of gates and wires. The simplest model of a gate is determined by only two
"parameters": (a) Boolean function is to be calculated, (b) fixed
propagation delay. The simplest model of a wire is an ideal medium with
zero resistance and consequently, with zero delay. Such simple models allow
circuit design procedures which are a sequence of elementary steps easily
realized by a computer.
When logic circuits designed by using the simplest models expose
unreliable operation as in the case of gate delay variations, designers
introduce less convenient but more realistic models with arbitrary but
finite delay. Using more complicated models may produce logic circuits that
are called speedindependent [1].
In speedindependent circuits transition duration can be arbitrary.
So a centralized clock cannot be used. Instead special circuitry to detect
output validity is applied. Besides, additional interface circuitry is
needed to communicate with the environment in a handshaking manner. A speed
independent circuit can be seen as a module consisting of combinational
logic (CL) proper, CL output validity detector (OVD) and interface
circuitry (Fig.1). To enable OVD to distinguish valid output data from
invalid ones, the redundant coding scheme was proposed [2]. The main idea
of the scheme is to enumerate all possible input and output data, both
valid and invalid. The OVD must be provided with appropriate information on
data validity. To realize the idea of redundant coding some constraints on
CL design are imposed [3]:
[pic]
(i) CL must be free of delay hazards, i.e. CL output data word must not
be dependent on the relative delay of signal paths through CL.
(ii) In changing between input states, any intermediate or transient
states that are passed through must not be mapped by CL onto valid output
states.
When these constraints were formulated, the circuit designers
realised that not every Boolean description could be implemented in a speed
independent style. Other approaches to speedindependent module design were
needed.
SIM design as a science has two branches: logical and physical. For
a long time physical branch was overshadowed in spite of its
competitiveness. The main properties of physical approach to SIM design
are:
(a) Arbitrary coding scheme.
(b) Conventional procedure of operational unit design.
(c) Races of signals in SIM do not affect on its proper operation.
In this paper we propose an approach based on the physical nature of
transitions in CL. We believe that each transition is actually a transfer
of energy which can be naturally detected by physical methods.
From the viewpoint of a radio engineer CL behaves like a radio
transmitter. It emits radio frequencies in the 1081010Hz band modulated by
signals of 106108Hz. Obviously, the carrier wave is produced by gate
switchings during transitions in CL. The modulating wave is produced by
control schemes (OVD and interface circuitry) that detect transition
completion and inform the environment about the readiness of CL. OVD is a
kind of radio receiver that extracts the modulation envelope and enhances
the received signal. The main properties that OVD circuit must expose from
a radio engineer's point of view are selectivity and high gain. Since the
useful signals can propagate through nonconducting medium, OVD circuits
can be coupled with CL indirectly.
Advances in semiconductor technology gave birth to two methods of
transition detecting based on two kinds of the information carrying signal,
namely electromagnetic radiation and current consumption. Frequency of the
signal produced by switching logic gates is determined by gate delay.
For instance, CMOS network of 1ns gates produces 1GHz signal, ECL
array of 100ps gates gives 10GHz radiation. Logic circuits consisting of
10ps gates will emit infrared radiation. That signal could be easily
detected by photosensitive devices.
2. Background
Let us have a closer look at the structure of speedindependent
modules (SIM) as presented in Fig.1. All input data are processed in CL,
all output data are obtained from CL, too. So, CL is the only unit in SIM
which is involved in proper data processing. The result of that processing
is specified by Boolean functions. Algorithms for calculating the Boolean
function are realised by the internal structure of CL. Generally, its
structure is seriesparallel as well as algorithm implemented.
When nbit data word is put into the CL, n or more signal
propagation paths (SPPs) can be activated concurrently. So, one can say
that the calculation of a Boolean function by CL is of parallel nature. On
the other hand, each SPP is a gate chain which processes data in a serial
manner. So, calculation in CL is also of sequential nature.
The OVD circuit is intended for detecting transient and steady
"states" of CL. If any SPP in CL is still "active", CL is in transient
state, otherwise it is in steady state. Each gate switching results in both
logical and electromagnetic effects on its surrounding medium. The logical
effects of switching has been heavily investigated; we consider physical
one.
To provide speedindependence of the module the OVD and interface
circuitry must also work in a speedindependent mode. This means that any
arbitrary but finite transistor or wire delay cannot impair proper
operation of OVD and interface circuitry.
The interface circuitry is a mediator between OVD and environment of
SIM. It implements any kind of signalling convention, commonly a two or
fourcycle one [4] based on request Req and acknowledgement Ack signal
using. The interface circuitry receives the output validity (OV) signal
from the OVD circuit, a Req signal from the environment and transmits an
Ack signal to the environment (Fig.1).
Consider an algorithm of operation for interface circuitry realizing
speedindependent fourcycle signalling convention (FCSC). In accordance
with FCSC the control signals must go in the following sequence: Req+OV
Ack+ReqAck where "+" corresponds to rising the signal and "" corresponds
to falling the signal. All signals are assumed to adhere to positive logic.
Initially the signals Req and Ack are low, the signal OV is high. If the
environment state changes, the Req signal rises and transient state of CL
occurs (OV). Upon completion of the transitions in CL, signal OV rises and
the interface circuitry generates the Ack signal rising. After that the
environment produces a falling Req signal and then the interface circuitry
transmits the falling Ack signal to the environment. All the signals have
to be reset into the initial state.
To develop the interface circuitry a circuit designer must take into
account that any OVD circuit has finite (nonzero) turnon delay ton. This
means that OVD cannot respond on transitions of short duration t tr< ton .
An example of interface circuitry is shown in Fig.2. It contains a
flipflop, a NORgate, an asymmetrical delay and an inverter as an output
stage [5].
[pic]
The asymmetrical delay is intended for delaying Req rising signal
for + period where + > ton . Delaying Req falling signal noted  is to be
as short as possible. Note that speedindependent operation of interface
circuitry is vulnerable to delay + variation. If + becomes less than ton ,
proper operation of SIM can not be guaranteed. Otherwise, if + is much
more than ton , performance of SIM will be significantly reduced. To
provide exact accordance of + and ton a circuit emulator can be used.
Such an emulator is either an exact copy of OVD or its functional
copy, i.e. resistivecapacitive model of OVD's critical path. In the chip
the emulator must be placed next to active OVD circuit in order to ensure
identical conditions of fabrication and operation.
In this example we use a simplified asymmetrical delay implemented
as an asymmetrical CMOS inverter chain (Fig.3). Contrary to the common
inverter an asymmetrical one has nonequal rise and fall times of output
signal.
[pic]
A time diagram for interface circuitry is presented in Fig.4 for two
cases: (a) ttr < ton and (b) ttr ton. In case (a) the signal sequence
Req+Ack+ is formed for (++tNOR) period where tNOR is a NORgate delay. In
case (b) the above sequence is formed for (ttr +toff+tNOR) duration where
toff is a turnoff delay of OVD circuit. When the SIM returns to the
initial steady state, the signal sequence ReqAck is formed for (+tNOR)
interval.
[pic]
After considering the SIM in operation it is obvious that the main
problems of the module design are in the area of CL and OVD interaction.
This includes (a) kind of signal used as a carrier of information about CL
output validity, and (b) method of OVD circuit design.
4. Current consumption detection
Using current consumption of CMOS CL for output validity detection
was proposed in 1990 [7]. Contrary to the method of EMR detection this one
is based on introducing direct coupling of source and receiver. While CL is
in steady state it consumes current of about 109108A which does not
allow OVD switching. The interface circuitry gets information on CL output
validity and in turn informs the environment about CL readiness to input
data processing. When an input data arrives CL changes its state to
"transient", current consumption increases to 104102A, which switches
the OVD, thus informing the interface circuitry about output invalidity.
The latter lets the environment know about CL business.
After the computations in the CL are finished, the current consumption
decreases down to the steady state value, and the OVD sends a signal of
output validity.
4.1 Information carrying signal
Current consumption by CMOS CL contains useful information on CL
state. CMOS CL is a network of CMOS gates, so the current consumed by CL is
a superposition of currents consumed by CMOS gates included in the CL.
Each CMOS gate contains PMOS transistor and NMOS transistor networks
(Fig.5). While a gate is in a steady state either the PMOS or the NMOS
network is in a conducting mode. When a gate switches the nonconducting
transistor network becomes conducting. There is usually a short period in
switching time when both networks are in a conducting mode.
[pic]
Generally, current consumed by a CMOS gate includes three
components [9,10]:
(a) leakage current Ilk passing between power supply and ground due
to finite resistance of nonconducting transistor network;
(b) shortcircuit current Isc flowing while both networks are in a
conducting mode;
(c) load capacitance CL charge current ILC flowing while a CMOS gate
is switching from low to high output voltage via conducting PMOS network
and CL .
SPICE simulation has shown [5] that amplitude of current consumed by
a typical CMOS inverter depends on CL and is limited by the nonzero
resistance of the conducting PMOS network (Fig.7). The integral of consumed
current is proportional to CL . When a gate switches from high to low
output voltage, the component ILC is negative by direction and negligible
by value (Fig.7b). It is evident, the switchings from high to low output
voltage occur at the expense of energy accumulated in CL during the
previous switching from low to high output voltage. The component Isc does
not depend on direction in which a gate switches.
[pic]
[pic]
The component ILC equals to ILC = CLVdd f where Vdd is a power
supply voltage, f is a gate switching frequency. Veendrick has investigated
the component Isc dependencies on CL and risefall time of input potential
signal [10]. He showed that if both input and output signal have the same
risefall time, the component Isc cannot be more than 20 percent of summary
current consumption [10]. However, when the output signal risefall time is
less than input one, the component Isc can be of the same order of
magnitude as ILC. In that case it must be taken into account. As to the
component Ilk, it entirely depends on CMOS process parameters and for state
of the art CMOS devices Ilk is about 1015 1012 A.
So, the analysis of CMOS gate current consumption allows us to
conclude that in transient state a CMOS gate consumes a current I=
Ilk+Isc+ILC and in steady state it consumes only IlkVref, the
comparator output signal equals to logical "one" which means that the
outputs are invalid.
As it follows from the OVD circuit configuration,
[pic] [pic]
where Vdd is a voltage of power supply.
Equations (4) and (5) allow us to calculate the threshold voltage
drop V of the CVC circuit:
since [pic], so [pic] [pic]
If 0750mV, the diode D1 is in active mode and while rb 20, the component under
the sign of summation in Equation (7) can be much larger than the
component Cin. Due to voltage drop V the effective power supply voltage is
reduced and CL performance is decreased by about 35 percent [7].
In order to make SIM operating faster special attention must be paid
to reducing the capacitance introduced by CL.
4.3 Speedindependent address bus
The simplest case of CL is a scheme degenerated into a set of wires
called a multibit bus. Let us develop the OVD circuit for such a CL.
Multibit bus consists of several lines. Each line can be
considered as a medium for signal propagating from one end of the chip
to another. Delay of signal propagation through a line depends on several
factors:
(a) output impedance and symmetry of driver circuit;
(b) initial state of the line: if driver is symmetrical, line switching
from high to low voltage lasts shorter than reverse switching;
(c) electrical properties of the line as a signal propagation medium
(resistance of conducting layer and capacitances between the line and other
wires next to it);
(d) length of the line;
(e) input impedance and sensitivity of receiving circuit.
Since different lines of the bus operate in different conditions
(a)(e), signal propagation delays are different, too. From the standpoint
of environment the bus behaves like any other more complicated CL.
Asynchronous RAM designers use a bus transition detector since 1980s
[1315]. Such a detector is usually based on doublerail address coding
and two series connected transistors for each address bit [15]. One of
the transistors receives the true address signal and the other receives
the complementary address signal of the particular address bit. For any
steady state condition one of the transistors will be turned on and one
will be turned off. There will be a finite rise and fall time during a
transition of the address bit. There is a short time during which both
transistors are conducting. The establishment of the conductive path
provides the detection of the address transition. In the first
asynchronous RAMs the output signal of the transition detector is used for
bit line precharging and for enabling/disabling sense amplifiers and
peripheral circuitry.
Selftimed RAM announced in 1983 [14] used transition detectors not for
address transition only but also for detecting read/write completion and
address/bit line precharge completion as well.
The CMOS transition detector was invented in 1986 [15]. This circuit
is also based on doublerail coding and uses a pair of seriesconnected
NMOS transistors (Fig.12). The scheme for nbit bus control contains n line
transition detectors (LTDs) and n ANDgates. Outputs of ANDgates are
united in node M forming wired OR. The output inverter serves as a pulse
shaper. Capacitors C1 and C2 are intended to prolong rise time of the LTD
output signal (true and complementary). This is necessary for reliable
detection.
[pic]
The main drawback of the circuit is speed dependence. One can see
that if true and complementary address bit signal have different
propagation delays, the conducting path via NMOS transistors will never be
formed.
Using the OVD circuit proposed in Section 4.2 as LTD we can avoid
this drawback.
Note that address transmission through the address bus is
unidirectional. So to detect completion of bus transition it is enough to
recognize the bus state at the destination end. For this purpose we modify
CL to consist of n lines. The modification means introducing n LTDs, each
actually a CMOS inverter chain. Each chain contains two inverters loaded
with a capacitance (Fig.13). Input of each LTD is connected with
corresponding line of the bus at the destination end. Power supply pads of
all LTDs are connected to the current input of the same OVD circuit.
[pic]
The parameters of the input current signal for the OVD circuit are
varied by
(i) value of capacitances C1 and C2 ;
(ii) dimensions of MOS transistors M1 M4 .
Since all transitions in CL are of the same duration and can be
lengthened to be outlast the OVD turningon time, we simplify the
interface circuitry by disallowing the asymmetrical delay.
Due to short duration of normal transition in this CL we must take
into account the integral nature of the sensitivity of the OVD circuit. OVD
sensitivity depends on both amplitude and width of input current pulse.
Simulated operation region of the OVD circuit for current pulses shorter
than 30ns is shown in Fig.14. It is obvious that in this case the threshold
of the OVD circuit must be determined by threshold charge Qth value. The
OVD input charge Q equals to [pic] where I is OVD input current, t is a
moment of time when transition occurs, w is a width of input current
pulse. Turningon condition for the OVD circuit is Q=Qth.
[pic]
When the LTD circuit shown in Fig.13 is used, the charge value Q is
determined by either C1 or C2. Namely, if the line goes from low to high
voltage, Q=VC2. If the line goes in the reverse direction then [pic] where
V is charging/discharging voltage, approximately equal to the effective
power supply voltage: VVdd V. Here Vdd is OVD power supply voltage and V
is CVC voltage drop.
The OVD circuit with typical parameters (See Table 1) has a
threshold charge value Qth =4.01012 C. When C1 =C2 =CL , the minimal value
of CL providing OVD capacity for operation is about 1.01012 F.
Influence of transistors M1 M4 dimensions on LTD delay d is
determined by approximation [17]:
[pic]
where ~ is a sign of proportionality, Gn and Gp are the conductances of
NMOS and PMOS transistors respectively (CL =C1 =C2.)
Since [pic] and [pic] where W and L are width and length of
transistor channels of the corresponding conduction type, the LTD delay d
is proportional to [pic].
It has been obtained that for [pic], [pic], CL=1.0pF and VddV=5.0V
the LTD delay d=7.6ns.
When LTD works jointly with the OVD in the speedindependent bus,
the real value of the LTD delay will increase by 3040 percent due to
OVD's R1 effect on the effective power supply voltage.
To determine the appropriate value of R1 in the OVD circuit we must
know threshold input current Ith corresponding to threshold voltage drop
Vth recommended to be equal to 400mV.
Average input current Iav in transient state of one line is
determined by the expression Iav =CLv where v is the average rate of
increase in the output signal for an inverter included in LTD. For typical
values v=1.0109 Volts per second and CL =1.0pF, Iav =1.0mA. Accepting Ith
=0.4mA and Imax=2.0mA we obtain R1=1k and rb=100.
Simulation has shown that in this case OVD turningon delay can be
approximated by an empirical expression:
ton[ns]=8.1+0.1n
where n is the address bus bit capacity. Total delay of recognizing address
transition ttot =dg+ton where g is a coefficient of the LTD delay increase
due to reducing power supply voltage. As we showed above g1.35. It can be
seen that if n=32, ttot=21.6ns.
4.4 Speedindependent adder
The circuit we use in this Section as a CL was a touchstone for
many speedindependent circuit designers for about four decades. We mean a
ripple carry adder (RCA) which is actually a chain of onebit full adders
(Fig.14).
[pic]
Each full adder calculates two Boolean functions: sum si=aibici and
output carry ci+1=aibi+bici+aici where ai, bi are summands, ci is input
carry and stands for XOR operation.
In 1955 Gilchrist et al. proposed speedindependent RCA with carry
completion signal [18]. In 1960s that circuit was carefully analyzed and
improved [1921]. In 1980 Seitz used RCA for illustrating his concept of
equipotential region and his approach to selftimed system design [4].
Now we use RCA as a CL for illustrating our approach to SIM design.
As it was shown in Section 4.2 the turnon and turnoff delays of
the OVD circuit are proportional to the equivalent capacitance Ceq
associated with OVD circuit input. Capacitance Ceq depends linearly on a
number of gates N in CMOS CL. To speed up a SIM it is necessary to reduce a
number N. This can be reached by structural decomposition CMOS CL into
subcircuits CL1, CL2, etc. Each subcircuit CLi is connected to its own
detecting circuit OVDi or directly to the power supply if this subcircuit
transition does not affect the transition duration in CL as a whole. Each
detecting circuit OVDi generates its own OV signal which is combined with
other OVDs' output signals via a multiinput OR (NOR) element. The output
signal of that element serves as OV signal of the CMOS CL.
Multibit RCA computation time is determined by length of maximal
activated carry chain. A lot of papers were devoted to analysis of carry
generation and carry propagation in RCA [1921], many of them contained
their own methods for estimation or calculation of average maximal
activated carry chain. We do not intend to add another one.
Let us have a look inside RCA. As it was mentioned above RCA
consists of onebit full adders and each full adder consists of two
parts: forming sum si part and forming carry ci+1 part (Fig.16).
In multibit RCA all forming sum parts do not interact with each
other and do not affect on transition duration in RCA. Each forming carry
ci+1 part receives ci signal from preceding forming carry part and sends
ci+1 signal to consequent one.
To decompose RCA we use three heuristic tricks:
(i) All forming sum parts we connect directly to power supply.
(ii) We divide each forming carry part into three subcircuits denoted in
Fig.16 by numbers 1,2 and 3. All subcircuits 1 we connect directly to
power supply because they do not contain input ci and so do not contain
carry propagation path.
(iii) All subcircuits 2 we connect to OVD1 and all subcircuits 3 we
connect to OVD2. Outputs of OVD1 and OVD2 are connected to twoinput
NORgate forming RCA OV signal in positive logic manner (Fig.17).
OVD1 and OVD2 input currents I1 and I2 curves for 6bit RCA and
longest transition duration are shown in Fig.18.
Accepting Vth1,2=400mV we calculated the OVD circuits parameters. It
was obtained R11=5k, Ith1=0.08mA, R12=3k, Ith2=0.13mA. OVD1 and OVD2 delay
dependencies on a number of bits in RCA are shown in Fig.19.
4.5 Comparison of SIMs with synchronous counterparts
Transition duration in CL is a random variable. Probability of
transition with duration D is determined by implemented Boolean function
and distribution of input logical combinations. Domain of possible values
for variable D occupies the interval [0;Dmax]. Here Dmax is a length of
critical path in CL.
Let [pic] is a mathematical expectation of transition duration in CL
where Di is a length of ith SPP in CL, pi is a probability of ith path
being the longest activated SPP.
When CL works in the synchronous mode, the cycle duration Ts is
chosen with regard to maximal transition duration Dmax. Certain margin must
be added to Dmax to provide reliable operation of CL in the case of CL
parameter variations: Ts =kDmax where k is a margin coefficient.
In SIM cycle duration is a random variable with expectation Tsi =
gDme+toff+tif where g is a coefficient of CL delay increasing due to
reducing power supply voltage, toff is turnoff delay of the OVD circuit,
tif is an interface circuitry delay.
We determine efficiency E for speedindependent mode of CL operation
as relative increase of SIM performance in comparison to its synchronous
counterpart:[pic].
Generally, speedindependent mode is more efficient than synchronous
one if Ts >Tsi or, in other words, [pic].
In the case of RCA [pic] where tc is a delay of carry forming part,
n is a number of full adders in RCA.
It has been shown [19] that in nbit RCA Dme tclog2(5n/4). Then, in
the case of speedindependent operation Tsi=gtclog2(5n/4)+toff+tif.
We have obtained dependencies of Ts , Tsi on a number of bits in
RCA that are shown in Fig.20. As it can be seen, speedindependent
operation of RCA is more efficient while n>8.
5.Conclusion
6.Acknowledgement
I would like to thank Igor Shagurin and Vlad Tsylyov of the Moscow
Physical Engineering Institute for helpful discussions of this work. I am
also grateful to Chris Jesshope of University of Surrey and Mark Josephs of
Oxford University who kindly provided the latest material on their research
in the area of delayinsensitive circuit design.
References
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eds., Introduction to VLSI Systems (AddisonWesley, New
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approach to CMOS module selftiming, Electronics Letters 26 (22)
(1990) 18351836.
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Appendix
SPICE2G.6: MOSFET model parameters
      
    VALUE  
 Name Parameter Units PMOS NMOS 
1 level model index  3 3 
2 VTO ZEROBIAS THRESHOLD VOLTAGE V 1.337 1.161 
3 KP TRANSCONDUCTANCE    
  PARAMETER A/V2 2.3105 4.6105 
4 GAMMA BULK THRESHOLD PARAMETER [pic] 0.501 0.354 
5 PHI SURFACE POTENTIAL V 0.695 0.660 
6 RD DRAIN OHMIC RESISTANCE OHM 333 85 
7 RS SOURCE OHMIC RESISTANCE OHM 333 85 
8 CBD ZEROBIAS BD JUNCTION    
  CAPACITANCE F 1.9810146.91015 
9 CBS ZEROBIAS BS JUNCTION    
  CAPACITANCE F 1.9810146.91015 
10IS BULK JUNCTION SATURATION    
  CURRENT A 3.4710159.221015
11PB BULK JUNCTION POTENTIAL V 0.8 0.8 
12CGSO GATESOURCE OVERLAP CAPACI    
  TANCE PER METER CHANNEL WIDTHF/M 6.7010103.301010
13CGDO GATEDRAIN OVERLAP CAPACI    
  TANCE PER METER CHANNEL WIDTHF/M 6.7010103.301010
14CGBO GATEBULK OVERLAP CAPACITANCE   
   F/M 1.90109 2.60109 
  PER METER CHANNEL LENGTH    
15RSH DRAIN AND SOURCE DIFFUSION    
  SHEET RESISTANCE OHM/SQ55 30 
16CJ ZEROBIAS BULK JUNCTION    
  BOTTOM    
  CAPACITANCE PER SQ METER OF F/M2 3.53104 1.24104 
  JUNCTION AREA    
17MJ BULK JUNCTION BOTTOM GRADING    
  COEFFICIENT  0.5 0.5 
18CJSW ZEROBIAS BULK JUNCTION SIDE   
      
  WALL CAPACITANCE PER METER OFF/M 1.7110103.201011
      
  JUNCTION PERIMETER    
      
