Sequential CMOS and NMOS Logic Circuits Sequential logic circuits contain one or more combinational logic blocks along with memory in a feedback loop with the logic –The next state of the machine depends on the present state and the inputs –The output depends on the present state of the machine and perhaps also on the inputs Mealy machine: output depends only on the state of the machine Moore machine: the output depends on both the present state and the inputs Topics to be studied in this section: –Basic memory cell operation –SR Latch –JK Latch –D Latch –Flip-Flops –Clocked CMOS Logic –Cascode Voltage Switch Logic –Clock Distribution R. W. Knepper SC571, page 5-27 From Chapter 8 in Kang and Leblebici, and portions of Chapter 5 in West and Eshraghian

Logic Circuit Classification: Sequential Circuit Types Sequential circuits (also called regenerative circuits) fall into three types: –Bistable –Monostable –Astable Bistable circuits have two stable operating points and will remain in either state unless perturbed to the opposite state –Memory cells, latches, flip-flops, and registers Monostable circuits have only one stable operating point, and even if they are temporarily perturbed to the opposite state, they will return in time to their stable operating point Astable circuits have no stable operating point and oscillate between several states –Ring oscillator R. W. Knepper SC571, page 5-28

Memory Cell: Two-Inverter Basic Bistable Element A memory cell is comprised of two inverters connected back-to-back, i.e. output of one to input of the other and vice-versa. The memory cell (or latch) has two stable states where the dc voltage transfer curves cross at the V OH and V OL points, but also exhibits an unstable state where the VTCs cross near their V th switching points. –In actual physical circuits the memory cell will never stay at the unstable point, since any small electrical noise in the circuit will trigger it to one side or the other –In numerical simulation (Cadence Spectre) the circuit may actually remain in the unstable state (assuming no noise source) The CMOS SRAM cell at the left will either be in state 0 with V01 at GND and V02 at V DD or in state 1 with V01 at V DD and V02 at GND. R. W. Knepper SC571, page 5-29

CMOS SR Latch: NOR Gate Version The NOR-based SR Latch contains the basic memory cell (back-to-back inverters) built into two NOR gates to allow setting the state of the latch. –If Set goes high, M1 is turned on forcing Q low which, in turn, pulls Q high S=1 Q = 1 –If Reset goes high, M4 is turned on, Q is pulled low, and Q is then pulled high R=1 Q = 1 –If both Set and Reset are low, both M1 and M4 are off, and the latch holds its existing state indefinitely –If both Set and Reset go high, both Q and Q are pulled low, giving an indefinite state. Therefore, R=S=1 is not allowed The gate-level symbol and truth table for the NOR-based SR latch are given at left To estimate Set time, add time to discharge Q + time to charge Q (pessimistic result) R. W. Knepper SC571, page 5-30

Depletion Load NMOS SR Latch: NOR Version A depletion load version of the NOR-based SR latch is shown at left. –Functionally same as CMOS version The latch is a ratio circuit –Low side conducts dc current, causing higher standby power than CMOS version R. W. Knepper SC571, page 5-31

CMOS SR Latch: NAND Gate Version A CMOS SR latch built with two 2-input NAND gates is shown at left –The basic memory cell comprised of two back-to-back CMOS inverters is seen The circuit responds to active low S and R inputs –If S goes to 0 (while R = 1), Q goes high, pulling Q low and the latch enters Set state S=0 Q = 1 (if R = 1) –If R goes to 0 (while S = 1), Q goes high, pulling Q low and the latch is Reset R=0 Q = 1 (if S = 1) –Hold state requires both S and R to be high –S = R = 0 if not allowed, as it would result in an indeterminate state R. W. Knepper SC571, page 5-32

Clocked SR Latch: NOR Version Shown at left is the NOR-based SR latch with a clock added. –The latch is responsive to inputs S and R only when CLK is high –When CLK is low, the latch retains its current state Timing diagram shows the level-sensitive nature of the clocked SR latch. –Note four times where Q changes state: When S goes high during positive CLK On leading CLK edge after changes in S & R during CLK low time A positive glitch in S while CLK is high When R goes high during positive CLK R. W. Knepper SC571, page 5-33

Clocked CMOS SR Latch: AOI Implementation CMOS AOI implementation of clocked NOR-based SR latch shown at left with logic symbol circuit below –Only 12 transistors required –When CLK is low, two series legs in N tree are open and two parallel transistors in P tree are ON, thus retaining state in the memory cell –When CLK is high, the circuit becomes simply a NOR-based CMOS latch which will respond to inputs S and R R. W. Knepper SC571, page 5-34

Clocked CMOS JK Latch: NAND Version The SR latch has a problem in that when both S and R are high, its state becomes indeterminate The JK latch shown at left eliminates this problem by using feedback from output to input, such all states in the truth table are allowable –If J = K = 0, the latch will hold its present state –If J = 1 and K = 0, the latch will set on the next positive-going clock edge, i.e. Q = 1, Q = 0 –If J = 0 and K = 1, the latch will reset on the next positive-going clock edge, i.e. Q = 1 and Q = 0 –If J = K = 1, the latch will toggle on the next positive-going clock edge Note that in order to prevent the JK Latch above from oscillating continuously during the clock active time, the clock width must be kept smaller than the switching delay time of the latch. Otherwise, several oscillations may occur before the clock goes low again. In practice this may be difficult to achieve. R. W. Knepper SC571, page 5-35

JK Master-Slave Flip-Flop A Flip-Flop is defined as two latches connected serially and activated with opposite phase clocks –First latch is the Master; Second latch is the Slave –Eliminates transparency, i.e. a change occurring in the primary inputs is never reflected directly to the outputs, since opposite phase clocks are used to activate the M and S latches. A JK master-slave flip-flop (NOR-based version) is shown below: –The feedback paths occur from Q and Q slave outputs to the master inputs AOI gates –does not exhibit any tendency to oscillate when J = K = 1 no matter how long the clock period, since opposite clock phases activate the master and slave latches separately. –The NOR-based version can be done with four AOI CMOS gates, requiring 28 transistors –Can be susceptible to ones catching, i.e. a positive glitch in either the J or K input while the CLK is high, which can change the state of the master latch (and the slave latch on next edge) R. W. Knepper SC571, page 5-36

CMOS D-Latch Implementation A D-latch is implemented, at the gate level, by simply utilizing a NOR-based S-R latch, connecting D to input S, and connecting D to input R with an inverter. –When CLK goes high, D is transmitted to output Q (and D to Q) –When CLK goes low, the latch retains its previous state The D latch is normally implemented with transmission gate (TG) switches, as shown at the left –The input TG is activated with CLK while the latch feedback loop TG is activated with CLK –Input D is accepted when CLK is high –When CLK goes low, the input is open-circuited and the latch is set with the prior data D R. W. Knepper SC571, page 5-37

CMOS D-Latch Schematic View and Timing A schematic view of the D-Latch can be obtained using simple switches in place of the TGs –When CLK = 1, the input switch is closed allowing new input data into the latch –When CLK = 0, the input switch is opened and the feedback loop switch is closed, setting the latch Timing diagram: –In order to guarantee adequate time to get correct data at the first inverter input before the input switch opens, the data must be valid for a given time (T setup ) prior to the CLK going low. – In order to guarantee adequate time to set the latch with correct data, the data must remain valid for a time (T hold ) after the CLK goes low. –Violations of T setup and T hold can cause metastability problems and chaotic transient behavior. R. W. Knepper SC571, page 5-38

Alternate CMOS D-Latch Implementation An alternate (preferred) version of the CMOS D-Latch (shown at left) is implemented with two tri-state inverters and a normal CMOS inverter. Functionally it is similar to the previous chart D-Latch –When CLK is high, the first tri-state inverter sends the inverted input through to the second inverter, while the second tri-state is in its high Z state. Output Q is following input D –When CLK is low, the first tri-state goes into its high Z state, while the second tri-state inverter closes the feedback loop, holding the data Q and Q in the latch. R. W. Knepper SC571, page 5-39

CMOS Static Latches with Single Phase Clock Various types of D latch circuit with single phase clocks –(a) shows the use of a weak inverter with long L (low W/L) devices to allow removal of feedback loop X-gate but retain static latch function –(b) D latch ckt with input inverter buffer –(c) implementation of (b) utilizing tri- state buffer/inverter circuits with clocks at center of tri-state –Alternate schematic of (c) indicating layout convenience due to common tie point at output of tri-state buffers Clock skew problems can be solved on- chip by using buffering in clock nets –Inverter buffers to generate neg clk –Transmission gate buffers for true clk R. W. Knepper SC571, page 5-40

Construction of D Register (Flip-Flop) in CMOS Two level-sensitive latches are combined to form a positive edge- triggered register, as is used to build a D register –(a) shows negative level sensitive latch (valid when clock is negative) –(b) shows positive level sensitive latch (valid when clock is positive) –(c) shows positive edge-triggered D register (also called a Flip-Flop) comprised of a negative latch feeding a positive latch First latch is the Master Second latch is the Slave D register timing: –Output Q valid at T q (clock-to-Q) delay after clock edge –Data must be valid T s (setup time) prior to clock edge and T h (hold time) after clock edge R. W. Knepper SC571, page 5-41

CMOS D Flip-Flop: Falling Edge-Triggered Shown below is a D Flip-Flop, constructed by cascading two D-Latch circuits from the previous chart –Master latch is positive level sensitive (receives data when CLK is high) –Slave latch is negative level sensitive (receives data Qm when CLK is low) The circuit is negative-edge triggered –The master latch receives input D until the CLK falls from high to low, at which point it sets that data in the master latch and sends it through to the output Qs R. W. Knepper SC571, page 5-42

Clocked CMOS Logic (C 2 MOS) Clocked CMOS logic has been used for very low power CMOS and/or for minimizing hot electron effect problems in N-FET devices Clocking transistors allow valid logic output only when clk is high Clocking transistors may be at output end of logic trees (maximum performance) or at power supply end of logic trees (maximum protection from hot electrons) R. W. Knepper SC571, page 5-43

Cascade Voltage Switch Logic (CVSL) CVSL is a differential type of logic circuit whereby both true and complement inputs are required –For example, true inputs are applied to left pull-down leg below and complement inputs are applied to right leg N pull-down trees are the dual of each other P pull-up devices are cross-coupled to latch output Both true and complement outputs are obtained Input pull-down trees may be intermixed, depending on the logic to be implemented R. W. Knepper SC571, page 5-44

Clocked CVSL (Cascade Voltage Switch Logic) Clocked CVSL circuit type shown at left Example in (b) is an implementation of Q = a XOR b XOR c XOR d –Some similarity to a current steering circuit in bipolar –When clock is low, nodes are precharged; When clock goes high, one leg pulls the internal output to ground, while opposite leg is non-conducting –Complement outputs Q and –Q are obtained R. W. Knepper SC571, page 5-45

Sample-Set Differential Logic (SSDL): Dynamic CVSL with a Latching Sense Amp SSDL utilizes a latching sense amplifier to latch output when clock goes high, much like a DRAM sense amplifier When clock is low P1 & P2 precharge while N1 pulls down the N tree logic causing a differential voltage on the internal output nodes When clock goes high, N3-N5 cause the sense amplifier to set and latch R. W. Knepper SC571, page 5-46

CMOS Schmitt Trigger Circuit The Schmitt Trigger circuit, shown at left, has a dc transfer characteristic like an inverter, but with different switching thresholds depending on whether Vin is increasing or decreasing –Hysteresis effect If Vin is increasing, high V th If Vin is decreasing, low V th SPICE simulated VTC waveforms with increasing and decreasing input voltage are shown at right. increasing Vin V th = 3.5 V decreasing Vin V th = 1.4 V R. W. Knepper SC571, page 5-47

Clocking Strategies for Finite State Machine & Pipelined Systems VLSI systems universally make use of storage elements and states, with clock(s) to control the sequencing (a) shows a Finite State Machine –at positive clock edge, the next state bits get stored as the current state bits and the current state bits combined with inputs generate new next state bits (b) shows a pipelined system indicative of todays microprocessors and logic systems R. W. Knepper SC571, page 5-48

Timing a Pipelined System Pipelined system typically has registers separated by combinational logic Minimum cycle time Tc obtainable given by T c = T q + T d + T s –Tq is the clock-to-Q output delay of Register A –Td is the total worst case delay through the combinational logic –Ts is the set-up delay time of Register B In order to increase frequency in superpipelined and superscalar machines, long combinational logic blocks can be split into smaller combinational blocks and latches used to separate the blocks (rather than full registers) –Improves overall frequency of processor, but adds some delay penalty due to added latches R. W. Knepper SC571, page 5-49

The Effect of Clock Skew on a Pipeline Design of a pipelined machine assumes that clock edges will appear at each register at a precise time t o If delay occurs in clock distribution due to RC wire delays, LC ringing on the clock nets, or buffer delay, the pipeline timing will be skewed. –Can cause a latch or register to be set with incorrect data (as shown at right) Example: –Register M1 is set by the clock at Tc1, providing data inputs to the combinational logic and then to register M2 –Register M2 is supposed to latch in old data at the same clock edge –But, if the delay to Tc2 > Tc1 + Tq1 + logic delay, M2 will incorrectly store the new data rather than the previous data. R. W. Knepper SC571, page 5-50

Clock Synchronization Using Phase Locked Loops Phase Locked Loop (PLL) is used to synchronize an on-chip generated clock with a system clock at some point on the chip –Reduces clock skew to zero at the sensing point (a) no PLL – clock skew (b) with PLL on chip (c) using PLL with divide by 4 scheme to achieve 4X freq on-chip (d) use of PLL approach to synchronize data across several chips R. W. Knepper SC571, page 5-51

Charge Pump Phase Locked Loop One implementation of a PLL using a phase detector and charge pump ckt with a freq multiplier of n times –A phase difference is measured between the reference clock and the on- chip clock –Charge pump pumps a ref voltage up or down depending on phase difference –Loop filter to clean up voltage –Variable Controller Oscillator (VCO) is used to obtain exact frequency clock (b) VCO frequency is a function of control voltage applied to N pull-downs and current mirror (c) another approach to control frequency by using a variable delay line –MOS capacitance load on each stage is varied by NFET gate voltage R. W. Knepper SC571, page 5-52

Latch Metastability Consider the problem of setting a latch when the data is late and/or has a very long rise/fall time and is still changing during the clock transition –if data change is delayed and overlaps clock edge (below), latch may set with new data rather than valid prior data Data delay = 2.2 ns latch sets correctly at Q=1 Data delay = 2.3 ns latch hangs momentarily at metastable point, but then sets correctly at Q=1 Data delay = 2.4 ns latch hangs momentarily and sets incorrectly at Q=0 Metastable point: non stable point in a latch where V left = V right (neither 0 or 1) –thermal noise will cause latch to move off metastable point and set at a 0 or a 1 How to fix? –speedup the data (register-based synchronizer) –delay the clock (introduce an intentional clock delay ---- risky!!) R. W. Knepper SC571, page 5-53

Clock Tree Distribution To prevent clock skew problems on a chip, clock distribution networks are designed very carefully Example shown: linear (E-W) clock tree distribution network –Clock is buffered several times before driving FO=3 –Each FO3 buffer drives another high FO (FO=4 shown) buffer –Finally another single buffer is used for each linear clock line to drive across chip or functional island on a chip H tree distribution network often used on chips with area pads (solder bumps) –Master clock is brought on board chip near central part of chip and driven outward with large H interconnection arrangement R. W. Knepper SC571, page 5-54

JK MS Flip-Flop Problem: Ones Catching Although the JK Master-Slave Flip- Flop can be considered edge-triggered in regards to a change in Qs at the negative CLK edge, it is actually level sensitive in regards to noise on J (or K) during the CLK high interval. –Note positive glitch in J which erroneously Sets the Master latch at Qm = 1 during the CLK high interval and then also reflects itself in Qs = 1 at the negative-going CLK edge. Called Ones Catching –Same problem can occur with a glitch in K during CLK high, causing a Reset operation –Since the master latch actually sets and latches on the noise glitch, the error is then transmitted to the slave latch during CLK R. W. Knepper SC571, page 5-36a

A Positive-Edge Triggered D Flip-Flop Why is the NAND-based D Flip-Flop shown at left edge-triggered and not level sensitive? –There is no master latch –The two NAND gate pairs are clocked with opposite phase clocks, and therefore act similar to the transmission gates in the TG-based D flip-flop The 1 st NAND pair is clocked with CLK The 2 nd NAND pair is clocked with CLK = CLK Result: –Q changes on the positive-going CLK edge –NAND #1 pair locks in the valid data at the negative CLK edge –The master latch is essentially dynamic, holding the state as charge at the inputs of the two inverters R. W. Knepper SC571, page 5-36b

Monostable Multivibrator Circuit with Depletion Loads The monostable multivibrator circuit at left has only one stable state (Vout low) Operation (Reset): –Pull Vin high momentarily to reset to Vout high, pulling left node down to some VOL –Capacitor immediately pulls gate of right NMOS down turing it OFF –Vout immediately charges high to Vdd Operation (Timed return to Set): –Capacitor slowly charges to roughly Vdd through NMOS depletion transistor –After capacitor charges sufficiently towards Vdd, right-most NMOS transistor turns ON and switches the latch back to Vout low R. W. Knepper SC571, page 5-47b

An NMOS Schmitt Trigger Circuit Transistor Sizes: –W/L) M1 = 1 –W/L) M2 = 0.5 –W/L) M3 = 10 –W/L) M4 = 1 How does the circuit work? R. W. Knepper SC571, page 5-47a

Clocked SR Latch: NAND Version NAND version of clocked SR latch with active high clock is shown –Circuit is implemented with four NAND gates, not with an AOI or OAI 16 transistors required –The latch is responsive to S or R only if CLK is high –When CLK is low, the latch retains its present state R. W. Knepper SC571, page 5-31c

JK Register Implemented in CMOS JK register is implemented by adding logic to front of a D register Operation: –If JK = 01, Q goes to 0 on rising clock edge –If JK = 10, Q goes to 1 on clock edge –If JK = 00, Q retains prior state –If JK = 11, Q toggles on clock edge –When clock is down, Q and QN hold prior state R. W. Knepper SC571, page 5-47

T Register CMOS Implementation T (Toggle) Register is shown below (note error in Weste & Eshraghian text) –Output Q toggles (changes state) on each positive clock edge –Used as a divide by two counter –Comprised of D register with –Q connected to the input –Clear function is added to the T reg below by replacing 1 st inverter in slave latch with a NAND Operation: –When clk goes up, output Q is complemented (and master latch is set) –When clk goes down, slave latch is set. No change occurs to Q –When clear goes high, QM is set to a 1 (Q to a 0) R. W. Knepper SC571, page 5-46

D Register Implementation in CMOS CMOS implementation of the popular positive edge-triggered D register is shown –Comprised of four transmission gates and five inverters Operation –Clock = 0: Master latch is connected to input to receive new D data Slave latch is holding previous data on output and is isolated from input –Clock = 1: Master latch stops sampling input, latches up the D data at the positive clock edge, and sends it through to the output Q R. W. Knepper SC571, page 5-45

CMOS Latch Symbolic Layouts Typical layouts of D latch circuits –(a) layout of tri-state buffer D latch shown on previous slide –(b) & (c ) two alternate layouts of conventional D-latch circuit R. W. Knepper SC571, page 5-51

Static Registers Based on CVSL and SRAM Structures (a) shows a static D register designed around CVSL circuits –Latch #1 is formed with PFETs cross coupled and is clocked with NFETs pulling down to cause latch to set –Latch #2 is formed with NFETs cross coupled and uses PFET transistors to pull the output nodes toward Vdd to set the latch when Clk is low (b) is a D latch based on an SRAM cell with NFET pull-downs to set latch when Clk goes high R. W. Knepper SC571, page 5-51a

Differential Split-Level CVSL Vref is applied to gates of cascode NFETs to isolate the logic signal inputs from the high speed output nodes –Vref is designed to be approximately equal to (Vdd/2) + Vtn –By using Vref, the signal swing on the N logic trees connected to nodes d and –d is reduced to a small voltage of roughly Vdd/2 P pull-up devices are wired as a cross-coupled latch designed with a full Vdd signal –Gates of P pull-up transistors are connected below the N cascode transistors for maximum speed (a) shows a simple inverter (b) shows open drain pull-down complementary outputs –N logic trees connect to d and -d R. W. Knepper SC571, page 5-36

Registers with Asynchronous Set and Reset Asynchronous Set and Reset are added to the D Register by replacing inverter gates with 2-input NAND gates in forward or feedback latch legs (as shown at left) –(a) Asynchronous Reset only –(b) Asynchronous Set and Reset –Both registers have an input buffer added R. W. Knepper SC571, page 5-52

Dynamic Latches with a Single Clock Dynamic latches eliminate dc feedback leg by storing data on gate capacitance of inverter (or logic gate) and switching charge in or out with a transmission gate –Minimum frequency of operation is typically of the order of KHz so as not to lose data due to junction or gate leakage from the node –Can be clocked at high frequency since very little delay in latch elements Examples: –(a) or (b) show simple transmission gate latch concept –(c ) tri-state inverter dynamic latch holds data on gate when clk is high –(d) and (e) dynamic D register R. W. Knepper SC571, page 5-53