VLSI and Embedded System Technical Library: Clocking Strategies in VLSI Systems

Clock - key to synchronous systems

Clocks help the design of FSM where outputs depend on both input and previous states.

Clock signals provide reference points in time - define what is previous state, current state and next state:

Latch vs Flip-Flop

Clock for timing synchronization

Clocks serve to slow down signals that are too fast

Flip-flops / latches act as barriers
With a latch, a signal can’t propagate through until the clock is high
With a Flip-flop, the signal only propagates through on the rising edge
All real flip-flops consist of two latch like elements (master and slave latch)

Latch Timing Parameters

Flip-flop Timing Parameters

Typical Clock System

Clocking Overhead

Latches and flops slow down the slow signals

Flip-flop delays the slowest signal by the setup + clk-q delay

Latches delay the late arriving signals by the delay through the latch

Problem of Clock Skew

Not all clocks arrive at the same time

Some clocks might be gated (ANDed with a control signal) or buffered There is an RC delay associated with clock wire

Clock Skew and Jitter

Clock skew

Spatial variation in temporally equivalent clock edges; deterministic + random, tSK

Clock jitter

Temporal variations in consecutive edges of the clock signal; modulation +random noise - Cycle-to-cycle (short-term) tJS Long term tJL

Both skew and jitter affect the effective cycle time

Only skew affects the race margin

Longest Logic Path - Edge Triggered

Unger and Tran, Trans. On Comp. 10/86

Shortest Path Constraint

If launching edge is early and receiving edge is late:

Clocking Strategies

Trade off between overhead / robustness / complexity

Constraints on the logic vs. Constraints on the clocks

Look at a number of different clocking methods:

Pulse mode clocking
Edge triggered clocking
Two phase clocking
Single phase clocking

We will only look at system level strategy - consider clocked circuits in the next lecture

Pulse Mode Clocking

Two requirements:

All loops of logic are broken by a single latch
The clock is a narrow pulse

It must be shorter than the shortest path through the logic

Timing Requirements

Used in the original Cray computers (ECL machines)

Advantage is it has a very small clocking overhead

One latch delay added to cycle

Leads to double sided timing constraints

If logic is too slow OR too fast, the system will fail

Pulse width is critical

Hard to maintain narrow pulses through inverter chains

People are starting to use this type of clocking for MOS circuits

Pulse generation is done in each latch.
Clock distributed is 50% duty cycle
CAD tools check min delay

Not a good clocking strategy for a beginning designer

Edge Trigger Flip-flop

Popular TTL design style

Used in many ASIC designs (Gate Arrays and Std Cells)

Using a single clock, but replaces latches with flip-flops

Timing Constraints

If skew is large enough, still have two sided timing constraints

Two phase clocking

Use different edges for latching the data and changing the output

There are 4 different time periods, all under user control:

Φ1 high
Φ1 falling to Φ2 rising
Φ2 high
Φ2 falling to Φ1 rising

Look at shift register again:

If there is a large skew on the Φ2x clock, then the spacing between Φ1 and Φ2
can be increased to make sure that even with the skew, the Φ2 latch closes
before the Φ1 latch lets the new data pass.
For some setting of the timing of the clock edges, the circuit will work!

Stable signal type

We will give signals timing types, so it will be easier to know which latchto use:

Output of a Φ1 latch is stable Φ2 (_s2) – good input to Φ2 latch

Output of a Φ2 latch is stable Φ1 (_s1) – good input to Φ1 latch

Signal is called stable2, since it is stable for the entire Φ2 period

General two phase system

Combination logic does not change the value of timing types.

No static feedback in the combination logic is allowed either. This makes the system not sensitive to logic glitches.

Why two phase clocking?

It is a constrained clocking style:

Synchronous design
Two clocks
Constrained composition rules

But gives this guarantee:

If you clock it slow enough (with enough non-overlap between edges)

It will be a level sensitive design
no race, glitch, or hazard problems
no skew problems
One sided timing constraints
Impossible for logic to be too fast

Mealey and Moore Machines

More on latch timing

Look a little more closely at latches, to come up with a more complete set of timing types (more than _s1 _s2 signals) that we can use in our synchronous designs.

Look at a latch since this the critical element

What is the weakest requirement on the input to a latch?

Signal must settle before Φ1 falls, and not change for some time after Φ1 falling, even for a skewed Φ1 (this is usually called the setup and hold times of the latch)

Valid Signal Type

The weakest input to a latch is called a valid signal (_v1 _v2)

For a valid signal we need to be sure we can guarantee it meets the setup and hold requirements of the latch

To do this we need to have the signal settle off an edge that comes before Φ1 falling. The closest edge is Φ1 rising.

The signal should not change until an edge occurs that comes after Φ1 falling. The closest edge is Φ2 rising.

If we changed the input on Φ1 falling, most of the time the circuit would work fine. But if it failed, we can’t change the clock timing to make the circuit work -- Φ1 falling controls the changing of the input, and the closing of the latch. Since we can’t guarantee it would be ok a signal that changes on Φ1 falling would not be a _v1 signal.

Use of valid signal

Very useful for precharged logic

Is not needed for standard combinational logic with latches

This should always give stable signals

Can’t use stable signals if you want to drive two signals/cycle on a wire (multiplex the wire), since the value has to change twice. There are many wrong ways to do it, and only one right way, which is shown below. The values become _v signals.

Stable Signals

a. Have even larger timing margins than valid signals

b. A _s1 signal starts to change sometime after Φ2 rises

c. A _s1 signal settles sometime after Φ2 rises

d. Input to the latch must be a _v2 (settles after Φ2 rises)

e. Output of a latch settles some small delay after input settles

Please note that combinational logic does not change the value of the timing type, even though it does increase the delay of the signal path. The timing types have to do with the clocking guarantee that we are trying to keep. This promise is that the circuit will work at some frequency. A _s1 signal might not settle until after Φ1 rises when the part is run at high-frequency, but the label means that you can make that signal stabilize before Φ1 rises if you need to by slowing the clock down.

Qualified (Gated) Clocks

These are signals that have the same timing as clocks, but they don’t occur every cycle. They are formed by ANDing a ‘_s1’ signal with Φ1 giving _q1, or ANDing a ‘_s2’ signal with Φ2 giving a _q2 signal.

The control signal needs to be a stable signal to prevent glitches on the qualified clocks.

Qualified clocks can only be used as the clock input to a latch

Summary of Clock Types

The figure shows the timing of all the signals we have discussed with little arrows that indication with clock edge caused the signal to change. Remember the pictures, and the timing types are what the signals look like at slow clock frequencies

Disadvantages of two phase clocking

1. Need four clocks in general

Need true and complement of both clocks

2. Still need low skew for good performance

The skew increases the cycle time of the machine
Need low skew between all the clocks for good performance
Want to have Φ1 and Φ2 close to coincident

3. Many systems use clock and its complement instead of 2 phases

Needless to say they are very careful about clock skew
For these systems it is still useful to maintain 2 phase timing types, since it ensures you connect all logic to the right latches
Call Clk - Φ1 and Clk - Φ2, and go from there.
Note in this class we will use Φ1 and Φ2 for clocks)

Advantage of Latches Over Flip-Flops

1. If you are going to use Clk and Clk_b and control skew, why not go back

to flip-flops?

2. Many people do: