Iterative Versus Sequential Circuits

Iterative Versus Sequential Circuits

Iterative Versus Sequential Circuits

Synchronous Design Methodology Synchronous systems – all flip-flops

are clocked by the same common clock.

To ensure reliable operation: Minimize and determine the amount of

clock skew in the system. Ensure that flip-flops have positive setup

and hold time margins, including allowance for clock skew.

Identify asynchronous inputs and synchronize them with the clock. Ensure that the synchronizers have low probability of failure.

Clock Skew

Difference between arrival times of the clock at different devices.

Clock Skew

Asynchronous Inputs Synchronizer – circuit that samples an

asynchronous input and produces an output that meets the setup and hold times required in a synchronous system.

Asynchronous Inputs

Asynchronous Inputs

Synchronous System Structure

Design Example Shift and add multiplier

MPY/LPROD – Initially stores the multiplier, and accumulates the low-order bits of the product.

HPROD – Initially cleared, and stores the high-order bits of the product.

MCND – Stores the multiplicand.

If low-order bit of MPY/LPROD

Is 1 then F = 9-bit sum of HPROD and MCND.

Is 0 then F = HPROD extended to 9 bits.

Design Example LDMCND_L=0 enables the

multiplicand register U1 to be loaded.

LDHP_L=0 enables the HPROD register U6 to be loaded.

MPSY[1,0] = 11 enables the MPY/LPROD

register U2 to be loaded. = 01 it shifts right during

multiplication. = 00 at other time to preserve

register content. SELSUM=1 multiplexers U7 and

U8 select adders U4 and U5 output otherwise it selects HPROD.

CLEAR=1 the output of multiplexers U7 and U8 is cleared.

Design Example LDMCND_L=0 enables the

multiplicand register U1 to be loaded.

LDHP_L=0 enables the HPROD register U6 to be loaded.

MPSY[1,0] = 11 enables the MPY/LPROD

register U2 to be loaded. = 01 it shifts right during

multiplication. = 00 at other time to preserve

register content. SELSUM=1 multiplexers U7 and

U8 select adders U4 and U5 output otherwise it selects HPROD.

CLEAR=1 the output of multiplexers U7 and U8 is cleared.

Design Example

Algorithmic State Machines - ASM Partition the system into two parts:

Controller - ASM. Controlled architecture – data processor.

ASM

Algorithm is a well defined procedure consisting of a finite number of steps to the solution of a problem.

Controller is a hardware algorithm or Algorithmic State Machine.

ASMs can serve as stand-alone sequential network model.

ASM Conditional outputs – Mealy model. State outputs – Moore model. State time:

Transition period. Stable period.

ASM State box.

Represents one state in the ASM.

May have an optional state output list.

Single entry. Single exit to

state or decision boxes.

ASM Decision box.

Provides for next alternatives and conditional outputs.

Conditional output based on logic value of Boolean expression involving external input variables and status information.

Single entry. Dual exit, denoting if

Boolean expression is true or false.

Exits to decision, state or conditional boxes.

ASM Conditional output

box. Provides a listing of

output variables that are to have a value logic-1, i.e., those output variables being asserted.

Single entry from decision box.

Single exit to decision or state box.

ASM Blocks Consists of the interconnection of a

single state box along with one or more decision and/or conditional boxes.

It has one entry path which leads directly to its state box, and one or more exit pathes.

Each exit path must lead directly to a state, including the state box in itself.

A path through an ASM block from its state box to an exit path is called a link path.

ASM Block Example

ASM Blocks An ASM block describes the operation of the

system during the state time in which it is in the state associated with the block.

The outputs listed in the state box are asserted.

The conditions indicated in the decision boxes are evaluated simultaneously to determine which link path is to be followed.

If a conditional box is found in the selected path then the outputs found in its output list are asserted.

Boolean expression may be written for each link path. The selected link paths are those that evaluate to logic-1.

ASM Blocks