1. Sept. 2005EE37E Adv. Digital Electronics Lesson 4 Synchronous Design Architectures: Control Unit Design (part three)

2. Sept. 2005EE37E Adv. Digital Electronics Topics Introduction Hardwired vs. Microprogrammed Control Hardwired Control Microprogrammed Control High-level Synthesis of Control Units

3. Sept. 2005EE37E Adv. Digital Electronics 1. Introduction The control subsystem is a (synchronous) sequential machine. That has the following characteristics: –INPUTS: control inputs to the system and conditions from the data subsystem –OUTPUTS: control signals –ONE STATE PER STATEMENT IN REGISTER- TRANSFER SEQUENCE –TRANSITION FUNCTION CORRESPONDS TO SEQUENCING –OUTPUT FOR EACH STATE CORRESPONDS TOCONTROL SIGNALS

4. Sept. 2005EE37E Adv. Digital Electronics 2. Hardwired vs. microcoded control Hardwired control has a state register and “random logic.” A microcoded (microprogrammed) machine has a state register which points into a microcode memory. Styles are equivalent; choice depends on implementation considerations.

5. Sept. 2005EE37E Adv. Digital Electronics 3. Hardwired Control Two types of sequencing: –UNCONDITIONAL: only one successor to a state –CONDITIONAL: several possible successors depending on the value of a condition Because unconditional case is much more frequent in most execution graphs, a favored implementation of the sequencer consists of using a counter in which consecutive states in the sequence are assigned to the counter.

6. Sept. 2005EE37E Adv. Digital Electronics If the number of states is small, structures with one flip-flop per state result is straightforward mapping between the sequence (state diagram) and implementation is recommended. These also have the advantage that no decoder is required for the generation of the control signals.

7. Sept. 2005EE37E Adv. Digital Electronics Let us discuss the correspondence among the state diagram and the one flip-flop per state implementation: –The simplest case is the state with one predecessor, as shown in Figure 4.3.1a, the corresponding implementation is in Figure 4.3.1a’. –A more general case is depicted in Figures 4.3.1b and 4.3.1b’

8. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.1: PRIMITIVES FOR THE “one ip-op per state" APPROACH.

9. Sept. 2005EE37E Adv. Digital Electronics Control signals They depend on the state, conditions, and the external inputs to the system. The conditional control signals can be implemented in two ways, corresponding to the Mealy and Moore models of sequential systems. Consider the following statement: IF (sign = ‘0’) then A <= B; Else C<=D; End If; Let C_1 and C_2 the control signals for loading A and C, respectively. Figure 4.3.2. Illustrates the two alternatives.

10. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.2: IMPLEMENTATION ALTERNATIVES FOR CONTROL SIGNALS: a) Moore-type implementation; b) Mealy-type implementation. Often a control signal is active during several consecutive states. Instead of generating such a signal in each relevant state, it’s possible to introduce a clocked cell that can be set when the signal becomes active and cleared when it become inactive, as illustrated in figure 4.3.3.

11. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.3: CLOCKED CELL: a) STATE DIAGRAM; b) IMPLEMENTATION OF SIGNALS c 1 and c 3; c) TIMING DIAGRAM.

12. Sept. 2005EE37E Adv. Digital Electronics Design Example Let us now illustrate the use of some of the components discussed previously. A VHDL specification is given in Figure 4.3.4. For an implementation we need to developed a structural description having as components the data and control subsystems.

13. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.4: Block diagram and entity declaration

14. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.5: Architecture specification

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16. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.6: DATA SUBSYSTEM FOR DESIGN EXAMPLE

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25. Sept. 2005EE37E Adv. Digital Electronics Figure 4.3.6: IMPLEMENTATION OF CONTROL SUBSYSTEM.