Patent Publication Number: US-5834961-A

Title: Gated-clock registers for low-power circuitry

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electronic circuitry, and more particularly to a method and apparatus for reducing power usage by registers in low-power circuits. 
     2. Description of Related Art 
     Low power electronic circuits are commonly used in battery-operated products such as cellular telephones, calculators, computers, etc. Much effort has been devoted by the electronics industry to provide greater functionality for such circuits while reducing power requirements. 
     Various types of low power circuits use a plurality of registers (also known as register files) to store multiple bits of data. For example, a microprocessor or microcontroller circuit often has multiple registers for storing data values and instructions. FIG. 1 is a block diagram of a typical prior art microprocessor showing a set or bank of 8 registers 1 are coupled to input selectors 2 and output selectors 3 as shown. Other elements, such as multiplexers 4, 5, barrel shifter 6, and arithmetic logic unit 7, are coupled in a loop to the inputs selectors 2 and output selectors 3 for the registers 1. Typically, one to three registers are involved in any one processing cycle. A microinstruction controlling a particular processing cycle specifies which registers are involved. 
     Registers are generally designed to be fast and static (i.e., do not require refreshing), and thus generally comprise a plurality of static storage elements, each element comprising a plurality of transistor elements. Static storage elements can consume a fairly large amount of power. It would be desirable to reduce power consumption by registers used in low power circuits. 
     The present invention provides a method and apparatus for accomplishing this goal. 
     SUMMARY OF THE INVENTION 
     The invention is based on the observations that (1) a characteristic of static storage elements is that they draw some power when clocked (i.e., when a synchronizing signal is applied to each element to transfer input to output, if the element is enabled), but draw substantially more power when the data within the elements is changing instead of static, and (2) not all registers are used in each processing cycle. Accordingly, the present invention analyzes each microinstruction to determine which registers in a processing cycle are not involved in the processing cycle, and prevents those registers from being clocked during such processing cycle. Hence, inactive registers during a processing cycle do not consume power at the level of active registers, thus lowering overall power usage by any system employing such gated-clock registers. 
     The details of the preferred embodiment of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a typical prior art microprocessor showing a set of 8 registers coupled to input selectors and output selectors. 
     FIG. 2 is a block diagram showing the preferred embodiment of the invention. 
     Like reference numbers and designations in the various drawings indicate like elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than as limitations on the present invention. 
     Overview 
     A characteristic of static storage elements is that they draw some power when clocked (i.e., when a synchronizing signal is applied to each element to transfer input to output, if the element is enabled), but draw substantially more power when the data within the elements is changing instead of static. In conventional circuitry using registers (e.g., microprocessors or microcontrollers), all registers are clocked even when no change is to be made to the contents of each register. Further, not all registers are used in each processing cycle. Accordingly, the present invention analyzes each microinstruction to determine which registers in a processing cycle are not involved in the processing cycle, and prevents those registers from being clocked during such processing cycle. (As used herein, &#34;microinstruction&#34; means any collection of bits controlling processing cycles). 
     FIG. 2 is a block diagram showing the preferred embodiment of the invention. A set of registers 20, similar to the register bank 1 shown in FIG. 1, have their clock inputs 21 coupled through corresponding AND gates 26 to a clock signal from a clock source 24. Also coupled to each AND gate 26 is decode logic 28, which in turn is coupled to an instruction source 30 (which may be, for example, a ROM). While FIG. 2 shows the functional aspects of the invention most clearly, in practice it may be preferred to directly couple the clock source 24 to the decode logic 28, and incorporate the AND gates 26 directly into the decode logic 28. 
     The decode logic 28 determines which of the registers 20 are involved in each processing cycle by examining the appropriate fields or bit positions of the microinstruction controlling that particular processing cycle. That is, each microinstruction has at least one indication of which of the plurality of registers are active during each processing cycle. If a register is not involved in a processing cycle, then the decode logic 28 blocks the clock signal from the clock source 24 from being applied to the clock input 21 of that register 20 through the corresponding AND gate 26. (In the preferred embodiment, the output control signal of the decode logic 28 is synchronized to the falling edge of the clock to avoid clock &#34;glitches&#34; or noise). Conversely, if a register is involved in a processing cycle, then the decode logic 28 allows the clock signal from the clock source 24 to pass to the clock input 21 of that register 20. Hence, unused (inactive) registers during a processing cycle do not consume power at the level of used (active) registers, thus lowering overall power usage by any system employing such gated-clock registers. 
     EXAMPLE 
     The following commented code sets forth one example of the invention. The code may be directly compiled to generate circuitry that embodies the functionality of the decode logic 28, using, for example, &#34;Design Compiler&#34; from Synopsys, Inc., Mountain View, Calif. However, one of skill in the art will recognize that the decode logic 28 may also be implemented in conventional combinatorial logic, as a programmable logic array, or in equivalent circuitry. 
     
         ______________________________________                                    
Entity which decodes the instruction word to determine which              
register (if any) is to be updated based on the current                   
instruction                                                               
library                                                                   
      IEEE;                                                               
use   IEEE.std.sub.-- logic.sub.-- 1164.all;                              
entity reg.sub.-- update.sub.-- control is                                
 port (                                                                   
- Update specified register if logic 1:                                   
  update.sub.-- reg.sub.-- a : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- b : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- c : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- d : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- e : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- f : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- g : out std.sub.-- logic;                      
  update.sub.-- reg.sub.-- h : out std.sub.-- logic;                      
- Current processor instruction:                                          
  instruction : in std.sub.-- logic.sub.-- vector(23 downto 0);           
- Result of instruction condition check:                                  
  condition : in std.sub.-- logic                                         
 );                                                                       
end reg.sub.-- update.sub.-- control;                                     
architecture behave of reg.sub.-- update.sub.-- control is                
- Declaration of register mapping in instruction fields                   
 constant REGISTER.sub.-- A : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;000&#34;;                                                                 
 constant REGISTER.sub.-- B : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;001&#34;;                                                                 
 constant REGISTER.sub.-- C : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;010&#34;;                                                                 
 constant REGISTER.sub.-- D : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;011&#34;;                                                                 
 constant REGISTER.sub.-- E : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;100&#34;;                                                                 
 constant REGISTER.sub.-- F : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;101&#34;;                                                                 
 constant REGISTER.sub.-- G : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;110&#34;;                                                                 
 constant REGISTER.sub.-- H : std.sub.-- logic.sub.-- vector(2 downto 0)  
:= &#34;111&#34;;                                                                 
- Declaration of instruction format:                                      
- Format 1 allows register updating                                       
- Format 2 does not                                                       
 constant INSTR.sub.-- FORMAT.sub.-- 1 : std.sub.-- logic := `0`;         
 constant INSTR.sub.-- FORMAT.sub.-- 2 : std.sub.-- logic := `1`;         
- Explanatory constants to enhance readability:                           
 constant DONT.sub.-- UPDATE                                              
                : std.sub.-- logic := `0`;                                
 constant CONDITION.sub.-- MET                                            
                : std.sub.-- logic := `1`;                                
- Local signals:                                                          
 signal target.sub.-- reg : std.sub.-- logic.sub.-- vector(2 downto 0);   
 signal update.sub.-- bit : std.sub.-- logic;                             
 signal update.sub.-- reg : std.sub.-- logic;                             
begin                                                                     
- The target register is read from the current instruction                
- in format 1 commands and defaults to register A otherwise.              
 target.sub.-- reg &lt;= instruction(2 downto 0)                             
  when (instruction(23) = INSTR.sub.-- FORMAT.sub.-- 1)                   
 else REGISTER.sub.-- A;                                                  
- The update bit is read from the current instruction                     
- in format 1 commands and defaults to &#34;don&#39;t update&#34; otherwise           
 update.sub.-- bit &lt;= instruction(3)                                      
  when (instruction(23) = INSTR.sub.-- FORMAT.sub.-- 1)                   
 else DONT.sub.-- UPDATE;                                                 
- A register will update if the bit in the instruction is set,            
- and the condition for execution was met (this is evaluated              
- elsewhere)                                                              
 update.sub.-- reg &lt;= update.sub.-- bit                                   
 when (condition = CONDITION.sub.-- MET)                                  
 else DONT.sub.-- UPDATE;                                                 
- The individual update controls combine the overall update               
- control and the target register obtained earlier;                       
- note that all control signals are combinational and                     
- must be stabilized elsewhere                                            
 update.sub.-- reg.sub.-- a &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- A)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- b &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- B)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- c &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- C)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- d &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- D)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- e &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- E)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- f &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- F)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- g &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- G)                                                      
 else DONT.sub.-- UPDATE                                                  
 update.sub.-- reg.sub.-- h &lt;= update.sub.-- reg when (target.sub.-- reg  
= REGISTER.sub.-- H)                                                      
 else DONT.sub.-- UPDATE                                                  
end behave;                                                               
______________________________________                                    
 
    
     The following commented code sets forth one example of a register 20, including the AND gate 26, as well as the circuit that synchronizes the decode logic 28 to the falling edge of the clock. 
     
         ______________________________________                                    
Entity which implements a 16 bit data register with an update             
control supplied by a module which decodes an instruction                 
library                                                                   
      IEEE;                                                               
use   IEEE.std.sub.-- logic.sub.-- 1164.all;                              
entity data.sub.-- register is                                            
 port (                                                                   
- register value:                                                         
  data.sub.-- value : out std.sub.-- logic.sub.-- vector(15 downto 0);    
- new value to load:                                                      
  next.sub.-- value : in std.sub.-- logic.sub.-- vector(15 downto 0);     
- control signal from reg.sub.-- update.sub.-- control:                   
  do.sub.-- update : in std.sub.-- logic;                                 
- system clock:                                                           
  clock : in std.sub.-- logic;                                            
- active low asynchronous reset:                                          
  reset.sub.-- b : in std.sub.-- logic                                    
 );                                                                       
end data.sub.-- register;                                                 
architecture behave of data.sub.-- register is                            
- Explanatory constants to enhance readability:                           
 constant RESET.sub.-- VALUE                                              
                : std.sub.-- logic.sub.-- vector(15 downto 0)             
  := (others =&gt; `0`);                                                     
 constant CLOCK.sub.-- FALLING                                            
                : std.sub.-- logic := `0`;                                
 constant CLOCK.sub.-- RISING                                             
                : std.sub.-- logic := `1`;                                
- Local signals:                                                          
 signal stable.sub.-- update                                              
                : std.sub.-- logic;                                       
 signal masked.sub.-- clock                                               
                : std.sub.-- logic;                                       
 signal register.sub.-- output                                            
                : std.sub.-- logic.sub.-- vector(15 downto 0);            
begin                                                                     
- The clock to the register which stabilizes the update signal            
- operates on the system clock falling; this is because the               
- instruction is fetched on the system clock rise, so the update          
- controls are unstable during system clock high; the information         
- needed to determine whether to fire the register is needed              
- around the subsequent system clock high, and is                         
- thus optimally maintained by a falling edge register.                   
- During reset, register updates are disabled; it requires the            
- assertion of the update control from the reg.sub.-- update.sub.--       
control                                                                   
- module to enable the clock to the main register bank.                   
 process (clock, reset.sub.-- b)                                          
  begin                                                                   
  if reset.sub.-- b = `0` then                                            
   stable.sub.-- update &lt;= `0`;                                           
  elsif (clock&#39;event and clock = CLOCK.sub.-- FALLING) then               
   stable.sub.-- update &lt;= do.sub.-- update;                              
  end if;                                                                 
 end process;                                                             
- The clock to the main register bank is enabled only when the            
- stabilized control signal is active high; in this event, the            
- system clock is passed to the data registers themselves. Since          
- the stabilized control signal is a constant value from falling          
- edge to falling edge of the system clock, there is no danger of         
- introducing glitches.                                                   
 masked.sub.-- clock &lt;= stable.sub.-- update and clock;                   
- The data is held in a bank of registers as in prior art, except         
- that the clock is controlled in the manner described above.             
 process(masked.sub.-- clock, reset.sub.-- b)                             
 begin                                                                    
  if reset.sub.-- b = `0` then                                            
   register.sub.-- output &lt;= RESET.sub.-- VALUE;                          
  elsif (masked.sub.-- clock&#39;event and masked.sub.-- clock = CLOCK.sub.-- 
RISING)                                                                   
   then register.sub.-- output &lt;= next.sub.-- value;                      
  end if;                                                                 
 end process;                                                             
- The register value is the data output from the module:                  
 data.sub.-- value &lt;= register.sub.-- output;                             
end behave;                                                               
______________________________________                                    
 
    
     Power savings using the invention depends on the implementing semiconductor technology, the style of the storage element circuitry comprising each register, and the width of each register. In one embodiment in which the invention was used for a 16-bit register, power savings from clocking only active registers was about 20% to 25%. For a smaller register width, the percentage saved would be lower, while for a wider register, the percentage saved is potentially considerably higher. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.