AVR Embedded Tutorial - Shiftregister

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Shift Registers

The sample code is for an ATtiny44 and the shift registers used are the 74HC165 for reading and the 74HC595 for writing.

Warning: The read routine is not compatible with Arduino shiftIn (...! The Arduino read function is for other shift registers. The clock must be controlled differently.

If you want to swap MSB to LSB, you simply have to swap downto to to.

Port access

For how the shift register commands access the ports, see here: GPIO output.

Do not forget to set the outputs as outputs (DDRx).

Port constants

The pins used for the ATtiny44 and the shift register. With the AVR you can use any pins. I have declared the ports, except the data port, globally, so access from the shift registers is a little faster. If you declared them locally, everything would be more complicated because we use direct port access. Arduino does this differently, but it is about 10 times slower.

IC	Attiny44		74HC165		74HC595
	Pin No.	Description	Pin No.	Description	Pin No.	Description
dataOutPin	13	PA0			14	DS
dataInPin	12	PA1	10	DS
latchPin	11	PA2	1	PL	12	ST_CP
clockPin	10	PA3	2	CP	11	SH_CP

A variable is declared for the port, this has the advantage that you do not have to change all ports in the shift register functions.

  var
   shiftPORT : byte absolute PORTA;
   shiftDDR  : byte absolute DDRA;
   shiftPIN  : byte absolute PINA;
 const
   dataOutPin = 0;
   dataInPin = 1;
   latchPin = 2;
   clockPin = 3;

Shift register write 74HC595

procedure shiftOut595(val : byte);
  var
    i : byte;
begin
  for i : = 7 downto 0 do // MSB FIRST
    begin 
      if(val and (1 shl i)) <> 0 then 
        begin
          shiftPORT := shiftPORT or (1 shl dataOutPin);
        end 
      else 
        begin
          shiftPORT := shiftPORT and not (1 shl dataOutPin);
        end;

      shiftPORT := shiftPORT or (1 shl clockPin);
      shiftPORT := shiftPORT and not (1 shl clockPin);
    end;
end;

Shift register read 74HC165

function ShiftIn165 : byte;
 var
   i : byte;
 begin
   Result := 0;

   for i := 0 to 7 do  // LSB FIRST
     begin 
       Result := (Result shl 1) or ((shiftPIN and (1 shl dataInPin)) shr dataInPin);
       shiftPORT := shiftPORT or (1 shl clockPin);
       shiftPORT := shiftPORT and not (1 shl clockPin);
     end;
 end ;

Arduino shift register

This version corresponds to Arduino's shiftIn.

function ShiftIn : byte;
  var
    i : byte;
begin
  Result := 0;

  for i := 0 to 7 do  // LSB FIRST 
    begin 
      shiftPORT := shiftPORT or (1 shl clockPin);
      Result := (Result shl 1) or ((shiftPIN and (1 shl dataInPin)) shr dataInPin);
      shiftPORT := shiftPORT and not (1 shl clockPin);
    end;
end;

Shift register examples

Read from a shift register

Before the shift register can be read out, the port status must be written to the latch register, this is done via the LatchPin.

Example for a 74HC165

 var
   b : byte;
 begin

   // Latch data
   shiftPORT := shiftPORT and not (1 shl latchPin);
   shiftPORT := shiftPORT or (1 shl latchPin);

   // Read shift register
   b := ShiftIn74HC165;

Example for cascaded 74HC165

 var
   b0, bn : byte;
 begin

   // Latch data
   shiftPORT := shiftPORT and not (1 shl latchPin);
   shiftPORT := shiftPORT or (1 shl latchPin);

   // Read shift register
   b0 := ShiftIn74HC165;

   // ...

   bn := ShiftIn74HC165;

Write to a shift register

If the data is written into the shift register, you have to add the latch clock that the current outputs become active.

Example for a 74HC595

 var
   b : byte;
 begin
   b := 123;

   // Write shift register
   shiftOut595(b);

   // Latch data
   shiftPORT := shiftPORT and not (1 shl latchPin);
   shiftPORT := shiftPORT or (1 shl latchPin);

Example for cascaded 74HC595

 var
   b0, bn : byte;
 begin
   b0 := 123;

   // ...

   bn := 789;

   // Write shift register
   shiftOut595(b0);
   
   // ...
   
   shiftOut595(bn);

   // Latch data
   shiftPORT := shiftPORT and not (1 shl latchPin);
   shiftPORT := shiftPORT or (1 shl latchPin);

See also

• AVR Embedded Tutorials - Overview
• SPI shift register - control shift register 74HC595 via SPI (very fast).