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.
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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).
From the Free Pascal wiki, page "AVR Embedded Tutorial - Shiftregister", as it stood on 2023-08-02 after a change by Zave2023, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.
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