Arduino•2mo ago

Need help optimizing 74hc595 updates

Hi i need a way to make my 595 update faster currently it takes about 750 to 850 ms for 8bytes (8bits per iteration) to transfer using shiftout neeh help with to either make shiftout faster or update 595 over spi

37 Replies

Onigiri_CloudOP•2mo ago

board: pro micro with atmega32u4

AnonEngineering•2mo ago

might be a code issue, Googling indicates shiftOut takes about 150 us (on a ~~much slower~~ Uno)

Onigiri_CloudOP•2mo ago

i am assuming u talking about per byte i am doing this 8 times per loop so it gets little high like 8 times that

AnonEngineering•2mo ago

8 x 150 us is still only about a millisecond

Onigiri_CloudOP•2mo ago

well i still require it being faster the delay of that adds up with other things making it like 3 miliseconds which is lot! and shift out adds the most of it to whole runtime

AnonEngineering•2mo ago

800 seemed long, 3 ms I can picture 🙂

Onigiri_CloudOP•2mo ago

so.... any idea how to make it faster?

AnonEngineering•2mo ago

apparently folks have gained some speed driving the 595 with SPI, I haven't ever done it myself

Onigiri_CloudOP•2mo ago

that seemed to be the most obvious answer but it's not working 595 output pins act kinda when i do spi and i am not sure if am doing it right or some bug? cause work fine with shiftout

#include <SPI.h>

const int latchPin = 14; 

void setup() {
  pinMode(latchPin, OUTPUT);
  digitalWrite(latchPin, LOW);

  SPI.begin();
}

void loop() {

  sendToShiftRegister(0b10101010); 
  delay(500);
  sendToShiftRegister(0b01010101); 
  delay(500);
}

// Function to send one byte to the 74HC595 via SPI
void sendToShiftRegister(uint8_t data) {
  digitalWrite(latchPin, LOW); 
  SPI.transfer(data);         digitalWrite(latchPin, HIGH);
}

#include <SPI.h>

const int latchPin = 14; 

void setup() {
  pinMode(latchPin, OUTPUT);
  digitalWrite(latchPin, LOW);

  SPI.begin();
}

void loop() {

  sendToShiftRegister(0b10101010); 
  delay(500);
  sendToShiftRegister(0b01010101); 
  delay(500);
}

// Function to send one byte to the 74HC595 via SPI
void sendToShiftRegister(uint8_t data) {
  digitalWrite(latchPin, LOW); 
  SPI.transfer(data);         digitalWrite(latchPin, HIGH);
}

this is what i did

pseud0•2mo ago

How did you connect each pin?

AnonEngineering•2mo ago

something like this? https://wokwi.com/projects/441920157134367745

Onigiri_CloudOP•2mo ago

VCC to vcc SER to arduino 16 as data OE to ground RCLK to 14 as latch SRCLK to 15 as clock SRCLR to VCC

pseud0•2mo ago

And what about pins like master reset

AnonEngineering•2mo ago

isn't that SRCLR?

pseud0•2mo ago

oh right that's your serial clear. GND to GNd technically.

Onigiri_CloudOP•2mo ago

it is the same wait going to try the same code again

AnonEngineering•2mo ago

yeah, i just changed the latch pin

Onigiri_CloudOP•2mo ago

i think... it got updated once or twice not sure... it's on 0b01010101

AnonEngineering•2mo ago

in other words your real life circuit isn't like the sim?

Onigiri_CloudOP•2mo ago

void loop() {

  sendToShiftRegister(0b10101010);
  Serial.println("0b10101010");
  delay(1000);
  sendToShiftRegister(0b01010101);
  Serial.println("0b01010101");
  delay(1000);
}

void loop() {

  sendToShiftRegister(0b10101010);
  Serial.println("0b10101010");
  delay(1000);
  sendToShiftRegister(0b01010101);
  Serial.println("0b01010101");
  delay(1000);
}

yea can say that too also installed the spark fun promicro board defination just to make sure it wasn't the cause of issue

AnonEngineering•2mo ago

how about not using 14 for latch (it's MISO)

Onigiri_CloudOP•2mo ago

it has pinMode(latchPin, OUTPUT); think that disables 14 as miso? but yea will try that wow that fixed it! so turns out... this not true thanks for all the help! so i might have lost my last digital io pin but spi works! didn't have any plans of using that as for now so.. guess its fine?

AnonEngineering•2mo ago

so it was using MISO as latch that was slowing it down?

wtf•2mo ago

use SPI oh you are

Onigiri_CloudOP•2mo ago

i think? latch definitely wasn't working right with miso as latch

AnonEngineering•2mo ago

but it runs at milliseconds now rather than 100's of ms?

Onigiri_CloudOP•2mo ago

i am using the pin the same way as before just changed the physical pin to 10 and 8bytes takes 80us now allows my whole to to run in about 1ms after further optimizations ofc i also had to speead up analog read clock a little

AnonEngineering•2mo ago

ok, 3 orders of magnitude is a good improvement 😉

Onigiri_CloudOP•2mo ago

it's actually more cause in the previous 3ms there were not analog reads involved the current code at slow speed as before would surely be more

AnonEngineering•2mo ago

well, thanks for bringing it up, i'd only ever used shiftOut with 595s before, TIL 🙂 can't figure out how to change the bit order though...

Onigiri_CloudOP•2mo ago

shift out works great ofc but if your are doing things that are like time critical it's has to be on spi i wanted my code to run under 1 ms so that does the job great think u just do MSBFIRST or LSBFIRST? for most/least significant bit never tried it myself it was always on MSBFIRST

AnonEngineering•2mo ago

yes, i was trying SPI.beginTransaction(SPISettings(15000000, LSBFIRST, SPI_MODE0)); but it seems to always do MSB

Onigiri_CloudOP•2mo ago

weird manually reverse all the bits u want to send? xD

AnonEngineering•2mo ago

there's always that!

Onigiri_CloudOP•2mo ago

it's a dirty way ofc but when nothing works yea

AnonEngineering•2mo ago

it works fine with shiftOut the SPI docs on Arduino are all over the place, seems the core changed in 2014 but most docs out there pre-date that one day, if i need the speed, and LSB, i'll dig more

Onigiri_CloudOP•2mo ago

yea i coudn't find much about it this was huge help

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Need help optimizing 74hc595 updates

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