Step 3: Exspearimintation

In this experiment, I sought to find out how long it took the digitalWrite() command to execute 1000 times, and then how long it took the “true c” style command to execute 1000 times. The code is fairly simple, and shown below:
void setup()
void loop()
 int initial = 0;
 int final = 0;
 initial = micros();
 for(int i = 0; i < 500; i++)
 final = micros();
 Serial.print("Time for digitalWrite(): ");
 initial = micros();
 for(int i = 0; i < 500; i++)
    PORTB |= _BV(PB5);
    PORTB &= ~_BV(PB5);
 final = micros();
 Serial.print("Time for true c command: ");
Feel free to try this out yourself. Here are the results I got:
(Seeeduino with ATMega168)
Time for digitalWrite(): 3804
Time for true c command: 348
So each style turned on the pin 500 times and turned it back off 500 times. digitalWrite() took 3804 microseconds, while the true c commands took just 348 microseconds. The true c commands are 10 times faster than the digitalWrite()!

Try out this experiment for yourself, all you need is an arduino and a computer.
<p>another way si use single instruction macro with CBI and SBI...</p><p>on bottom of post...</p><p>http://playground.arduino.cc/Main/AVR</p>
<p>An example (assuming an inductor of proper value is connected to the CLKIN pin and corresponding IO pin) ...</p><p>PORTB |= 0b00100000; // if needed repeat this operation for total of 32 cycles<br>PORTB &amp;= ~0b00000000; // if needed repeat for 32 cycles and so on ...</p><p>Then (in theory) after flashing this simple code the uC should be clocked to the RFID's carrier and also powered by it as well :D</p>
<p>Does anyone know how many actual clock cycles it takes to set pin states using this 'pure c' method? I found a very interesting post on emulating RFID tags using nothing but a PIC and a single radial coil. The same could be done with an ATTiny. It exploits the uC's internal capacitance and clamping diodes on the IO pins. Essentially, the RF modulation of the RFID reader supplies the oscillator frequency and just enough RF/induced current to even parasitically power the chip! One end of the coil is connected to the GP/CLKIN and the other end to a GP/IO pin. If the right amount of clock cycles are used in between switching pin states, it emulates a RFID card (simply switching low/high/low). There must be 32 clock cycles between each state per Manchester encoding. So, if you know how many cycles a pin state operation takes then you can achieve this kind of extremely-simple emulation. Here's the article - http://www.t4f.org/projects/open-rfid-tag/the-simplest-possible-rfid-emulator/</p>
If you count down rather than up in the loop it will be even faster! On my Arduino Mega changing the loop to count down for the True C commands reduced the time taken from 288microseconds to 192 microseconds, big difference if you need it to be as fast as possible!
<p>Counting down is more efficient, but only if you are comparing to zero. It is always more efficient to compare a value to zero than to another value! Similarly, do{}while(); loops are more efficient than for() loops. There are lots of pre-optimization tricks that you can do to squeeze your code into the smallest of chip spaces.</p>
<p>I am glad someone else finally realized the dirty secret behind Arduino... all of those libraries you love you use are horribly inefficient because they have to remap function calls to different pins on different devices.<br><br>You also have very little control over how the internal hardware is used (which timer, at what speed, etc). I regularly squeeze my optimized code into 99% of the space available on AVR chips. I want to get everything out of it that I can - no room for fluff.</p>
<p>If you know how to program C on an AVR, then it's likely Arduino's are not for you to begin with. However, I regularly program AVRs, but still find Arduino's useful, inefficient or not. The sheer number of libraries for almost every peripheral device imaginable makes it appealing for getting projects quickly off the ground and/or prototyping. If the project provides fruitful, I'll then port it to an AVR or write my own C library for the Arduino.</p>
<p>There is nothing wrong with that approach... in fact, I'd say that is probably the most intelligent use of Arudino possible - rapid proof of concept with the intent to optimize the code later.<br><br>However, that still doesn't solve the issues many people come across with multiple libraries try take control of the same hardware peripherals or IO Pins, and the user has no idea how any of it works, so they just give up.</p>
<p>&quot;Note that | can be found to the left of the backspace key, on the same key of the backslash.&quot;</p><p>On a Windows PC, with a U.S.-style keyboard.</p><p>Different machines will vary.</p>
<p>&quot;0b10000000 is an 8-bit binary number, you can convert it to hex for a cleaner look. Doing it manually is a pain&quot;</p><p>That's not true, every four bits can be directly translated to hex: 1000 is binary for 8 and 0000 is a 0, so the output will be 0x80. 0b11110101: 1111 is decimal 15 and hexadecimal F, 0101 is binary for 5 (dec and hex). Putting these numbers together gives 0xF5. If you get the hang of it, it is ways faster than looking up every single number.</p>
Thanks for sharing that !
It's better change just the part after the logic operator:<br><br>PORTB |= 0b00100000;<br>PORTB &amp;= ~0b00000000;<br><br>If you don't use the logic operators, but just the equal &quot;=&quot; sign, all the pins are going to be set like the byte you sent, not just pin B5. In the case where you set B5 high, you would set all ohter B pins to low.
Part of the reason this works is that the DigitalWrite() and DigitalRead() do some error checking before it sets the register bit--in particular, it checks and turns off PWM if it's enabled for that pin.&nbsp; Directly accessing the bit yourself is fine, but you're also assuming responsibility for skipping the check.<br /> <br /> This is pretty safe if your sketch is simple and doesn't do much multitasking or multiplexing of pins, but can get perilous if you've got a complex one.<br />
&nbsp;I wonder why I get this error:<br /> <br /> In function void loop():<br /> Error: 'PB5' was not declared in this scope.<br /> <br /> I use 0018 software.<br /> <br /> <br /> I used form PORTB = B.....;<br /> <br /> and it took 3808 vs 284 microseconds.<br /> <br /> I'm trying to get TLC5945 communicating with arduino, no luck with digitalWrites, hopefully this version works.<br />
&nbsp;I have the same problem. The example code is obviously missing something. Where and how should PB5 be declared?&nbsp;<br /> <br /> Me, I'm trying to PWM on all the pins of an Arduino Duemilanove to dim some LEDs.&nbsp;
&nbsp;Hmm, compiles fine for me on 0015, it may be something with the new software.<br /> <br /> If anyone has problems, replace<br /> &quot; PORTB |= _BV(PB5);<br /> &nbsp; PORTB &amp;= ~_BV(PB5);&quot;<br /> <br /> with:<br /> &quot;PORTB = 0b00100000;<br /> PORTB = 0b00000000;&quot;<br /> <br /> Which should toggle PB5 like the original code. PB5 shouldnt have to be declared, as winAVR should recognize it as a pin, but arduino 0018 may be doing something funky with that.<br />
Thanks this helped a lot becaus its the only way to programm the 16 extra pins on the Seeduino Mega<br />
Thanks for this instructable. Not only does this save a lot of time, it also knocks the program size down quite a bit.&nbsp; You can also you the registers DDRx an PINx (where x is the letter of the port) to change a port's input/output status, and to check what the input value is.<br />
&nbsp;So do you then use the AVR Studio 4?&nbsp;<br /> Or how do you program an Arduino in this way?<br /> Can you use the Arudino's usb ftdi connection?<br /> <br /> Please elaborate. Thanks!<br />
I use Arduino with the &quot;true c&quot; style commands.<br /> <br /> You can use AVR Studio 4, however it is quite complicated and you cannot use the usb connection, you will need a ISP programmer to burn the programs.<br />
Maybe using <a href="http://sites.google.com/site/funlw65/electronics/jaluino-pinguino-28-pins-starting-bo/freejalduino">this</a> &quot;Arduino&quot; ?<br /> <br /> <br />
Actually, there is a very nice writeup on &quot;Direct Port Manipulation&quot; (which is the Arduino folk's name for what you are doing) on the Arduino site.&nbsp; Here's the link:<br /> <br /> &nbsp; <a href="http://www.arduino.cc/en/Reference/PortManipulation">http://www.arduino.cc/en/Reference/PortManipulation</a><br /> <br /> The description at the top of the page says &quot;Port registers allow for lower-level and faster manipulation of the i/o pins of the microcontroller on an Arduino board.&quot;<br /> <br /> Wayne<br />
Nice tutorial.&nbsp; You should enter it into the Arduino contest.<br />
Some explanation of C &quot;bitwise&quot; operators might be helpful, too... And maybe break it down without using compound assignment ops first.<br /> <br /> Here's my fav bitwise link:<a href="http://www.avrfreaks.net/index.php?name=PNphpBB2&amp;file=viewtopic&amp;t=37871"> AVR bit tutorial</a><br /> <br /> Other than that, it's a good 'ible...<br />
Thanks! That may be a bit complicated, but I&nbsp;added a final step with that link.&nbsp;
*facepalm*<br /> horrible pun was unintended
How come you didnt you Hardware SPI instead of Bit Banging the commands to the PWM chip? I checked briefly and it seems it use SPI&nbsp;commands.&nbsp;Looking at the Atmega168 Data sheet, It Does have support for hardware SPI, and if you used hardware, it would much Much faster than bit Banging SPI Commands since you can usually get up to 20Mhz Data Transfer depending on your internal clock (or external, but usually it requires you to go with the max clock rate for the chip) since it would Free up the CPU to do other things.<br />
SPI uses the USART&nbsp;doesnt it? And thats pre-wired on the arduino to support the bootloader<br />
True, actually there is a TLC5940 library for Arduino that uses SPI, however I wanted to see what was actually going on so I just did it the &quot;manual&quot; way by bit banging. <br /> <br /> SPI doesn't use the USART, it uses SS/MOSI/MISO/SCK, the latter 3 are used for ISP programming also.&nbsp;
does this work for arduino mega? (chip is atmega1280<br />
&nbsp;Yes it does, however I&nbsp;cannot find a chart that shows which arduino pin goes to which pin... you may have to take a peek in to the schematic of the Arduino Mega.
reading and writing pwm pins will also be somewhat slower than writing non-pwm pins.<br /> <br /> hmmm, i notice that there's no longer any range checking on pin numbers.&nbsp; although this speeds things up i wonder what havoc a pinMode or digitalWrite to some out of range value might wreak.<br />
Good to know that there is such an overhead!<br /> How does the ShiftOut() command compare to this? Looking at the arduino reference it seems to be dedicated for this task (if the SPI pins are available..)<br /> Would be nice if someone posts some numbers, since i do not own an arduino to test with..<br /> <br />
There's a pretty extensive discussion on pin-toggling on the arduino forums here: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1230286016<br /> <br />

About This Instructable


157 favorites

Bio: Check out razorconcepts.net for some projects.
More by RazorConcepts: Start an Online Microbusiness Joule Thief - use LEDs with only one AA battery! Arduino is Slow - and how to fix it!
Tags: arduino
Add instructable to: