Unless that HTML is wrapped around Serial.print() or Client.print(), you aren’t going to be able to use the F() macro. Storing HTML pages is another example where you probably want to keep the bulk of the text in PROGMEM. #3: Not good for big blocks of text (like HTML) Which makes this reference on PROGMEM from the site a great read. If that’s the kind of thing you are trying to store, then you’ll need to use the traditional PROGMEM statements to do so. While it is useful to use PROGMEM to store stuff like bit-patterns for characters, this macro isn’t going to help with that. The only code that can change PROGMEM is code stored in the boot partition, which is where the bootloader lives.Īnother tradeoff is that this macro only works for strings. Anything stored in PROGMEM cannot be changed by the running program. The key tradeoff to using the F() Macro is that you can’t use it for data you want to change. Tradeoffs to Consider #1: Can’t Change Data However, printing strings over Serial or to a LCD is a really slow process, so a few extra clock cycles really won’t matter. There’s a small performance overhead for this extra work. Then when it is time to access it, one byte of the data is copied to RAM at a time. The F() macro tells the compiler to leave this particular array in PROGMEM. Which means, all strings need to be put into RAM before they can be used. Since arrays are based around pointers, the compiler needs to put the array into RAM so that pointers work correctly. But, that technique won’t work with c-style strings or,well, arrays. Depending on the optimizations used, the avr-gcc compiler will avoid putting that value into RAM since it knows it’ll never change. Even the Uno’s big brother, the Mega2560, only has 8K of RAM.įrom the #define vs const post, the const keyword will tell a compiler that a variable is a constant and can’t change. This microcontroller only offers 2,048 bytes of RAM. Remember that the Arduino Uno (and it’s cousins) are based on the ATmega328. Simply wrap your string (const character array) with F(). Here’s an example of how you would use the F() macro with Serial.print() or Lcd.print(). That long string of code tells the compiler to keep a string inside of PROGMEM and not allow it to consume RAM. WString.h:#define F(string_literal) (reinterpret_cast(PSTR(string_literal))) I keep mixing the terms “macro” and “function.” F() really isn’t a function, it is a #define macro which lives in WString.h Which is unfortunate because it is one of the most powerful functions, which was added with the 1.0 release of the IDE. Search the entire Arduino Reference Page, and you won’t find a single mention of the F() macro. Which can be a problem because 2,048 bytes of RAM doesn’t allow for much. Which are “constant character arrays.” In order for those arrays to work properly, they get loaded into the ATmega’s RAM before your code starts running. What’s going on? You’ve run out of RAM!Īll of those Serial.print() statements are composed of c-style strings. You sprinkle Serial.print() statements everywhere you can think of, and then that’s when all hell breaks loose: Your code randomly locks up, the LEDs go crazy, and you’ve had it. It’s 3am but you are not going to bed until you squash this last bug.
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