Jeff Epler's blog

30 April 2014, 21:14 UTC

Towards fast I/O in Arduino for AVRs

So I've been working on something. I've only looked at assembler output so far, but it looks right and promising.

Basically, I'd like to let you name I/O pins via typedef, then efficiently read and write the pins. And I'd like to make Arduino's digitalWrite just as efficient when the pin argument is a constant.

All instruction counts are in Debian Wheezy's avr-gcc version 4.7.2 with -mmcu=atmega328p -Os specified on the commandline.

// You can declare ports (they use zero RAM)
typedef IOPIN<PORT_B, 0> iob0;
typedef IOPIN<PORT_B, 1> iob1;
typedef OCPIN<PORT_B, 1, true> ocb1; // emulate an open collector port

// .. and use them efficiently (instruction counts never include ret)
int en() { iob0::output(); }                // 1 instruction
int fn() { iob0::toggle(); }                // 1 instruction
int gn() { ocb1::set(); }                   // 2 instructions
int hn() { ocb1::clear(); }                 // 2 instructions
int jn(bool b) { iob0::write(b); }          // 5 instructions
int kn() { iob0::write(1); }                // 1 instruction

// you can use fastDigitalWrite just like digitalWrite (but faster and inline)
int c(int i) { fastDigitalWrite(1, i); }    // 5 instructions
int d() { fastDigitalWrite(1, 1); }         // 1 instruction

// these have a variable port so they still turn into calls to digitalWrite
int a(int i, int j) { fastDigitalWrite(i, j); }
int b(int i) { fastDigitalWrite(i, 1); }

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27 December 2011, 20:35 UTC

It's gratifying when the compiler is right

I've been playing with clang because I'd like to use its static analyzer on the code at $DAY_JOB. Unfortunately, it frequently bombs while building our code. Fortunately, many of the bombs are due to our bugs, not theirs. Here's one example:

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