I'm using an Attiny84 with the Tiny Core. I'm try to use port manipulation to write the lower four bits of port A. I'm using i2c which occupies bits 5 and 7.

How can I write only to the lower 4 bits to avoid interfering with the i2c?

  • Have you tried doing it, and it has not worked?
    – Nick Gammon
    Mar 10 '18 at 5:40

I don't think you need to worry. A simple piece of code:

void setup() 
 PORTA |= bit (0);

void loop() { }


00000044 <setup>:
  44:   d8 9a           sbi 0x1b, 0 ; 27
  46:   08 95           ret

In other words, the compiler generates code to set that bit, ignoring all the other bits.

  • Presumably PORTA |= bit (0000); would set the 4 least significant bits? Mar 10 '18 at 20:04
  • Not at all. Zero is zero, no matter how many leading zeroes you put there. You could do this: PORTA |= 0b00001111;
    – Nick Gammon
    Mar 10 '18 at 20:52
  • The bit macro sets a bit by shifting the number 1 left the number of bits in the argument, so bit (0) is 1 << 0 which is the same as 1.
    – Nick Gammon
    Mar 10 '18 at 20:53
  • OK, so I understand now that I can set the bits to 1 without touching the i2c pins. Is it the same, but I suppose with the OR operator to set back to 0? Mar 10 '18 at 21:25
  • No, you use the AND operator. PORTA &= ~0b00001111; The ~ operator takes the ones-complement, and you AND that into the port. Alternatively: PORTA &= 0b11110000;
    – Nick Gammon
    Mar 10 '18 at 22:00

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