IO with port manipulation while not interfering with i2c

Question

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?

• Nick Gammon: Have you tried doing it, and it has not worked?

Answer 1

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

void setup() 
{
 PORTA |= bit (0);
}
void loop() { }

Generates:

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.

• Charlie Miller: Presumably PORTA |= bit (0000); would set the 4 least significant bits?
• Nick Gammon: Not at all. Zero is zero, no matter how many leading zeroes you put there. You could do this: PORTA |= 0b00001111;
• Nick Gammon: 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.
• Charlie Miller: 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?
• Nick Gammon: No, you use the AND operator. PORTA &= ~0b00001111; The ~ operator takes the ones-complement, and you AND that into the port. Alternatively: PORTA &= 0b11110000;