ATTINY84 Port Mapping and Output Control

Using Arduino Programming Questions
#1

I need to use PA 0,1,2,3 for operation of a small stepper motor (via a ULN2803A) controller by I2C
The I2C SCL and SDA are on PA 4 and PA6

Obviously I do not want to upset their status as DI/DO

So I have mapped PA 0 to 3 as outputs with

DDRA = DDRA | 0b00001111

My problem is that I then want to cycle the outputs to the stepper using an array of steps
ie

byte Step1 = 0x9; //0b 0000 1001
byte Step2 = 0xC; //0b 0000 1100
byte Step3 = 0x6; //0b 0000 0110
byte Step4 = 0x3; //0b 0000 0011

byte UnitStep[4]= {UnitStep1,UnitStep2,UnitStep3,UnitStep4};

Normally I would do this with successive calls to the array
PORTA = Step*;*
However this call also sets the state of the PA4,5,6,7, disrupting the I2C
I have tried several ways including PORTA =PORTA|Step without success.
Any suggestions gratefully received

#2

if you only want to toggle the lower four bits of PORTA

PORTA = PORTA & (0xF0) | Step

You need to clear the lower 4 bits ( & with 0xF0) and than add in (or in) your Step values

If you just OR in your step value, it won't clear any set bits.

#3

Thanks for that.. Works brilliant!

Pity this is not included in the Port Mapping examples in Arduino reference

#4