how to access the lower 4 bits of port b in one go

guest · Guest

Hi to all.
Hope I can explain what I am trying to do do. I am using a pic16f876.
I have portb set up to drive 4 relays with the bottom 4 bits(Rb 0 to 3) and the top 4 bits are used for digital outputs.
How can I access the bottom 4 bits without affecting the to 4 bits , in 1 instruction.

For example , I want to turn on relays 1 and 3. I don't want to say:
relay1 = 1;
relay2 =0;
relay3 =1;
relay4 = 0;

I created a structure like this :
struct {
unsigned char relay1:1;
unsigned char relay2:1;
unsigned char relay3:1;
unsigned char relay4:1;

}relays;
#locate relays = 6.0

When I say relays = 5;

The correct relays are turned on , but the top bits are also affected , ie if
they were all 1 they are now 0.
Is there a way around this problem , or am I barking up the wrong tree altogether.
Cheers
Rob

Ttelmah · Guest

In your example, the problem is that the instruction 'relays = 5', will overwrite the entire 'carrier' byte, since the compiler does not know there is another potential variable there...
Try something like:

guest · Guest

Hi there.
Thanks for the help.
Cheers
rob