Hy Sylvan,

I used also a mechanical knob. Yes you need a debouncing on these.
On my knob I get all 4 phases on every step. E.g. if you connect it to P1.0 and P1.1 you get the following sequences:

CW: 0-1-3-2-0-1-3-2-0-1
Val:5-6-6-6-6-7-7-7-7-8

CCW: 0-2-3-1-0-2-3-1-0
Val: 5-5-5-5-4-4-4-4-3

To read it out, I scan it with the timer interrupt every 256 cycle (T0 in mode 3). And if the last reading was 0 and the current was 1 the value was incremented, but if changing from 1 to 0 the value was decremented.


But how was the debouncing done ?
Its already included !

A mechanical contact vibrate and can give the following reading:

CW: 0-1,0,1-3,1,3-2,3,2-0,2,0

You can see 2 changes 0-1, and also 1 change 1-0. This result in 2 increasing and 1 decreasing and result so only in 1 pulse, how it was wanted.


The C program:

char val;

void digital_pot(void) interrupt 1
{
static char old = 4;
char tmp;
tmp = P1 & 3; // mask P1.0 and P1.1
if( old == 0 && tmp == 1 ) val++;
else if( old = 1 && tmp == 0 ) val--;
old = tmp;
}


Peter