Direct Port Reading [solved]

Projects Programming
1

I'm trying to read an entire port at once, but for some reason it isn't working.
AD[0-7] are pins 22-29 on the MEGA (PortA)

If I do:

byte Value=0;
//Set PORTA to inputs
DDRA=0x00;
//no pullups
PORTA=0x00;
//Read Inputs
Value=PINA;

Value is always 0xFF

if I change the last line to:

for(int i=0;i<8;i++)
{
  Value = Value|digitalRead(AD[i]<<i);
}

It works fine.

What gives?

2

if I change the last line to ... It works fine.

Let's take a closer look...

Unrolling the loop and adding some whitespace we get...

Value = Value | digitalRead( AD[0] << 0 );
Value = Value | digitalRead( AD[1] << 1 );
Value = Value | digitalRead( AD[2] << 2 );
Value = Value | digitalRead( AD[3] << 3 );
Value = Value | digitalRead( AD[4] << 4 );
Value = Value | digitalRead( AD[5] << 5 );
Value = Value | digitalRead( AD[6] << 6 );
Value = Value | digitalRead( AD[7] << 7 );

After performing the array lookup and reducing the expression in parenthesis...

Value = Value | digitalRead( 22 );
Value = Value | digitalRead( 46 );
Value = Value | digitalRead( 96 );
Value = Value | digitalRead( 200 );
Value = Value | digitalRead( 416 );
Value = Value | digitalRead( 864 );
Value = Value | digitalRead( 1792 );
Value = Value | digitalRead( 3712 );

Does your board really have a pin 3712?

The second thread in this section is titled Read this before posting a programming question. If you had read that thread, you would have found this item...
Post your complete sketch (program code)!

3

My bad for the stupid error. The funny thing is that the result from that very wrong loop is what I expected to see, but I know why now.

Also, I know the "Post your complete sketch (program code)!" rule, but in this case it involves thousands of lines across 5 files and isn't really applicable when the problem is obviously in 3 lines of it.

Thanks for finding my facepalmer, it's been a long day with lots of code.

4

Hillridge:
Also, I know the "Post your complete sketch (program code)!" rule, but in this case it involves thousands of lines across 5 files and isn't really applicable when the problem is obviously in 3 lines of it.

I would rephrase that as "Post a complete sketch that demonstrates the problem". If your actual sketch contains a significant amount of code not relevant to the problem, write a test sketch to demonstrate the problem in the simplest possible way. Most likely, the very act of doing that will reveal a false assumption or enable you to spot the problem for yourself; it's a very useful investigative technique, even if you aren't planning to ask for help.